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Anesthesia & Analgesia:
doi: 10.1213/ANE.0000000000000287
Anesthetic Pharmacology: Special Article

Coagulation, Flocculation, and Denaturation: A Century of Research into Protoplasmic Theories of Anesthesia

Perouansky, Misha MD

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From the Department of Anesthesiology, University of Wisconsin School of Medicine and Public Health, Madison, Wisconsin.

Accepted for publication February 26, 2014.

Funding: No external funding.

The author declares no conflicts of interest.

This report was previously presented, in part, at the Society for Neuroscience.

Reprints will not be available from the author.

Address correspondence to Misha Perouansky, MD, Department of Anesthesiology, University of Wisconsin School of Medicine and Public Health, B6/310 CSC 600 Highland Ave., Madison, WI, 53711. Address e-mail to mperouansky@wisc.edu.

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Within two decades of the discovery of anesthesia, the physicochemical concept of colloid and the biological concept of protoplasm had emerged. Fusion of these concepts into a theoretical framework, which has been largely forgotten decades ago, promised to uncover fundamental biological truths and determined research into anesthetic mechanisms for a century after “Ether Day.” Observations of optical changes in unstained tissue were condensed into a theory of anesthesia by coagulation of protoplasm in the 1870s. The underlying hypotheses, conformational changes of proteins within the protoplasm cause all behavioral effects of anesthesia, continued to be pursued well into the 20th century. The goal was to explain anesthesia using physical chemistry within a fundamental cell biological framework. This large body of work, swept aside during the decades of lipid membrane hegemony, has remained in obscurity even after proteins in excitable membranes became firmly established as mediators of the immediate anesthetic effects. This article is a reminder of the prolonged interdisciplinary research effort dedicated to “protoplasmic theories” at a time when attention is increasingly directed toward examining the nature of (un)consciousness well as noncanonical consequences of anesthetic exposure that are not easily accounted for within conventional pharmacological concepts.

“…the mechanism behind the anesthetic reaction, resulting in a reversible gelatinization of the protoplasm… may be viewed as gelatinization, incipient coagulation, thixotropy, coacervation, or reversible protein denaturation.”

—HI. Virgen

The arcane-sounding “protoplasmic coagulation” theories of anesthesia and their descendants deserve a distinguished place in the history of anesthetic research. Not unlike the contemporary role of anesthetics as tools within the larger consciousness research endeavor, protoplasmic theories not only strove to explain anesthesia per se but also used anesthetics to probe the fundamental material essence of life itself. These theories emerged from the identification of the physical basis of life with cellular protoplasm,a an idea that took the scientific world by storm in the 1860s.1 When, within 1 decade, it was discovered that anesthetics caused visible changes in animal tissues, it seemed obvious that not only did anesthetics act directly on protoplasm (“coagulation” being one of the terms used to describe the effect) but that these agents could be used to investigate what “being alive” actually meant. The subsequent rise of these theories was driven by an intense dialogue between natural scientists from all disciplines, especially cell biologists and physical chemists involved in the rapidly expanding field of colloidb chemistry. As a result, protoplasm-based theories of anesthesia benefitted from the theoretical and experimental advances in colloidal sciences, which generated models capable of guiding research and accommodating experimental results within a larger scientific framework. These physicochemical, protoplasm-oriented theories of anesthesia were pursued by leading scientists across political divides in Europe, the Soviet Union, and the United States and enjoyed remarkable longevity. As late as the 1950s, >70 years after their initial formulation, a major review journal described anesthetic mechanisms largely in terms of colloid chemistry and coagulation-like changes in protoplasm (see introductory quote).2

As biological membranes became accessible to investigation, the popularity of protoplasm science and protoplasm-based theories of anesthesia declined after the second World War. However, delayed and long-lasting effects of general anesthetics are not easily accommodated within conventional notions of anesthetic pharmacology.3 Targets and mechanisms invoked by protoplasmic theories attempted to address the complexity of water,4 lipid,5 and protein6 interactions in the biological milieu referred to as protoplasm and are worth revisiting today.

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By the middle of the 19th century, the interpretation of Robert Hooke’s (1635–1703)c,7“cellulae had evolved from that of mere communication channels to that of essential building blocks of all organisms. The two competing concepts of the cell were the cell wall/cell membrane theory and the protoplasm doctrine.1 Theodor Schwann (1810–1882) saw the containing wall or membrane of both cell and nucleus as of primary physiological importance. The respective content, a homogeneous, transparent, “watery” fluid was relegated to a marginal role. The alternative view emphasized the biological essentiality of protoplasm, a gelatinous, mysteriously cohesive substance endowed with spontaneous streaming movements (later termed cyclosis) and therefore appearing to be enigmatically “alive” on its own.8

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Proponents of the protoplasmic view (e.g., Max J. S. Schultze [1825–1874] and Ernst W. von Brücke [1819–1892]) claimed that the cell itself was simply a mass of protoplasm with a nucleus, denied the necessity of a cell wall or a membrane and even argued in favor of abandoning the Schwann-Schleiden concept of the cell altogether.1

The term protoplasm became a household world almost overnight in 1868 when, in a public lecture reprinted in popular magazines, the English biologist Thomas Henry Huxley (“Darwin’s bulldog,” 1825–1895) proclaimed that protoplasm, of which little was known beyond that it was “proteinaceous” and gelatinous, was indeed “the physical basis of life.”9 While Huxley neither discovered nor contributed substantially to investigating the properties of protoplasm per se, he espoused the concept and captured the imagination of the thinking public. In one rhetorically brilliant phrase, Huxley summarized decades of research by many individuals: the concept of protoplasm filled the void of Hooke’s cellulae with life itself. In the debate on the primacy between cell wall and cell content, public opinion decidedly shifted in favor of the latter and protoplasm came to be viewed as “the original active substratum of all vital phenomena,” a view enthusiastically embraced also by none other than Friedrich Engels (1820–1895).e,10 This shift had decisive consequences for theories of anesthesia well into the 20th century. For this to occur, however, a theoretical framework and experimental tools for inquiry into the nature of protoplasm were necessary.

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It is difficult to imagine that the idea of protoplasm as the essential seat of vital activities could have captured the imagination of experimental and theoretical biology so thoroughly and for so long if its conceptualization had not coincided with the discovery of colloids and the subsequent flourishing of colloid chemistry. The original coagulation theories of anesthesia were based on the interpretation of changes in protoplasm observable with light microscopy or even with the naked eye. Therefore, the fact that colloid systems display dynamic changes in their optical properties (e.g., the alteration of their color, turbidity, and birefringence) because the wavelength of visible light lies in the colloidal size range11 played an important role for mechanistic theorizing about the nature of anesthetic-induced changes in protoplasm.

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The term colloid was coined by Thomas Graham early in the 1860s (Fig. 1A). Before that, pseudosolutions of sulfur, silver chloride, Prussian blue, and colloidal gold in water were reported by Francesco Selmi (1817–1881) and Michael Faraday (1791–1867) in 1845 and 1857, respectively,11 but colloid gold solutions had already been prepared by alchemists in the 17th century.12 Colloid solutions are present as dispersed systems and are characterized by slow diffusion and, under normal gravity, slow to negligible sedimentation, as well as unusually dynamic optical (Fig. 1B) and rheological properties. Depending on the nature of dispersed substance, dispersion medium and longevity, colloidal systems are termed dispersions, emulsions, gels, suspensions, or aerosols. These systems are held in equilibrium by a balance of repulsive electrostatic forces opposing the London-van der Waals attractive forces, and thus, their properties are considerably influenced by particle size (typically 1 nano–1 micrometer), shape (spherical versus nonspherical), and charge. As a result of this stable but fragile equilibrium, dramatic changes can be rapidly induced by seemingly innocuous alterations: gold solutions prepared by Faraday more than a century ago and potentially stable indefinitely can be precipitated on the spot by adding salt.12

Figure 1
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Terms like flocculation, aggregation, precipitation, dispersion, coacervation,f and coagulation have all been used to describe these state changes depending on reversibility, exact circumstances, and preferred terminology of the time. Concepts like gelation (understood as “partial” coagulation) that captured reversible changes in the state of a colloidal system were particularly useful as models to explain coagulation-like effects of anesthetics on the protoplasm. Concentrated colloids also have remarkable rheological properties termed thixotropy and dilatancy. In nature, these latter properties are present in quicksands, which break down when agitated slowly but act like a solid under sudden impact.13 In essence, these frequently confused terms describe the capacity for sudden, isothermic increases, or decreases in viscosity, depending on the intensity of shear stress, for example, dense ketchup suddenly flowing freely from a bottle when shaken vigorously. Thixotropy was a property explicitly ascribed to protoplasm13 and also thought to be influenced by anesthetics. The laws governing the behavior of colloids are extraordinarily complex and occupied leading scientists for another century.g As the study of life’s defining principles was understood by many as the physicochemical analysis of protoplasm, this aspect of biological inquiry became a natural part of colloid chemistry. As a result, in addition to developing into important commercial industries, colloid chemistry inspired the study of anesthetics, supplied theoretical models, provided the terminology to articulate results, and thereby profoundly influenced the whole research enterprise.

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“…a very fundamental problem is the maintenance in protoplasm of two apparently incompatible properties: the capacity to flow and the possession of structural qualities necessary to satisfy elasticity and tensile strength.”

—William Seifriz

It is not surprising that this novel branch of chemistry influenced biologists seeking to define the nature of life: simple combinations of inert components resulted in mixtures displaying novel, complex, and unpredictable properties. The analogy to protoplasm was tempting. Moreover, if one accepted the colloidal essence of protoplasm, the development of colloid chemistry offered increasingly sophisticated approaches for the experimental dissection of its properties and hence, arguably, of life itself. The mysteriousness of colloids only increased their apparent similarity with and hence their appeal as valid working models of protoplasm. Conversely, scientists primarily interested in colloids found it tempting to apply their models, experience and knowledge to the solution of the enigmatic life-suspending phenomenon of anesthesia. This mutual fertilization is illustrated by the publication of articles dealing with anesthetic mechanisms in specialized colloid chemistry journals (see below) and by the contributions of prominent colloid chemists, for example, Herbert Freundlich (1880–1941) to hypotheses about anesthetic mechanisms.

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Visible Changes in Protoplasm

Within a decade after Thomas Huxley’s proclamation, Heinrich Ranke (1830–1909), Carl Binz (1832–1913), and Claude Bernard (1813–1878) observed that addition of anesthetics to muscle and brain tissue, either in the form of filtered extract or as fresh slices, caused an opacification of the preparation.14–16 All 3 believed this observation to have a causal relationship to anesthesia. Binz and Bernard interpreted the change in appearance of cortical and muscle tissue as “semicoagulation” of the protoplasm and saw in them a microscale analog of in vivo chloroform rigidity (“rigidité chloroformique” or “Chloroformstarre”), a recognized clinical phenomenon.h Bernard illustrated the underlying logic with an intuitive analogy: anesthetized protoplasm was to normal protoplasm what ice was to water. In modern parlance, the claim was that anesthetics caused a phase transition of protoplasm which, in any organism as a whole, presented as anesthesia. This theory of anesthesia intertwined the dispute over the physical essence of life with the most important pharmacological discovery of the era. Indeed, Bernard proposed that anesthetics were assays separating “true life” from “mere chemistry.”17

After Ranke, Bernard, and Binz, theories explaining anesthesia as coagulation of protoplasmic proteins went dormant for a while until the development of colloidal chemistry provided an impetus for their resurgence toward the end of the 19th century. At this point, close interaction between protoplasm-oriented cell biology (both plant and animal) and colloid chemistry resulted in the quick adoption of newly discovered colloidal properties as models for novel theoretical variation of the protoplasmic theory proposed by Bernard. One of the earliest theories of narcosis that clearly integrated Thomas Graham’s (Fig. 1A) ideas was Raymond Dubois’ “dehydration theory” of narcosis that he developed experimenting on plants in the 1880s. In his opinion, replacing water with anesthetics turned the normal hydrogel of protoplasm into ether, alcohol, or chloroform gel. These anesthetic-based gels were denser than the physiological hydrogel and suspended vital processes, an analogy to what desiccation does for plant seeds.18

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Lipids Fit into the Colloidal Concept

Partly fueled by Meyer and Overton’s well-known publications, the renewed interest in lipids (termed lipoids in contemporaneous literature), led to experiments testing whether lipids in solution shared anesthetic sensitivity with proteins. Lipid particles dissolved in fat solvents remain true molecular solutions but tend to coalesce in aqueous media. Lipids were therefore considered to be of interest within the coagulation framework of anesthetic interaction with cell contents. Fortuitously, similar to Ranke’s proteinaceous preparations, addition of chloroform turned clear lipid-containing solutions into cloudy suspensions.19 Chloroform’s action in this context was explained by its aggregating effect on minuscule dissolved lipid particles leading to the formation of “coacervates” (in later terminology) that reached a size visible under the microscope and hence clouded the suspension medium. In the wake of these findings, both proteins and lipids could be accommodated within one protoplasm-colloidal paradigm of anesthetic action especially because lipids were not yet associated with any membranes. This mechanistic framework was also accepted by Hans Meyer (of the Meyer-Overton rule) who saw the protoplasm as a colloidal emulsion of proteins and lipid particles whose scaffold (“Gefüge”) and hence its internal surface resistances (“innere Oberflächenwiderstände”) were altered by narcoticsi in a way that somehow resulted in anesthesia.20 This integrative colloidal hypothesis of the cell persisted into the 1930s, when complex coacervates of proteins, lipids, carbohydrates, and nucleic acids would be interpreted as “static (equilibrium) models of cells” while living cells were the same mixture of components but in a state of nonequilibrium.21

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Prescient Consideration of “Clinical Concentrations”

Remarkably few scientists paid close attention to considerations of pharmacokinetics and “clinical” concentration ranges when studying anesthetics in vitro. In undeservedly forgotten articles communicated to the Royal Academy in 1904 and 1906 by Charles S. Sherrington (1857–1952, Nobel prize for Physiology or Medicine in 1932), Benjamin Moore and Herbert Roaf22,23 reported detailed and foresighted experiments quantifying the uptake of chloroform into various solutions (water, saline, serum, and whole blood), measuring the resulting partial pressures and explicitly addressing not only clinically relevant concentrations but also the effect of temperature, drug kinetics, and substrate on uptake. They were the first to note that blood and its components took up much more chloroform than did water or electrolyte solutions. They also observed that serum exposed to 1% chloroform in air (corresponding to approximately 3.9 mM at 37°C) acquired “a peculiarly opalescent and fluorescent appearance, but remained quite transparent to transmitted light.” Only at higher concentrations well above the clinical range (MAChuman = 0.5%) did it precipitate.24 In their opinion, chloroform formed chemical compounds with proteins, be it in blood, serum, or protoplasm. Because these anesthesia-inducing protein-anesthetic combinations were unstable, they existed only as long as chloroform partial pressure was maintained in the tissue, thereby elegantly explaining rapid emergence from anesthesia. In the opinion of Moore and Roaf,22 opalescence was a fully and easily reversible precursor of Bernard’s coagulation or Binz’s granulation that had escaped the attention of the original investigators. As for the mechanism of action, these unstable anesthetic-protein compounds were thought to “limit the chemical activities of the protoplasm” and thereby lead to narcosis.22 What chemical activities could that be? Jacobj25 echoed widely held ideas when he proposed that anesthetic agglutination of protein molecules caused “cellular suffocation:” respiration was considered an essential activity of protoplasm and asphyxia-based theories were very popular at that time.

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Mysterious Changes in Viscosity

The dynamic changes in viscosity of protoplasm puzzled biologists from the very beginning. Cycles of liquefaction (solation) and solidification (gelation) accompanied fundamental biological processes, for example, fertilization and mitosis observed in sea urchin eggs. It seemed like the healthy, normal protoplasm was balanced in a delicate “Aristotelian” equipoise between the excesses of structureless fluidity and inflexible rigidity as illustrated in the holistic conception in Figure 2.

Figure 2
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Anesthetic effects on viscosity were first described in artificial colloidal systems.26 Once ingenious experimental models pioneered by Heilbronn27 had been developed, investigations in living organisms followed. Heilbronn observed and quantified the speed of particles moving through protoplasm of plants (starch granules) and of slime mold (minuscule iron rods) under the influence of gravity and magnetic force, respectively.j He noted that chloroform and ether decreased and increased protoplasm viscosity depending on circumstances.27,28 Others found that the viscosity of protoplasm was generally reduced by stimulants (e.g., caffeine) and increased by depressants, including anesthetics.29 The consensus was that increased and decreased fluidity corresponded to the excitatory and depressed states of anesthesia, respectively, while the irreversible gelation was the cellular correlate of in vivo chloroform rigidity. Again, it seemed only natural to conclude that a slight increase in viscosity matched the reversible phase of Bernard and Binz’s coagulation, but the problems of reversibility and of the high concentrations required to achieve measurable effects on viscosity nevertheless remained. Molecular hypotheses of how anesthetic-induced changes in the state of a colloidal system were sorely needed.

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Searching for a Molecular Explanation

How can alterations in refractive properties, aggregation or precipitation of proteins, changes in thixotropy, or in other colloidal properties of protoplasm by anesthetics be explained on a molecular level? Colloid chemistry paired with abundant theorizing provided the necessary mechanistic constructs. The general idea was that in the appropriate colloidal system anesthetics could have at least a dual mode of action (on the dispersion medium and on the dispersed particles) that could be additive, thereby providing a high degree of explanatory flexibility.30 For example, reduction of the dielectric constant of some solvents by narcotics could increase the attractive forces between particles and hence promote their aggregation, an argument buttressed by the fact that alcoholic reduction of the dielectric constant followed the rule of the homologous series.k Similarly, proaggregating effects could result from a reduction of the surface tension of the solvent around the dispersed particles, for example, by narcotic adsorption at the interface between the particle and the medium. Depending on the nature of the particles and the narcotics, their interaction could strip the particles of their hydrating envelopes, thereby facilitating aggregation, not an implausible scenario.31 Many different interactions were theoretically possible and could be molded to account for almost any experimental finding. For example, Loewe32 proposed that anesthetics eagerly adsorb to hydrophobic particles rendering them hydrophilic. Hydrophilic colloids, in turn, were less stable and therefore tended to aggregate or even precipitate. At about the same time, Traube33 proposed the so-called Haftdruck (adhesive pressure) theory of narcosis that was similarly based on narcotic-induced reduction of surface tension around colloidal particles and also supported by correlations, in this case between anesthetic potency and reduction of surface tension. Otto Warburg34 (1883–1970, Nobel prize for Physiology or Medicine in 1931) used aggregation of respiratory enzymes, which he called “reduction in the degree of dispersion” (commonly used terminology in colloid chemistry), and the resulting reduction of the enzymatically active surface area to explain his version of the anesthesia-by-cellular-asphyxia idea. Similar conclusions, for example, inhibition by reduced dispersion of enzymatic proteins were reached for nucleoproteins,35 for anesthetic inhibition of digestive enzymes, and for the anesthetic effect of magnesium.18

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Coagulation-Based Theories After World War I: Ferrocyanide, Infusoria, Copepods, and Slime Mold

Despite the unresolved problems of supraclinical concentrations required for visible effects and the issue of reversibility, Bernard’s coagulation theory continued to provide rich soil for theorizing, with individual scientific backgrounds shaping the line of argument.

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The Chemists

At one end of the spectrum were chemists. Half a century after Bernard’s death, Wilder D. Bancroftl and George H. Richter of Cornell University attempted to rehabilitate Bernard’s theory in its original coagulating version.36 The importance that colloidal chemistry had gained in providing concepts for anesthetic action is apparent in their argument that Bernard simply lacked the necessary knowledge (in colloid chemistry) to reject the main objections against his theory himself. In a remarkably simplistic approach, they refuted criticism of Bernard’s theory by showing that in the presence of electrolytes copper ferrocyanide was precipitated by low concentrations of alcohol, Brownian motion of yeast ceased, and the cells flocculated in anesthetic-containing medium (Fig. 3). The reversibility of these effects in their hands was called by the authors “a great triumph for the theory of Claude Bernard.”

Figure 3
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The Biologists

In the Soviet Union, Dimitry N. Nasonov (1895–1957) and his school took a diametrically opposite approach to Bancroft’s to investigate the nature of protoplasmic responses to environmental agents.m Nasonov was a biologist of the living cell and the living protoplasm within it. His respect for life is evident in the following quote about cytologists working on fixated preparations: “the cytologist acts like an archeologist who reconstructs vanished life from sherds. The difference is that while the archeologist does not have anything but sherds at his disposal, the cytologist first breaks life into sherds and then attempts to reconstruct it.” Nasonov’s laboratory embarked on a decades-long investigation of a “nonspecific cellular reaction,” a fundamental cellular response to a variety of stressors and agents. The group focused specifically on protoplasm and used mostly light microscopy paired with vital dyes. Like Bernard in the 19th century, Nasonov et al. were interested in fundamental biological phenomena, in their case the response of the living substance to external stress, and interpreted anesthetic effects in this larger context. The work gave rise to a general denaturation theory of excitation, damage, and necrosis.37 They interpreted visible changes in the protoplasm as reversible denaturation of intracellular proteins, that is, changes in the tertiary and quarternary structures affecting their solubility (note that denaturation can be reversible).38 Using equilibrium distributions of vital dyes between the cell and its surroundings, Nasonov’s school collected a large amount of experimental data suggesting that partial, physiological protein denaturation, manifested by visible changes in the protoplasm within a spectrum of colloidal effects, was the common end result of different forms of natural cellular activity (muscle contraction, synaptic transmission, secretion, etc.), physiological stress, and anesthesia39 (Fig. 4). A contemporary rendition of this theory has been published.40

Figure 4
Figure 4
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The Botanists

Somewhere between Bancroft and Nasonov lies the chemical-biological approach to protoplasm and anesthetics represented by the botanists Vladimir V. Lepeschkin (1876–1956) and William Seifriz (1888–1955). Lepeschkin, a botanist with a strong physicochemical perspective fine-tuned in the laboratory of his mentor Friedrich Wilhelm Ostwald (1853–1932, Nobel prize in Chemistry 1909), had a long-standing interest in protoplasm throughout his life as an emigrant. Working at the Scripps Institute of Oceanography, Lepeschkin published experiments in copepods (small crustaceans) supporting, in his interpretation, protoplasmic coagulation theories of anesthesia. Taking advantage of the rapid equilibration of agents between surrounding water and the body of these minuscule animals, Lepeschkin studied the effects of chloroform, ether, and alcohol (Fig. 5). The inclusion of mercury bichloride (a toxin) and egg white (a model colloid) into the same figure as the copepods illustrates the holistic thinking of traditional naturalists striving to establish fundamental laws in nature by comparisons and analogies.41

Figure 5
Figure 5
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By comparing similarities and differences between the trajectories of the graphs, Lepeschkin concluded that anesthesia is brought about by a slight coagulation of protoplasm which, in his view, was different from that induced by the poison mercury chloride.

Seifriz, from the University of Pennsylvania, was another prominent botanist and scholar of protoplasm who advocated a protoplasmic coagulation theory of anesthesia. In addition to the changes in viscosity as a correlate of behavioral drug effects mentioned earlier, he presented the experimental basis of his protoplasm coagulation-based theory of anesthesia directly to the clinical target audience in original articles published in Anesthesiology in 194142 and 1950.43 In these studies, cessation of protoplasmic streaming in slime mold served as a model for the anesthetized state. Seifriz challenged Bernard’s unified paradigm according to which all anesthetics act via the same mechanism in all creatures to create an identical state. However, he did conclude that some agents, one notable exception being ether, did cause anesthesia by coagulation of protoplasm, which he termed “gelation” or “thixotropic setting” to accentuate its reversibility and set it apart from “coagulation,” which had the connotation of a terminal, irreversible process. As a mechanism for thixotropic settling, Seifriz proposed the readily reversible locking of linear protein molecules of what would be called today the cytoskeleton.

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Now Just Add Watern

An underappreciated aspect of inhaled anesthetic physical chemistry is the requisite water solubility and the consequences of the physical presence of high concentrations of anesthetics in this most important and active biological medium.4 Paucity or lack of water solubility characteristically delimits nonanesthetics (also known as nonimmobilizers) from true anesthetics.44 Anesthetics accumulate preferentially at interfaces between water and biological membranes,45 where a zone of bound water displays physicochemical characteristics that differ from that of conventional bulk water.46,47 The interaction of bound water with amino acid side chains determines the folding and unfolding of proteins into 3-dimensional shapes and multisubunit assemblies.4 Similarly, interaction with the surfaces of polymorphic lipid components determines the curvature and 3-dimensional arrangement of lamellar, hexagonal, or cubic lipid aggregates.48 Consequently, inhaled anesthetics present in interfacial water alter local protein dynamics, global protein stability,49,50 perturb lipid structures,51 and may also interact with nucleic acids. These activities of anesthetics at interfaces and subsequent nonlinear fluctuations in the arrangement of protoplasmic constituents (e.g., microscale phase transitions) may underlie some of the visible changes reported over decades by scientists working within the colloid-chemical framework whether or not they are of biological relevance.

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The triad of cell biology, physiology, and colloid chemistry created a powerful dynamic in life sciences in the second half of the 19th century ambitiously aimed at uncovering the secret of life. Biophysicists set out to reduce all biological processes to chemistry and Newtonian physics. Physiology sought to replace all vestiges of vitalism with a mechanistic understanding of biology and medicine. Colloid chemistry generated not only scientific models of alchemical miracles but also entire industries. Born into this era, the science of pharmacology in general and that of anesthesia in particular were poised to benefit from and contribute to an ambitious agenda aimed at understanding the very essence of life. The story of protoplasmic theories of anesthesia demonstrates not only the creative potential of interdisciplinary thinking but also the limitations of reductionist mechanistic research applied to biology: misinterpretation of correlation as causality, excessive simplification of poorly understood complex phenomena, extrapolations from simple experimental models to real life across layers of biological complexity, all of which remain perils for research into anesthetic mechanisms today.

Paradoxically, protoplasmic theories of anesthetic action may be receiving a new lease on life: structural protoplasmic proteins appear to be affected by some anesthetics.52,53 More subtle effects, easily reconcilable with current theories of anesthetic-protein interaction,54 may influence intracellular signaling simply by changes in protein flexibility potentially leading to delayed effects,55 and potential interactions with complex intracellular lipid structures remain unexplored. Whether these protoplasmic targets mediate any relevant biological consequences of anesthetics remains uncertain but is not impossible. The protean biological actions of “St. Anaesthesia” (Winston Churchill’s quip) continue to challenge us.

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Name: Misha Perouansky, MD.

Contribution: This author designed and conducted the study, analyzed the data, and wrote the manuscript.

Attestation: Misha Perouansky approved the final manuscript.

This manuscript was handled by: Marcel E. Durieux, MD, PhD.

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I express my gratitude to Professor Vladimir V. Matveev, PhD, Laboratory of Cell Physiology, Institute of Cytology, Russian Academy of Sciences, St. Petersburg, Russia, for alerting me to the important work of D. N. Nasonov, as well as insightful discussions and comments on an earlier version of this manuscript; to Luca Turin, PhD, Visiting Professor, Institute of Theoretical Physics, Universität Ulm, Ulm, Germany, for his help with biophysical interpretations of protoplasmic phenomena; and to Mathew I. Banks, PhD, Department of Anesthesiology, University of Wisconsin SMPH, Madison, WI, for critical comments. I am also grateful for the support of the Wood Library-Museum of Anesthesiology with biographical information on some of the personalities mentioned.

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a The term protoplasm was first used by Jan Purkyne (1787–1869) but acquired its mature meaning later in the works of Hugo von Mohl (1805–1872) and Ferdinand Cohn (1828–1898), as well as many others. The concept originally encompassed the cytoplasm with its “granularities” (gradually identified as a multitude of “organelles”) and the cytomatrix (cytoskeleton or ground substance)but not the nucleus which, being visible early on with light microscopy, has always been considered as a distinct body. Cited Here...

b A colloid is a substance dispersed in another substance. The properties of colloids are strikingly different from normal solutions or mixtures. Colloids are referred to by different terms depending on the nature of the particles and the solvent. See text for details. Cited Here...

c First Curator of Experiments of “The Royal Society of London for Improving Natural Knowledge”. A polymath credited with introducing the term “cellulae” but without realizing the true significance of his discovery. Cited Here...

d Life naked,” characterization of protoplasm attributed to Claude Bernard. Cited Here...

e German philosopher, historian, social critic, and journalist. Together with Karl Marx, Friedrich Engels developed the economic and social theory that later became known as Marxism. Cited Here...

f The propensity of assorted organic (specifically lipid) molecules to form microscopic spherical droplets that, under appropriate conditions, display colloid properties. Cited Here...

g Including Hermann von Helmholtz (1821–1894) and the Nobel Prize laureates Albert Einstein (1879–1955) and Leo L. Landau (1908–1968). Cited Here...

h In the original: “eine Trübung der Lösung ‘i.e., a darkening of the solution (Ranke); ‘… son contenu n’est plus transparent.. il est dans un état de semi-coagulation’ i.e., ‘… its contents is not transparent anymore … it is in a state of semi-coagulation….” (Bernard); “Dunkelung der Gehirnrindensubstanz … Zellen von trübem Protoplasma,” that is, “darkening of the substance of the brain cortex … cells with turbid protoplasm” (Binz). Cited Here...

i During the decades covered by this article, narcotics and anesthetics referred to essentially the same group of agents with subtle semantic differences between German and French speaking authors (see reference 1 for details), differences largely absent from the English-speaking literature. Cited Here...

j Heilbronn’s conclusions must be viewed with caution: analogous experiments on protoplasm were published in 1950 by FHC. Crick (Nobel Prize for Physiology or Medicine 1962) and AFW. Hughes and turned out to be difficult to interpret, confirming the experimental complexity of this type of measurements. Cited Here...

k The rule of the homologous series was described by BW. Richardson in 1869: the potency of alcohols increases with the length of the carbon backbone, up to a cutoff point. Compliance with this rule and the cutoff rule will enjoy long-lasting fame in mechanistic reasoning. Cited Here...

l Wilder D. Bancroft (1867–1952), colloid chemist, founded and edited the Journal of Physical Chemistry from 1896 to 1932. Among other honors, he was President of the American Chemical Society, member of the National Academy of Science, etc. During WWI, he worked on gas mask design, as well as on poison gases for military use. Cited Here...

m In 1928, Nasonov travelled to Columbia University as a fellow of the Rockefeller Foundation where he spent a year working in the laboratory of EB Wilson (1856–1939, generally acknowledged to be the “father of American cell biology”). Today such scientific exchanges are the norm. In a Soviet Union emerging from a bloody civil war, a fellowship of this kind was unusual and attests to the caliber of both scientists involved. Cited Here...

n The interested reader is referred to readable but potentially controversial books by Gerald H. Pollack for a refreshing view on water (The Fourth Phase of Water, Ebner and Sons 2013) and Gilbert N. Ling for a history of cell physiology (In Search of the Physical Basis of Life, Plenum Press 1984). The book by Ole G. Mouritsen presents an update on the role of lipids in biology (Life as a Matter of Fat, Springer, The Frontiers collection 2005). Cited Here...

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