Levosimendan, a novel Ca 2+ sensitizer, activates the glibenclamide-sensitive K + channel in rat arterial myocytes

Abstract

The electrophysiological effect of levosimendan, a novel Ca 2+-sensitizing positive inotropic agent and vasodilator, was examined on rat mesenteric arterial myocytes using the patch clamp technique. Resting potential was significantly hyperpolarized with levosimendan, with an EC 50 of 2.9 μM and maximal effect (19.5±3.5 mV; n=12) at 10 μM. Levosimendan (10 μM) significantly increased the whole-cell outward current. The currents intersected close to the calculated E K (−84 mV), suggesting that the activated current was a K + current. Hyperpolarization and stimulation of K + current by levosimendan were not prevented by 30 μM H-7 (a non-specific inhibitor of protein kinases) and 100 nM charybdotoxin (a blocker of Ca 2+-activated K + channels), but were abolished by 10 μM glibenclamide. In single-channel current recording in open cell-attached patches, two types of K + channels were observed having conductances of 26 and 154 pS. The 154 pS channels were not affected by levosimendan and glibenclamide. The 26 pS channels were evoked in one-fourth of the patches when 10 μM levosimendan (and 0.1 mM UDP) was added (at −60 mV) and channel activity was abolished by glibenclamide. The mean open probability of the 26 pS channels was 0.094±0.017 ( n=9), and the mean open time (at −60 mV) was 6.6 ms in the presence of UDP and levosimendan. Although significant hyperpolarization (4.7±1.5 mV, n=8) was observed at 1 μM levosimendan, the same concentration did not affect Ca 2+ channel currents ( n=10). In summary, levosimendan hyperpolarized the arterial myocytes, probably through activation of a glibenclamide-sensitive K + channel. This mechanism may contribute to the vasodilating action of levosimendan.