CALCIUM RELEASE FROM PITUITARY SECRETORY GRANULES - MODULATION BY THIOLS, DISULFIDES, AND DIHYDROPYRIDINE CALCIUM-CHANNEL BLOCKERS

The distribution of calcium in isolated bovine pituitary secretory granules was studied by atomic absorption. The total granule calcium (in 26 preparations) averaged 14.5 nmol/mg protein, or 21.2 ± 1.6% of the total pituitary homoge-nate calcium. Incubation of granules with KCl resulted in calcium release (78% at 15 mM and 100% at 50 mM, for example). Calcium release was also pH dependent, with greater release at acidic pH values; it was not influenced by either 500 MM strontium or 500 μM lanthanum. Release was augmented bv reduced glutathione (GSH), with significant release observable at thiol levels as low as 10 nM. In addition to GSH, cysteine alsostimulated release; mercaptoethanol and dithiothreitol were less potent. Interestingly, the disulfides cystine and oxidized glutathione also stimulated calcium release. Since the latter com-pounds are known to inhibit hormone release from granules, calcium and protein release appear to be regulated independently. A number of dihydropyridines were tested as potential blockers of calcium releasefrom granules. Nimodipine inhibited basal calcium release at high concentrations and potently inhibited GSH-stimulated calcium release, with an apparent Ki in the 10-20 nM range; it also inhibited K+-stimulated release but to a lesser extent. Nimodipine, however, did not significantly influence protein or hormone release. GSH-stimulated calcium release was also inhibited by nifedipine, and this inhibition was qualitatively and quantitatively similar to that by nimodipine. Nisoldipine and nitrendipine, however, displayed nosignificant inhibition. In summary, it appears that the release of secretory granule calcium in vitro is independent of protein release. Thiols and some disulfides stimulate calcium release, and its inhibition by dihydropyridines suggests that granule membranes may contain specific ion channels. The role of granule calcium in the cell remains to be defined. © 1990 by The Endocrine Society.