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Endogenous Cannabinoids are Candidates for Lipid Mediators of Bone Cement Implantation Syndrome

Motobe, Takashi*; Hashiguchi, Teruto; Uchimura, Tomonori; Yamakuchi, Munekazu; Taniguchi, Noboru*; Komiya, Setsuro*; Maruyama, Ikuro

doi: 10.1097/01.SHK.0000094766.36694.49
Clinical Aspects

Acute hypotension, hypoxemia, cardiac arrhythmias, cardiac arrest, (or a combination of these), and sudden death are well-recognized complications of the cemented hip arthroplasty procedure. Collectively, these are known as the bone cement implantation syndrome (BCIS). The endogenous cannabinoids, anandamide (ANA) and 2-arachidonylglycerol (2-AG), are reported to be strong vasodilators and play a role in the hypotension associated with hemorrhagic and septic shock. In the present study, a potential role for the endogenous cannabinoids in influencing hemodynamic variables in BCIS was investigated. Thirty-five patients (35 hips) entered a prospective, randomized clinical trial. The patients were divided into two groups. Group 1 comprised 16 patients who had the component inserted using a conventional cementing technique, whereas group 2 consisted of 19 patients who had the femoral component inserted without cement. Blood samples were taken at six consecutive time points: before anesthesia, after reaming the femur, 2 min after insertion of stems with or without cement into the femur, and 10 min, 20, and 30 min after stem insertion. In group 1 (with cement), the mean levels of ANA and 2-AG significantly increased after stem insertion. In a comparison of each group after stem insertion, mean ANA and 2-AG levels in group 1 also significantly differed from those in group 2. By contrast, in group 2 (without cement) neither ANA nor 2-AG levels exhibited a significant increase or change at any point in time. In conclusion, we have shown for the first time that endogenous cannabinoids are candidates for lipid mediators of BCIS.

Departments of *Orthopaedic Surgery and Laboratory Medicine, Faculty of Medicine, Kagoshima University, Kagoshima Japan

Received 11 Apr 2003;

first review completed 15 May 2003; accepted in final form 26 Aug 2003

Address reprint requests to Dr. Ikuro Maruyama, 8-35-1 Sakuragaoka, Kagoshima, 890-8520 Japan. E-mail: motobe@m.kufm.kagoshima-u.ac.jp, maruyama@m3.kufm.kagoshima-u.ac.jp.

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INTRODUCTION

Bone cement implantation syndrome (BCIS) is characterized by hypotension, hypoxemia, cardiac arrhythmias, cardiac arrest, or a combination of these and leads to death in 0.6–1% of patients (1). One of the mechanisms suggested to explain these complications is diffuse microembolization of the lungs as a consequence of extrusion of intramedullary content by pressurized bone cement (2,3). By reducing intramedullary pressure and changing operative technique, BCIS can be diminished, but death cannot be completely prevented (4).

The psychoactive properties of cannabinoids, or the biologically active constituents of the marijuana plant, are well recognized (5). Recent research has revealed that cannabinoids elicit not only neurobehavioral and immunological but also profound cardiovascular effects. Similar effects can be elicited by the endogenous cannabinoids, anandamide (ANA) and 2-arachidonylglycerol (2-AG; Ref. 6). These two endogenous cannabinoids can be generated by activated macrophages and platelets, respectively, during endotoxic shock, and have been proposed to play crucial roles in the induction of shock-related hypotension (7,8). ANA induces vasodilation through vanilloid receptors (9). It was recently reported that cirrhotic patients are endotoxemic and that activation of vascular cannabinoid CB1 receptors is implicated in endotoxin-induced hypotension. Rats with biliary cirrhosis have low blood pressure, and ANA was found to activate vascular CB1 receptors in the vasodilated state in advanced cirrhosis (10). Moreover, endogenous cannabinoids are strong vasodilators and influence hemodynamic variables in experimental models of acute myocardial infarction (11).

We have recently shown that ANA and 2-AG both in saline/ethanol solution and in serum were efficiently absorbed by a polymyxin B (PMB)-immobilized beads column and eluted with ethanol (12,13). We used PMB-immobilized beads to selectively absorb ANA and 2-AG, instead of an organic extraction method, for quantitative measurement of ANA and 2-AG (13). Here, we show the potential of endogenous cannabinoids to act as mediators of hypotension in endotoxemic shock. We hypothesize that endogenous cannabinoids play an important role in the hemodynamic changes occurring in BCIS.

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PATIENTS AND METHODS

Patients

We performed a prospective study in 35 patients with osteoarthritis/rheumatoid arthritis of the hip joint who underwent total hip arthroplasty (THA) or bipolar hemiarthroplasty for femoral neck fracture using a femoral component inserted with or without cement. Cases of osteoarthritis, rheumatoid arthritis, and femoral neck fracture were randomly assigned to one of two groups. Sixteen patients had the femoral component inserted with cement (group 1), and 19 without cement (group 2). All acetabular components were inserted without cement in all THA cases in group 1 (n = 5) and group 2 (n = 7;Fig. 1). The main criteria for exclusion from the study were metabolic bone disease, age less than 55 years, and cases requiring cement on the acetabular side. Informed consent was obtained from each patient with particular attention being paid to informing the patients about the advantages and disadvantages of installation of the femoral component with and without cement. The study was approved by the Investigational Review Committee of the hospital and was performed in adherence to the Declaration of Helsinki. All operations were performed by the senior author.

Fig. 1

Fig. 1

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Operations

The patients were placed in the supine position, and the hip joint was exposed through the lateral approach. A press-fit acetabular component was inserted without cement in all THA cases (12 hips). In group 1, the femoral canal was opened using an osteotome. Pulsatile high-volume lavage of the bone surface was performed with normal saline solution. The component was inserted with a second-generation cementing technique using a bone plug and cement gun. A Teflon tube was used to vent the intramedullary canal during insertion of the stem. In group 2, after broaching and suction, the stem of the largest size that provided a stable press-fit was then inserted. Neither powered instruments nor pulsatile lavage was used.

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Anesthesia

Thirty-five patients scheduled for operation were given general and/or epidural anesthesia. General anesthesia was induced with fentanyl and muscle relaxation with vecuronium. Anesthesia was maintained with one-third to one half the minimum alveolar concentration of isoflurane. All patients underwent endotracheal intubation and were mechanically ventilated to maintain a constant level of end-tidal carbon dioxide. In epidural anesthesia, epidural puncture was performed at the T10–L2 level and an epidural catheter was inserted into the epidural space. Four milliliters of 2% lidocaine was used as a test dose, and the main dose was 15–25 mL of 0.75% bupivacaine delivered via the catheter. Selection of the type of anesthesia for each patient was made by the anesthesiologist.

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Blood samples and blood pressure measurements

Blood samples were collected and systolic/diastolic blood pressure was measured at six consecutive time points: before anesthesia (t0), after reaming the femur (t1), 2 min after insertion of stems with or without cement into the femur (t2), 10 min after stem insertion (t3), 20 min after stem insertion (t4), and 30 min after stem insertion (t5). Blood was removed via an intraarterial canula. The arterial line was used for measuring blood pressure.

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Quantification of 2-AG and ANA

Taking advantage of the ability of polymyxin B (PMB) to bind to ANA and 2-AG, we measured ANA and 2-AG by using PMB-immobilized beads to selectively adsorb them in biological fluids, instead of using the organic solvent extraction method (12). The eluate from beads could be directly fractionated using reverse-phase high-performance liquid chromatography (HPLC). The fractions corresponding to authentic ANA and 2-AG were collected, derivatized with fluorogenic reagent, and subsequently quantified using HPLC with fluorometric detection. HPLC was performed according to a method described previously (13).

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Statistical analysis

Statistical analysis was performed using repeated-measures analysis of variance. Bonferroni adjustment was used for multiple comparisons. Patient profiles were analyzed using the paired t-test. Differences from corresponding baseline values with a P value of less than 0.05 were considered significant.

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RESULTS

General profiles of patients in group 1 and group 2

Complete data for all 35 patients were recorded. The profiles of patients are summarized in Table 1 There was no significant difference between the two groups of patients with respect to gender, age, body surface index, associated systemic disease, type of anesthesia, type of implant, or amount of blood loss. We measured blood pressure at six consecutive time points. Mean values of absolute changes of systolic/diastolic blood pressure at t1 and t2 in groups 1 and 2 are shown in Figure 2. Sixteen patients in group 1 exhibited a sudden decrease in systolic blood pressure of more than 20% at t2. However, 19 patients in group 2 did not exhibit significant changes in systolic blood pressure at t2 (P = 0.0015). Sudden decrease in diastolic blood pressure also differed significantly between group 1 and group 2 at t2 (P < 0.05).

Table 1

Table 1

Fig. 2

Fig. 2

All patients were admitted to the recovery room after operation for monitoring. There were no early adverse events. There was no significant difference in ANA or 2-AG level associated with the method of anesthesia, type of diagnosis or implant.

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Differences in cannabinoid levels in groups 1 and 2

In group 1, ANA and 2-AG levels increased quickly, with peaks at t3, and there was no tendency toward decrease until t5 (Figs. 3 and 4). In group 1, the mean ANA was 11.88 ± 2.69 pmol/mL at t1 and 50.43 ± 18.35 pmol/mL at t3 (P < 0.05;Fig. 3). In group 1, 2-AG was 69.41 ± 36.01 pmol/mL at t0 and 231.30 ± 66.45 pmol/mL at t5 (P < 0.05;Fig. 4). In addition, 2-AG at t1 (35.52 ± 8.83 pmol/mL) significantly differed from that at t5 (231.30 ± 66.45 pmol/mL; P < 0.008;Fig. 4). However, in group 2, neither the level of ANA nor 2-AG exhibited a significant increase or change at any point of time (Figs. 3 and 4).

Fig. 3

Fig. 3

Fig. 4

Fig. 4

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Differences in cannabinoid levels between groups 1 and 2

In a comparison among groups, the mean ANA at t3 in group 1 (50.43 ± 18.35 pmol/mL) also differed significantly from that in group 2 (15.20 ± 2.00 pmol/mL; P < 0.05;Fig. 3). 2-AG at t2 was 130.12 ± 33.22 pmol/mL in group 1 and 42.58 ± 13.59 pmol/mL in group 2 (P = 0.011;Fig. 4). In addition, 2-AG at t3 was 263.57 ± 65.97 pmol/mL and 53.98 ± 12.39 pmol/mL in groups 1 and 2, respectively (P = 0.001); 2-AG at t4 was 179.05 ± 57.39 pmol/mL and 67.67 ± 19.70 pmol/mL, respectively (P < 0.05); and 2-AG at t5 was 231.30 ± 66.45 pmol/mL and 93.10 ± 30.72 pmol/mL, respectively (P < 0.05;Fig. 4).

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DISCUSSION

The purpose of the present study was to investigate potential effects of endogenous cannabinoids in patients undergoing cemented hip arthroplasty. BCIS is characterized by hypotension, hypoxemia, cardiac arrhythmias, cardiac arrest, or a combination of these. Numerous hypotheses have been proposed to explain the pathophysiologic basis of BCIS. Powell et al. (14) first reported BCIS in 1970, 10 years after Charnley introduced the use of bone cement for hip arthroplasty surgery (15). Reaction to heat, direct toxic or vasodilating effects of the monomer, anaphylaxis, embolization of fat, air, or polymer, and apposition thrombi generated by impaction have all been implicated in the development of this syndrome (2,3,16). A number of investigators have stressed the important role of changes in intramedullary pressure in the genesis of intraoperative embolic phenomena (2,17,18). Conventional techniques of implantation with cement (19,20) provide a maximum bone-acrylic interlock by means of a combination of a lower viscosity of cement and pressurization. However, these techniques generate extreme intramedullary pressures within the femoral canal, which often exceed the pressures in the general venous circulation and cause disruption of the thin-walled medullary vessels. This allows intravasation and flow of bone fat, bone marrow, bone debris, and polymethylmethacrylate (PMMA) though the natural drainage system of the diaphysis, located along the linea aspera, and through the metaphyseal veins (21–23). As a new approach to avoid increase in intramedullary pressure and intravasation of fat and bone marrow, the venting hole and bone-vacuum technique was developed (3). A computer-assisted implantation technique is also under examination (24).

However, during percutaneous vertebroplasty, which involves consolidation of a vertebral body with PMMA and is used for the treatment of benign and malignant lesions of the spine, transient hypotension is induced by injection of 3–4 mL PMMA. No data exist concerning increase in intramedullary pressure and the potential release of bone marrow emboli during percutaneous vertebroplasty (25). Furthermore, methylmethacrylate monomer produced hypotension by direct relaxation of vascular smooth muscle of the rabbit in vitro (26). Although we agree that embolism is an important pathogenic factor of BCIS, we believe that the fall in blood pressure observed in this study is not attributable to any single factor, but is caused by combined effects of multiple factors.

Endotoxin is considered a major effector of hypotension in septic shock (27–29). It has recently been suggested that ANA and 2-AG exert cardiovascular effects and act as contributors to endotoxic and hemorrhagic shock-induced hypotension (6–8). Endogenous cannabinoids may be key players in the hemodynamic changes that accompany certain forms of shock (8). Rat macrophages and platelets contain the endogenous cannabinoids ANA and 2-AG, as identified by reverse-phase HPLC, gas chromatography and mass spectrometry. In vitro exposure of macrophages and platelets to bacterial lipopolysaccharide (200 μg/mL) markedly increases levels of ANA and 2-AG, and thus have been determined to be mediators of the hypotension associated with endotoxemia (8).

PMB is a cationic antibiotic that also stoichiometrically neutralizes the lipid A moiety of endotoxin (30,31). Very recently, we have shown that ANA and 2-AG in saline/ethanol solution and in serum were efficiently absorbed in a PMB-immobilized beads column and eluted with ethanol (12,13). This is a greatly simplified and highly reproducible method for simultaneous measurement of ANA and 2-AG. Macrophages and platelets generate endogenous cannabinoids (8), and our findings of increases in ANA and 2-AG levels up to 50 pmol/mL and 263 pmol/mL, respectively, indicated that these cannabinoids contributed to severe hypotension after cementing. Endogenous cannabinoids are potent vasodilators and elicit hypotension and bradycardia (7,8). Endogenous cannabinoids released from platelets and macrophages during bone cement implantation might act as activators of vascular CB1 receptors.

Effects of anandamide on systemic hemodynamics have been investigated in anesthetized rats (32), and it has been found that a 4 mg/kg injection of anandamide produces a profound, dose-dependent decrease in arterial pressure within 2 min. The effect of anandamide is most pronounced within the first 1–3 min after intravenous administration, and arterial pressure returns to baseline values within 10–15 min (32). In our study, 16 patients in group 1 exhibited a sudden decrease in systolic blood pressure of more than 20% at t2 associated with an increase in ANA and 2-AG levels. Blood pressure returned to baseline within 5 min in all of the patients with hypotension.

In our study, in group 1, ANA and 2-AG levels increased by t2, with a peak at t3, and no tendency to decrease was demonstrated at t5. However, the lowest mean arterial blood pressure was observed at t2 (Fig. 1). The properties of cannabinoids, the biologically active constituents of the marijuana plant, are well recognized (5). Tolerance may be an important contributor to these findings (5). Tolerance develops rapidly during continual administration of cannabinoids. Receptor desensitization or uncoupling has been consistently implicated as one of the molecular events underlying the onset of tolerance in many systems (33,34). Jin et al., in an experimental study of Xenopus oocytes, reported that desensitization of cannabinoid CB1 receptors was observed after prolonged (8-min) exposure to CB1 agonist (35). Prolonged exposure to cannabinoids with cement may result in tolerance and desensitization of cannabinoid receptors, and may be related to the recovery of blood pressure observed from t3.

Obviously, as shown in Figures 3 and 4, ANA and 2-AG tended to increase immediately after cement insertion. Analyzed individually, some cases were found to have high levels of ANA and 2-AG before anesthesia in both the cemented group and the cementless group. During surgery, ANA and 2-AG levels tended to increase with time in both the cemented and cementless groups. It is thus necessary to identify factors other than cement (anesthesia, complications, drugs, surgical invasion, etc.) responsible for the observed increase in ANA and 2-AG levels.

In addition, in our study, no measurements of CB1 receptors density were performed. It is possible that up- or down-regulation of endothelial CB1 receptors occurred and this is an area for future study.

In conclusion, we have demonstrated for the first time significant increases in levels of ANA and 2AG, members of a newly identified class of neurohumoral vascular mediators, in the course of cemented hip cement arthroplasty. This observation strongly suggests that ANA and 2AG are mediators of the hemodynamic variables associated with bone cement implantation shock. Therefore, targeting of the biosynthesis of, specific receptors for and biological degradation systems of endocannabinoids might be useful as new strategies for the prevention and clinical management of BCIS.

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REFERENCES

1. Duncan JA: Intra-operative collapse or death related to the use of acrylic cement in hip surgery. Anaesthesia 44:149–153, 1989.
2. Kallos T, Enis JE, Gollan F, Davis JH: Intramedullary pressure and pulmonary embolism of femoral medullary contents in dogs during insertion of bone cement and a prosthesis. J Bone Joint Surg Am 56:1363–1367, 1974.
3. Pitto RP, Koessler M, Kuehle JW: Comparison of fixation of the femoral component without cement and fixation with use of a bone-vacuum cementing technique for the prevention of fat embolism during total hip arthroplasty. J Bone Joint Surg Am 81:831–843, 1999.
4. Jensen JS, Trap B, Skydsgaard K: Delayed contact hypersensitivity and surgical glove penetration with acrylic bone cements. Acta Orthop Scand 62:24–28, 1991.
5. Adams IB, Martin BR: Cannabis: pharmacology and toxicology in animals and humans. Addiction 91:1585–1614, 1996.
6. Lake KD, Compton DR, Varga K, Martin BR, Kunos G: Cannabinoid-induced hypotension and bradycardia in rats mediated by CB1-like cannabinoid receptors. J Pharmacol Exp Ther 281:1030–1037, 1997.
7. Calignano A, La Rana G, Beltramo M, Makriyannis A, Piomelli D: Potentiation of anandamide hypotension by the transport inhibitor, AM404. Eur J Pharmacol 337:R1–R2, 1997.
8. Varga K, Wagner JA, Bridgen DT, Kunos G: Platelet- and macrophage-derived endogenous cannabinoids are involved in endotoxin-induced hypotension. FASEB J 12:1035–1044, 1998.
9. Zygmunt PM, Petersson J, Andersson DA, Chuang H, Sorgard M, Di Marzo V, Julius D, Hogestatt ED: Vanilloid receptors on sensory nerves mediate the vasodilator action of anandamide. Nature 400:452–457, 1999.
10. Batkai S, Jarai Z, Wagner JA, Goparaju SK, Varga K, Liu J, Wang L, Mirshahi F, Khanolkar AD, Makriyannis A, Urbaschek R, Garcia Jr, N Sanyal AJ, Kunos G: Endocannabinoids acting at vascular CB1 receptors mediate the vasodilated state in advanced liver cirrhosis. Nat Med 7:827–832, 2001.
11. Wagner JA, Hu K, Bauersachs J, Karcher J, Wiesler M, Goparaju SK, Kunos G, Ertl G: Endogenous cannabinoids mediate hypotension after experimental myocardial infarction. J Am Coll Cardiol 38:2048–2054, 2001.
12. Wang Y, Liu Y, Sarker KP, Nakashima M, Serizawa T, Kishida A, Akashi M, Nakata M, Kitajima I, Maruyama I: Polymyxin B binds to anandamide and inhibits its cytotoxic effect. FEBS Lett 470:151–155, 2000.
13. Wang Y, Liu Y, Ito Y, Hashiguchi T, Kitajima I, Yamakuchi M, Shimizu H, Matsuo S, Imaizumi H, Maruyama I: Simultaneous measurement of anandamide and 2-arachidonoylglycerol by polymyxin B-selective adsorption and subsequent high-performance liquid chromatography analysis: increase in endogenous cannabinoids in the sera of patients with endotoxic shock. Anal Biochem 294:73–82, 2001.
14. Powell JN, McGrath PJ, Lahiri SK, Hill P: Cardiac arrest associated with bone cement. Br Med J 3:326, 1970.
15. Charnley J: Anchorage of the femoral head prosthesis to the shaft of the femur. J Bone Joint Surg 42-B:28, 1960.
16. Fairman RP, Morrow C, Glauser FL: Methylmethacrylate induces pulmonary hypertension and increases lung vascular permeability in sheep. Am Rev Respir Dis 130:92–95, 1984.
17. Woo R, Minster GJ, Fitzgerald Jr, RH Manson LD, Lucas DR, Smith FE: Pulmonary fat embolism in revision hip arthroplasty. Clin Orthop 319:41–53, 1995.
18. Pape HC, Giannoudis PV, Grimme K, van Griensven M, Krettek C: Effects of intramedullary femoral fracture fixation: what is the impact of experimental studies in regards to the clinical knowledge (review)? Shock 18:291–300, 2002.
19. Timperley AJ, Ling RSM, Jones PR: The effect of surgical technique on the quality of hip arthroplasty. J Bone Joint Surg 74-B:139–140, 1992.
20. Weber BG: Pressurized cement fixation in total hip arthroplasty. Clin Orthop 232:87–95, 1988.
21. Draenert K: Modern cementing techniques. An experimental study of vacuum insertion of bone cement. Acta Orthop 55:273–295, 1989.
22. Pitto RP, Koessler M, Draenert K: Prophylaxis of fat and bone marrow embolism in cemented total hip arthroplasty. Clin Orthop 355:23–34, 1998.
23. Weissman BN, Sosman JL, Braunstein EM, Dadkhahipoor H, Kandarpa K, Thornhill TS, Lowell JD, Sledge CB: Intravenous methylmethacrylate after total hip replacement. J Bone Joint Surg 66-A:443–450, 1984.
24. Schmidt J, Steur G, Specht R, Kumm D: Computer-assisted experimental implantation technique for comparative quality assessment of various cementation techniques in cemented hip prosthesis implantation. Biomed Tech (Berl). 39:79–84, 1994.
25. Vasconcelos C, Gailloud P, Martin JB, Murphy KJ: Transient arterial hypotension induced by polymethylmethacrylate injection during percutaneous vertebroplasty. J Vasc Interv Radiol 12:1001–1002, 2001.
26. Karlsson J, Wendling W, Chen D, Zelinsky J, Jeevanandam V, Hellman S, Carlsson C: Methylmethacrylate monomer produces direct relaxation of vascular smooth muscle in vitro. Acta Anaesthesiol Scand 39:685–689, 1995.
27. Mailman D, Self A, Henry M: Time- and surgery-dependent effects of lipopolysaccharide on gut, cardiovascular and nitric oxide functions. Shock 12:208–214, 1999.
28. Murphey ED, Traber DL: Cardiopulmonary and splanchnic blood flow during 48 hours of a continuous infusion of endotoxin in conscious pigs: a model of hyperdynamic shock. Shock 13:224–229, 2000.
29. Mueller LP, Yoza BK, Neuhaus K, Loeser CS, Cousart S, Chang MC, Meredith JW, Li L, McCall CE: Endotoxin-adapted septic shock leukocytes selectively alter production of sIL-1RA and IL-1beta. Shock 16:430–437, 2001.
30. Fox ES, Wang L, Tracy Jr: TF Lipopolysaccharide and tumor necrosis factor-alpha synergy potentiate serum-dependent responses of rat macrophages. Shock 5:429–433, 1996.
31. Mayumi T, Takezawa J, Takahashi H, Kuwayama N, Fukuoka T, Shimizu K, Yamada K, Kondo S, Aono K: Low-dose intramuscular polymyxin B improves survival of septic rats. Shock 11:82–86, 1999.
32. Garcia Jr, N Jarai Z, Mirshahi F, Kunos G, Sanyal AJ: Systemic and portal hemodynamic effects of anandamide. Am J Physiol Gastrointest Liver Physiol 280:G14–G20, 2001.
33. Appleyard SM, Patterson TA, Jin WZ, Chavkin C: Agonist-induced phosphorylation of the kappa-opioid receptor. J Neurochem 69:2405–2412, 1997.
34. Kovoor A, Nappey V, Kieffer BL, Chavkin C: Mu and delta opioid receptors are differentially desensitized by the coexpression of beta-adrenergic receptor kinase 2 and beta-arrestin 2 in Xenopus oocytes. J Biol Chem 272:27605–27611, 1997.
35. Jin W, Brown S, Roche JP, Hsieh C, Celver JP, Kovoor A, Chavkin C, Mackie K: Distinct domains of the CB1 cannabinoid receptor mediate desensitization and internalization. J Neurosci 19:3773–3780, 1999.
Keywords:

Anandamide (ANA); 2-arachidonylglycerol (2-AG); hypotension; shock

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