Calcium Antagonists and Hormone Release. II. Effects of Verapamil on Basal, Gonadotropin-Releasing Hormoneand Thyrotropin-Releasing Hormone-Induced Pituitary Hormone Release in Normal Subjects

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Abstract
Although it has been well established that Ca2+ plays an essential role in the release of several hormones, very little is known of the interactions between Ca2+ and secretagogues in the process of pituitary hormone release. One possible way of studying the mechanism of action of hypothalamic releasing hormones is to study how organic calcium antagonist$ affect their action. Consequently, we infused the commonly used calcium antagonist, verapamil, into 20 normal subjects (10 men and 10 women; aged 19–37 yr) and studied its effects on both basal pituitary hormone levels and augmented hormonal release induced by gonadotropin-releasing hormone (GnRH) and TRH. Verapamil, infused at a rate of 5 mg/h for 3 h, induced a significant and marked suppression of circulating LH and FSH levels in both men and women. By the end of the infusion, the suppression of release was greater for LH (60%) than for FSH (54%). After the termination of the infusion, plasma gonadotropin concentrations returned progressively to basal levels within 2 h. Verapamil was also capable of blunting the peak incremental gonadotropin response to GnRH. Although the basal TSH concentration was apparently unaffected by verapamil, the incremental TSH response to TRH was significantly inhibited in both men and women. Verapamil infusion did not affect either the basal PRL concentration or the PRL response to TRH. Our data provide evidence that verapamil exerts different effects on the release of pituitary hormones in normal subjects. It inhibits the centrally mediated as well as the peripherally mediated gonadotropin release and blunts the TSH response to TRH. On the contrary, verapamil does not seem to affect basal or TRH-mediated PRL release. The use of organic calcium antagonists in experimental models in vitro as well as in vivo appears to offer a promising tool for further studies on the mechanism of action of secretagogues in the process of hormone release.