Bidirectional modulation of exocytosis by angiotensin II involves multiple G-protein-regulated transduction pathways in chromaffin cells

Angiotensin II (AngII) receptors couple to a multitude of different types of G-proteins resulting in activation of numerous signaling pathways. In this study we examined the consequences of this promiscuous G-protein coupling on secretion. Chromaffin cells were voltage-clamped at -80 mV in perforated-patch configuration, and Ca2+-dependent exocytosis was evoked with brief voltage steps to +20 mV. Vesicle fusion was monitored by changes in membrane capacitance (Delta C-m), and released catecholamine was detected with single-cell amperometry. Ca2+ signaling was studied by recording voltage-dependent Ca2+ currents (I-Ca) and by measuring intracellular Ca2+ ([Ca2+](i)) with fura-2 AM. AngII inhibited I-Ca (IC50 = 0.3 nM) in a voltage-dependent, pertussis toxin (PTX)-sensitive manner consistent with G(i/o)-protein coupling to Ca2+ channels. Delta C-m was modulated bidirectionally; subnanomolar AngII inhibited depolarization evoked exocytosis, whereas higher concentrations, in spite of I-Ca inhibition, potentiated Delta C-m fivefold (EC50 = 3.4 nM). Potentiation of exocytosis by AngII involved activation of phospholipase C (PLC) and Ca2+ mobilization from internal stores. PTX treatment did not affect AngII-dependent Ca2+ mobilization or facilitation of exocytosis. However, protein kinase C (PKC) inhibitors decreased the facilitatory effects but not the inhibitory effects of AngII on stimulus-secretion coupling. The AngII type 1 receptor (AT1R) antagonist losartan blocked both inhibition and facilitation of secretion by AngII. The results of this study show that activation of multiple types of G-proteins and transduction pathways by a single neuromodulator acting through one receptor type can produce concentration-dependent, bidirectional regulation of exocytosis.