Verapamil: Influence upon Basal and Stimulated Rat Growth Hormone and Prolactin Release in Vitro

Verapamil is an organic calcium antagonist which is believed to prevent the passage of calcium (Ca²⁺) across the cell membrane into the cell. In a rat pituitary perifusion-immunoprecipitation system, verapamil (50 μM) prevents the inhibitory effect of increased extracellular Ca²⁺ (5.4 mM) on basal and stimulated release of stored, prelabeled [³H]GH and [³H]PRL. [³H]GH release from pituitary explants perifused in standard medium (GIBCO Minimum Essential Medium: 1.8 mM Ca²⁺) is transiently increased by 50 μM verapamil while [³H]PRL release is suppressed. With continued exposure to 50 μM verapamil, [³H]GH release rates fall below (89.8 ± 2.1% of base) preverapamil levels while [³H]PRL release rates simply remain suppressed (48.2 ± 7.3% of base). With 250 μM verapamil, poststimulatory inhibition of [³H]GH release occurs more quickly, and after its withdrawal rebound release of both GH and PRL occur. Inhibition of [³H]GH release by 25 nM somatostatin (SRIF) and post-SRIF rebound [³H]GH release is not prevented by 50 μ^M verapamil. The early, rapid [³H]GH release phase of 1 mM dibutyryl cyclic AMP (dbcAMP) stimulation is potentiated by verapamil pretreatment, but only if the verapamil is continued during dbcAMP stimulation. Potassium (21 mM K⁺)-stimulated release of both ³H-labeled hormones is inhibited after similar pretreatment 50 μM verapamil. Conclusions: (a) verapamil antagonizes the inhibitory effects of increased extracellular Ca²⁺ on basal or dbcAMP-stimulated [³H]GH and [³H]PRL release; (b) in standard medium (1.8 mM Ca²⁺), 50 μM verapamil increases basal [³H]GH release suggesting either a direct effect or an antagonism of 1.8 mM extracellular Ca²⁺; (c) although verapamil-sensitive Ca²⁺ movement is not necessary for dbcAMP stimulation of [³H]GH release, verapamil potentiates dbcAMP-stimulated release; (d) because verapamil also inhibits K⁺-stimulated [³H]GH and [³H]PRL release, these observations support previous suggestions that K⁺- and dbcAMP-stimulated rapid hormone release occurs from different intracellular sites; and (e) because verapamil does not prevent any phase of SRIF action and since these two agents differentially alter K⁺- and cAMP-stimulated release, their mechanisms of action must partially differ.