K+-induced hyperpolarization in rat mesenteric artery: Identification, localization and role of Na+/K+-ATPases

Abstract

1. Mechanisms underlying K+-induced hyperpolarizations in the presence and absence of phenylephrine were investigated in endothelium-denuded rat mesenteric arteries (for all mean values, n = 4). 2. Myocyte resting membrane potential (m.p.) was-58.8±0.8 mV. Application of 5 mM KCl produced similar hyperpolarizations in the absence (17.6±0.7 mV) or presence (15.8±1.0 mV) of 500 nM ouabain. In the presence of ouabain +30 μM barium, hyperpolarization to 5 mM KCl was essentially abolished. 3. In the presence of 10 μM phenylephrine (m.p.-33.7±3 mV), repolarization to 5 mM KCl did not occur in the presence or absence of 4-aminopyridine but was restored (-26.9±1.8 mV) on addition of iberiotoxin (100 nM). Under these conditions the K+-induced repolarization was insensitive to barium (30 μM) but abolished by 500 nm ouabain alone. 4. In the presence of phenylephrine + iberiotoxin the hyperpolarization to 5 mM K+ was inhibited in the additional presence of 300 nM levcromakalim, an action which was reversed by 10 μM glibenclamide. 5. RT-PCR, Western blotting and immunohistochemical techniques collectively showed the presence of α1-, α2- and α3-subunits of Na+/K+-ATPase in the myocytes. 6. In K+-free solution, re-introduction of K+ (to 4.6 mM) hyperpolarized myocytes by 20.9±0.5 mV, an effect unchanged by 500 nM ouabain but abolished by 500 μM ouabain. 7. We conclude that under basal conditions, Na+/K+-ATPases containing α2- and/or α3-subunits are partially responsible for the observed K+-induced effects. The opening of myocyte K+ channels (by levcromakalim or phenylephrine) creates a 'K+ cloud' around the cells which fully activates Na+/K+-ATPase and thereby abolishes further responses to [K+]o elevation.