Stimulation of Ca2+‐independent exocytosis in rat pituitary gonadotrophs by G‐protein

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Abstract
1 We employed the whole-cell recording technique in conjunction with fluorometry to measure cytosolic Ca2+ concentration ([Ca2+]i) and exocytosis (capacitance measurement) in single, identified rat gonadotrophs. 2 Direct activation of G-protein (via intracellular dialysis of non-hydrolysable analogues of GTP, but not of GDP) triggered a slow rise in capacitance even in the presence of a fast intracellular Ca2+ chelator. 3 The broad-spectrum kinase inhibitors H7 and staurosporine did not prevent this Ca2+-independent exocytosis, ruling out the involvement of the cAMP and PKC pathways. 4 AlF4−, a potent stimulator of heterotrimeric G-proteins, failed to stimulate any exocytosis when the intracellular Ca2+ store was depleted, implicating the involvement of AlF4−-insensitive G-protein(s). 5 Maximal stimulation of Ca2+-independent exocytosis by GTP analogues did not reduce the number of readily releasable granules that were available subsequently for Ca2+-dependent release. 6 The last finding raises the possibility that the G-protein-stimulated Ca2+-independent exocytosis may regulate a pool of granules that is distinct from the Ca2+-dependent pool.