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Volatile Anesthetic-induced Efflux of Calcium from IP3-gated Stores in Clonal (GH3) Pituitary Cells.

Hossain, M. Delawar M.B.B.S., Ph.D.; Evers, Alex S. M.D.

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Background: Many hormones and neurotransmitters produce their effects by stimulating the generation of inositol 1,4,5-trisphosphate (IP3), a chemical second messenger that releases Ca2+ from intracellular stores. Interruption of this pathway is a potential mechanism through which volatile anesthetics might inhibit chemically mediated communication between cells. This study used GH3 cells (a clonal cell line) as a model system in which to characterize the effects of volatile anesthetics on IP3-induced mobilization of Ca2+ from intracellular stores.
Methods: Intracellular Ca2+ concentration ([Ca2+]]) was continuously monitored in suspensions of GH3 cells at 37[degrees]C using the fluorescent Ca2+ indicator Fura-2. Thyrotropin releasing hormone (TRH) was used to discharge IP3-sensitlve intracellular Ca2+ stores. The effects of halothane, isoflurane, and octanol on TRH-induced Ca2+ mobilization were assessed as a function of time and anesthetic concentration. To distinguish between anesthetic effects on Ca2+ uptake and Ca2+ release, experiments were performed using thapsigargin (a Ca2+-ATPase inhibitor) to inhibit Ca2+ uptake into IP3-sensitive stores.
Results: Halothane increased resting [Ca2+], and caused a time-and concentration-dependent inhibition of TRH-induced increases in [Ca2+], (IC50 = 0.6 mM). Thapsigargin, in concentrations that completely inhibit Ca2+ uptake by IP3-sensitive stores, also caused a time-dependent reduction in the [Ca2+], response to TRH; the time constant of this decay describes the rate of spontaneous leak of Ca2+ from IP3-sensltive stores (T = 98 +/- 9 s). In the presence of thapslgargln, halothane produced concentration-dependent Increases in the rate of leak from IP3-sensitive stores (T = 74 +/- 12 and 46 +/- 6 s at 0.5 and 1.0 min halothane, respectively). Isoflurane and octanol also produced concentration-dependent inhibition of the [Ca2+], response to TRH.
Conclusions: Halothane causes a concentration-dependent leak of Ca2+ from IP3-sensitive stores, leading to depletion of the stores and inhibition of IP3-lnduced increases in [Ca2+]., This effect occurs at clinically relevant concentrations of halothane (as well as isoflurane and octanol) and may be an important mechanism underlying some of the physiologic effects of volatile anesthetics.
(C) 1994 American Society of Anesthesiologists, Inc.
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