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The effect of modifications of extracellular calcium concentrations on -MSH release has been studied using perifused frog neurointermediate lobes. Increasing concentrations of calcium (from 2 to 10 mmol/l) gave rise to a dose-related stimulation of -MSH secretion, whereas reduction of Ca²⁺ from 2 to 15 mmol/l partially inhibited -MSH release. The direct effect of extracellular Ca²⁺ on -MSH secretion was confirmed by the dose-dependent stimulation of -MSH release induced by the calcium ionophore A23187. [GenBank] Perifusion with a calcium-free medium or blockade of Ca²⁺ channels by 4 mmol Co²⁺/l both resulted in an inhibition of spontaneous and TRH-induced -MSH release. Conversely, administration of verapamil or methoxy-verapamil (10 µmol/l each) did not alter basal secretion and had no effect on the response of the glands to TRH. Nifedipine (10 µmol/l), which was able to block KCl (20 mmol/l)-evoked -MSH release, induced a slight inhibition of basal -MSH secretion, indicating that extracellular Ca²⁺ levels may regulate -MSH release in part by Ca²⁺ influx through voltage-dependent Ca²⁺ channels. In contrast TRH-induced -MSH release was not affected by nifedipine or dantrolene (10 µmol/l), and BAY-K-8644 (1 µmol/l) did not significantly modify the response of neurointermediate lobes to TRH. Taken together, these results suggest that TRH-induced -MSH secretion is associated with calcium influx across the plasma membrane and that calcium entry caused by TRH may occur through nifedipine/verapamil-insensitive Ca²⁺ channels.