Synergistic action of NS-004 and internal Ca2+ concentration in modulating pulmonary artery K+ channels

Considering the singular vasomotor behavior of the pulmonary artery, we were interested to test NS-004 (1-(2'-hydroxy-5'-chlorophenyl)-5-trifluoromethyl-2(3H)-benzimidazolone) on pulmonary artery smooth muscle cells. Using the patch clamp technique, we identified a delayed rectifier K+ current and a Ca2+-activated K+ current. With a low free intracellular Ca2+ concentration ([Ca2+](i)), 10-50 mu M NS-004 activated a noisy outward current which was blocked by iberiotoxin. 50 mu M NS-004 also inhibited a smooth inactivating outward current. Under these conditions, 10 mu M NS-004 induced no change in the resting membrane potential. With a higher free [Ca2+](i), 10 mu M NS-004 was 3.5 times more efficacious in increasing the noisy current and it induced a hyperpolarization, We concluded that increasing free [Ca2+](i) induced potentiation of the NS-004-induced activation of high conductance Ca2+-sensitive K+ channels and of the NS-004-induced hyperpolarization of the cell. The delayed rectifier K+ channel was inhibited by NS-004 as well as by an increased free [Ca2+](i).