PINACIDIL RELAXES PORCINE AND HUMAN CORONARY-ARTERIES BY ACTIVATING ATP-DEPENDENT POTASSIUM CHANNELS IN SMOOTH-MUSCLE CELLS

We investigated the effect of the potassium channel opener pinacidil o n ATP-dependent K+ channels (K-ATP) in the relaxation of porcine and h uman coronary arteries by means of isometric contraction experiments i n arterial rings. We also measured whole cell currents in freshly isol ated porcine and human coronary artery vascular smooth muscle cells wi th patch clamp. We first characterized serotonin-induced precontractio ns in our vessels and proved that the contractions were mediated by Ca 2+ influx through voltage-dependent Ca2+ channels. Similarly, we obser ved that serotonin-induced contractions were strongly enhanced by smal l K+-induced depolarizations. Pinacidil completely relaxed rings preco nstricted with 5 mu M serotonin and produced dose-dependent relaxation s of 5 mu M serotonin-preconstricted rings, with an IC50 of 1.26 mu M. Similar results were observed (IC50 = 1.15 mu M) when the endothelium was removed. The K-ATP blocker glibenclamide (3 mu M), inhibited pina cidil-induced relaxations (5-10 mu M) by similar to 25% although the K -ATP blocker tetrapentylammonium (10 mu M), inhibited pinacidil-induce d (5-10 mu M) relaxations completely. Pinacidil 10 mu M had only a min imal effect on rings precontracted with a 50 mM external K+ concentrat ion (IC50 = 60 mu M). Porcine and human arterial rings did not differ qualitatively in their responses. Moreover, in the patch clamp experim ents pinacidil (1 mu M and 20 mu M) induced a large, nonrectifying, ou tward current in both human and porcine cells. The reversal potential was close to the K+ equilibrium potential, suggesting an induction of pinacidil-activated K+ current. The pinacidil-induced (1 mu M) current was strongly inhibited by glibenclamide (3 mu M). These data show tha t the relaxation of porcine and human coronary arteries by pinacidil i s primarily induced by an opening of K-ATP in smooth muscle cells. Fur thermore, the vasorelaxant effect of pinacidil is not endothelium depe ndent.