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

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). Myocyte resting membrane potential (m.p.) was −58.8±0.8 mV. Application of 5 m M KCl produced similar hyperpolarizations in the absence (17.6±0.7 mV) or presence (15.8±1.0 mV) of 500 n M ouabain. In the presence of ouabain +30 μ M barium, hyperpolarization to 5 m M KCl was essentially abolished. In the presence of 10 μ M phenylephrine (m.p. −33.7±3 mV), repolarization to 5 m M 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 n M ). Under these conditions the K+‐induced repolarization was insensitive to barium (30 μ M ) but abolished by 500 n M ouabain alone. In the presence of phenylephrine + iberiotoxin the hyperpolarization to 5 m M K + was inhibited in the additional presence of 300 n M levcromakalim, an action which was reversed by 10 μ M glibenclamide. RT–PCR, Western blotting and immunohistochemical techniques collectively showed the presence of α 1 ‐, α 2 ‐ and α 3 ‐subunits of Na + /K + ‐ATPase in the myocytes. In K + ‐free solution, re‐introduction of K + (to 4.6 m M ) hyperpolarized myocytes by 20.9±0.5 mV, an effect unchanged by 500 n M ouabain but abolished by 500 μ M ouabain. We conclude that under basal conditions, Na + /K + ‐ATPases containing α 2 ‐ and/or α 3 ‐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.British Journal of Pharmacology (2002) 136 , 918–926. doi: 10.1038/sj.bjp.0704787