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 Ca(2+)-dependent exocytosis was evoked with brief voltage steps to +20 mV. Vesicle fusion was monitored by changes in membrane capacitance (DeltaC(m)), and released catecholamine was detected with single-cell amperometry. Ca(2+) signaling was studied by recording voltage-dependent Ca(2+) currents (I(Ca)) and by measuring intracellular Ca(2+) ([Ca(2+)](i)) with fura-2 AM. AngII inhibited I(Ca) (IC(50) = 0.3 nm) in a voltage-dependent, pertussis toxin (PTX)-sensitive manner consistent with G(i/o)-protein coupling to Ca(2+) channels. DeltaC(m) was modulated bi-directionally; subnanomolar AngII inhibited depolarization-evoked exocytosis, whereas higher concentrations, in spite of I(Ca) inhibition, potentiated DeltaC(m) fivefold (EC(50) = 3.4 nm). Potentiation of exocytosis by AngII involved activation of phospholipase C (PLC) and Ca(2+) mobilization from internal stores. PTX treatment did not affect AngII-dependent Ca(2+) 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, bi-directional regulation of exocytosis.
Teschemacher
A G
AG
Department of Pharmacology, School of Medical Sciences, University of Bristol, Bristol BS3 1TD, United Kingdom.
Seward
E P
EP
D000313:Adrenal Medulla / Q000166:cytology / Q000502:physiology; D000804:Angiotensin II / Q000494:pharmacology*; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000502:physiology*; D002417:Cattle; D002478:Cells, Cultured; D019439:Chromaffin Cells / Q000166:cytology / Q000502:physiology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D005260:Female; D019204:GTP-Binding Proteins / Q000378:metabolism*; D008564:Membrane Potentials; D018408:Patch-Clamp Techniques; D037342:Pertussis Toxin; D011493:Protein Kinase C / Q000037:antagonists & inhibitors; D044140:Receptor, Angiotensin, Type 1; D044139:Receptor, Angiotensin, Type 2; D011945:Receptors, Angiotensin / Q000502:physiology; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D010738:Type C Phospholipases / Q000378:metabolism; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D004791:Enzyme Inhibitors; D044140:Receptor, Angiotensin, Type 1; D044139:Receptor, Angiotensin, Type 2; D011945:Receptors, Angiotensin; D010566:Virulence Factors, Bordetella; D000804:Angiotensin II; D002110:Caffeine; D037342:Pertussis Toxin; D011493:Protein Kinase C; D010738:Type C Phospholipases; D019204:GTP-Binding Proteins; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium