Mechanisms of vasopressin-induced intracellular Ca2+oscillations in rat inner medullary collecting duct

Abstract

Arginine vasopressin (AVP) causes increase in intracellular Ca2+concentration with an oscillatory pattern. Ca2+mobilization is required for AVP-stimulated apical exocytosis in inner medullary collecting duct (IMCD). The mechanistic basis of these Ca2+oscillations was investigated by confocal fluorescence microscopy and flash photolysis of caged molecules in perfused IMCD. Photorelease of caged cAMP and direct activation of ryanodine receptors (RyRs) by photorelease of caged cyclic ADP-ribose (cADPR) both mimicked the AVP-induced Ca2+oscillations. Preincubation of IMCD with 100 μM 8-bromo-cADPR (a competitive inhibitor of cADPR) delayed the onset and attenuated the magnitude of AVP-induced Ca2+oscillations. These observations indicate that the cADPR/RyR pathway is capable of supporting Ca2+oscillations and endogenous cADPR plays a major role in the AVP-induced Ca2+oscillations in IMCD. In contrast, photorelease of caged inositol 1,4,5-trisphosphate (IP3) induced Ca2+release but did not maintain sustained Ca2+oscillations. Removal of extracellular Ca2+halted ongoing AVP-mediated Ca2+oscillation, suggesting that it requires extracellular Ca2+entry. AVP-induced Ca2+oscillation was unaffected by nifedipine. Intracellular Ca2+store depletion induced by 20 μM thapsigargin in Ca2+-free medium triggered store-operated Ca2+entry (SOCE) in IMCD, which was attenuated by 1 μM GdCl3and 50 μM SKF-96365. After incubation of IMCD with 1 nM AVP in Ca2+-free medium, application of extracellular Ca2+also triggered Ca2+influx, which was sensitive to GdCl3and SKF-96365. In summary, our observations are consistent with the notion that AVP-induced Ca2+oscillations in IMCD are mediated by the interplay of Ca2+release from RyRs and a Ca2+influx mechanism involving nonselective cation channels that resembles SOCE.