Angiotensin II inhibits the electrogenic Na+/HCO3- cotransport of cat cardiac myocytes

Abstract

The Na+/HCO3- cotransporter (NBC) plays an important role in intracellular pH (pHi) regulation in the heart. In the myocardium co-exist the electrogenic (eNBC) and electroneutral (nNBC) isoforms of NBC. We have recently reported that angiotensin II (Ang II) stimulated total NBC activity during the recovery from intracellular acidosis through a reactive oxygen species (ROS) and ERK-dependent pathway. In the present work we focus our attention on eNBC. In order to study the activity of the eNBC in isolation, we induced a membrane potential depolarization by increasing extracellular K+ [K+]o from 4.5 to 45mM (K+ pulse). This experimental protocol enhanced eNBC driving force leading to intracellular alkalization (0.19±0.008, n=6; data expressed as an increase of pHi units after 14min of applying the K+ pulse). This alkalization was completely abrogated by the NBC blocker S0859 (-0.004±0.016*, n=5; * indicates p<0.05 vs control) but not by the Na+/H+ exchanger blocker HOE642 (0.185±0.04, n=4), indicating that we are exclusively measuring eNBC. The K+ pulse induced alkalization was canceled by 100nM Ang II (-0.008±0.018*; n=5). This inhibitory effect was prevented when the myocytes were incubated with losartan (AT1 receptor blocker, 0.18±0.02; n=4) or SB202190 (p38 MAP kinase inhibitor, 0.25±0.06; n=5). Neither chelerythrine (PKC inhibitor, -0.06±0.04*; n=4), nor U0126 (ERK inhibitor, -0.07±0.04*; n=4) nor MPG (ROS scavenger, -0.02±0.05*; n=8) affected the Ang II-induced inhibition of eNBC. The inhibitory action of Ang II on eNBC was corroborated with perforated patch-clamp experiments, since no impact of the current produced by eNBC on action potential repolarization was observed in the presence of Ang II. In conclusion, we propose that Ang II, binding to AT1 receptors, exerts an inhibitory effect on eNBC activity in a p38 kinase-dependent manner. © 2010 Elsevier Ltd.