Exposure of luminal membranes of LLC-PK1cells to ANG II induces dimerization of AT1/AT2receptors to activate SERCA and to promote Ca2+mobilization

Abstract

ANG II is secreted into the lumens of proximal tubules where it is also synthesized, thus increasing the local concentration of the peptide to levels of potential physiological relevance. In the present work, we studied the effect of ANG II via the luminal membranes of LLC-PK1cells on Ca2+-ATPase of the sarco(endo)plasmic reticulum (SERCA) and plasma membrane (PMCA). ANG II (at concentrations found in the lumen) stimulated rapid (30 s) and persistent (30 min) SERCA activity by more than 100% and increased Ca2+mobilization. Pretreatment with ANG II for 30 min enhanced the ANG II-induced Ca2+spark, demonstrating a positively self-sustained stimulus of Ca2+mobilization by ANG II. ANG II in the medium facing the luminal side of the cells decreased with time with no formation of metabolites, indicating peptide internalization. ANG II increased heterodimerization of AT1and AT2receptors by 140%, and either losartan or PD123319 completely blocked the stimulation of SERCA by ANG II. Using the PLC inhibitor U73122, PMA, and calphostin C, it was possible to demonstrate the involvement of a PLC→DAG(PMA)→PKC pathway in the stimulation of SERCA by ANG II with no effect on PMCA. We conclude that ANG II triggers SERCA activation via the luminal membrane, increasing the Ca2+stock in the reticulum to ensure a more efficient subsequent mobilization of Ca2+. This first report on the regulation of SERCA activity by ANG II shows a new mechanism for Ca2+homeostasis in renal cells and also for regulation of Ca2+-modulated fluid reabsorption in proximal tubules.