Quinidine inhibits prolactin secretion induced by thyrotropin-releasing hormone, high medium potassium or hyposmolarity in GH4C1 cells.

In cultured GH4C1 cells quinidine inhibited basal prolactin (PRL) secretion and that induced by 0.1 to 10 nM thyrotropin-releasing hormone (TRH), 30 mM medium K+ or 30% medium hyposmolarity but did not inhibit secretion induced by 100 nM 12-O-tetradecanoylphorbol 13-acetate. Inhibition of basal PRL secretion was highly correlated with the drug concentration between 30 microM to 1 mM quinidine; 50% inhibition of basal secretion occurred at 300 microM and at this concentration quinidine completely blocked PRL secretion stimulated by TRH, K+ and hyposmolarity. Significant inhibition of TRH-induced PRL secretion was produced by 15 microM quinidine, a concentration equivalent to that in plasma during standard antiarrhythmic therapy with quinidine in humans. In rats in vivo, a single injection of 2 mg i.p. of quinidine gluconate/100 g b.wt. 1 hr before TRH injection significantly inhibited induced TSH secretion by 15%. Quinidine inhibition of secretion may be caused by blocking depolarization of the cell membrane, thus depressing voltage-gated Ca++ channels and preventing a rise in intracellular Ca++ release.