HCO3-Dependent Impact of Na+,K+,2Cl- Cotransport in Vascular Smooth Muscle Excitation-Contraction Coupling.

In smooth muscles, inhibition of Na+,K+,2Cl- cotransport (NKCC) by bumetanide decreased intracellular Cl- content ([Cl-]i) and suppressed the contractions triggered by diverse stimuli. This study examines whether or not bicarbonate, a regulator of several Cl- transporters, affects the impact of NKCC in excitation-contraction coupling. Addition of 25 mM NaHCO3 attenuated the inhibitory action of bumetanide on mesenteric artery contractions evoked by 30 mM KCl and phenylephrine (PE) by 5 and 3-fold, respectively. In cultured vascular smooth muscle cells, NaHCO3 almost completely abolished inhibitory actions of bumetanide on transient depolarization and [Ca2+]i elevation triggered by PE. In bicarbonate-free medium, bumetanide decreased [Cl-]i by ∼15%; this effect was almost totally abrogated by NaHCO3. The addition of NaHCO3 resulted in 2-fold inhibition of NKCC activity and 3-fold attenuation of [Cl-]i. These data strongly suggest that extracellular HCO3- diminishes the NKCC-sensitive component of excitation-contraction coupling via suppression of this carrier. Copyright © 2009 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]