Calcium currents in a pituitary cell line (AtT-20): Differential roles in stimulus-secretion coupling

The purpose of the present investigation was to identify voltage-dependent calcium channel subtypes that control the release of ACTH in AtT-20 cells, a clonal mouse pituitary cell line. Using the perforated patch-clamp technique, we identified dihydropyridine (nimodipine)-, omega-Agatoxin IVA-, and omega-Conotoxin MVIIC-sensitive calcium currents. No omega-Conotoxin GVIA-sensitive currents are present in these cells. There also existed a considerable resistant component to the recorded inward current that was inhibited by cadmium, a nonselective calcium channel antagonist. Using RIA, we examined the contributions of each of the pharmacologically distinct calcium channel populations to CRH- or potassium chloride (KCl)-stimulated release of ACTH at various time intervals (10 sec to 60 min). We found that nimodipine markedly inhibited ACTH release at all intervals tested, whereas omega-Agatoxin TVA, omega-Conotoxin MVIIC, and omega-Conotoxin GVIA had no significant effect. Moreover, the inhibition by nimodipine was comparable to that seen after cadmium application, and the effects of these two antagonists were not additive. These data suggest that although AtT-20 cells possess dihydropyridine-, omega-Agatoxin IVA-, and omega-Conotoxin MVIIC-sensitive calcium channels as well as a considerable toxin-resistant current, only the dihydropyridine-sensitive calcium channels appear to be coupled to CRH- or KCl-induced ACTH release.