The action of halothane on stimulus-secretion coupling in clonal (GH₃) pituitary cells

The effect of halothane on the physiological response to excitatory stimuli was assessed in clonal (GH₃) pituitary cells. Halothane, at concentrations used to produce general anesthesia in animals (0.25-0.76 mu), inhibited thyrotropin-releasing hormone (TRH)-induced prolactin (PRL) secretion. The sustained (extracellular calcium-dependent) phase of PRL secretion was 70 ± 7% inhibited by the highest concentration of halothane tested (0.76 mM); 50% inhibition was produced by ≈0.4 mM halothane. The early (largely inositol trisphosphate-mediated) phase of secretion was less sensitive to halothane; 0.76 mM halothane produced 18 ± 2% inhibition of the early phase of secretion. Consistent with these observations, halothane inhibited (IC₅₀ ≈ 0.45 mM) the sustained phase of the TRH-induced rise in intracellular calcium ([Ca²⁺]_i) to a greater extent than the initial [Ca²⁺]_i peak. The sustained phase of the [Ca²⁺]_i elevation was inhibited by 75 ± 7% at the highest concentration of halothane tested (0.76 mM), whereas the peak [Ca²⁺]_i was only inhibited by 14 ± 5%, consistent with the observation that halothane did not inhibit TRH-stimulated inositide hydrolysis in these cells. Halothane (0.5 mM) did not inhibit phorbol ester- or ionomycin-induced PRL secretion, indicating that halothane has inconsequential effects on the secretory apparatus. Halothane (0.5 mM) also inhibited KCI-induced PRL secretion by 50-80% and the corresponding KCI-induced rise in [Ca²⁺]_i by 68 ± 6%. These data indicate that halothane inhibits secretagogue-stimulated PRL secretion by reducing the elevation of [Ca²⁺]_i produced by calcium (Ca²⁺) influx. The effects of halothane on average resting membrane potential and on the average KCI- and TRH-induced membrane depolarization were measured to determine if alterations in membrane potential might be responsible for the effect of halothane on the secretagogue-induced elevations of [Ca²⁺]_i. In nimodipine-treated cells, it was found that halothane neither altered resting membrane potential nor affected TRH- or KCI-induced depolarization. These findings support the view that halothane can act to inhibit the phys-iological response to excitatory stimuli by selectively reducing the elevation in [Ca²⁺]_i produced by cellular depolarization.