The action of volatile anaesthetics on stimulus‐secretion coupling in bovine adrenal chromaffin cells

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Abstract
The action of four volatile anaesthetics, ethrane, halothane, isoflurane and methoxyflurane on stimulus‐secretion coupling has been studied in isolated bovine adrenal medullary cells. All four agents inhibited the secretion of adrenaline and noradrenaline evoked by 500 μM carbachol at concentrations within the anaesthetic range. Total catecholamine secretion induced by stimulation with 77 mM potassium was also inhibited but at higher concentrations. All four agents inhibited the45 Ca influx evoked by stimulation with 500 μM carbachol and the45 Ca influx in response to K+‐depolarization. When total catecholamine secretion in response to potassium or carbachol was modulated by varying extracellular calcium or by adding halothane or methoxyflurane to the incubation medium, the amount of catecholamine secretion for a given Ca2+ entry was the same. The action of methoxyflurane on the relationship between intracellular free Ca and exocytosis was examined using electropermeabilised cells, which were suspended in solutions containing a range of concentrations of ionised calcium between 10−8 and 10−4 M. The anaesthetic had no effect on the activation of exocytosis by intracellular free calcium. Halothane and methoxyflurane inhibited the carbachol‐induced secretion of catecholamines in a non‐competitive manner. Halothane and methoxyflurane inhibited the increase in22 Na influx evoked by carbachol. For halothane and methoxyflurane this inhibition of Na influx appears to be sufficient to account for the inhibition of the evoked catecholamine secretion. We conclude that the volatile anaesthetics ethrane, halothane, isoflurane and methoxyflurane inhibit the secretion of adrenaline and noradrenaline induced by carbachol at concentrations that lie within the range encountered during general anaesthesia. In addition all four also inhibit the secretion of catecholamines induced by depolarization with 77 mM K+ but at much higher concentrations. The decrease in Ca influx caused by methoxyflurane accounts fully for the decrease in secretion in response to depolarization with potassium. Similar actions at synapses within the CNS may underlie the general anaesthetic effects of these agents.