Volatile anesthetic-induced efflux of calcium from IP3-gated stores in clonal (GH3) pituitary cells

Abstract

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+]i) 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-sensitive 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 Ca(2+)-ATPase inhibitor) to inhibit Ca2+ uptake into IP3-sensitive stores.

Results: Halothane increased resting [Ca2+]i and caused a time- and concentration-dependent inhibition of TRH-induced increases in [Ca2+]i (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+]i response to TRH; the time constant of this decay describes the rate of spontaneous leak of Ca2+ from IP3-sensitive stores (tau = 98 +/- 9 s). In the presence of thapsigargin, halothane produced concentration-dependent increases in the rate of leak from IP3-sensitive stores (tau = 74 +/- 12 and 46 +/- 6 s at 0.5 and 1.0 mM halothane, respectively). Isoflurane and octanol also produced concentration-dependent inhibition of the [Ca2+]i 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-induced increases in [Ca2+]i. 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.