Endothelin activation of an inwardly rectifying K+ current in atrial cells

Various tissues including heart express specific binding sites for endothelin. Endothelins have been reported to increase the force of contraction of cardiac muscle, presumably via specific receptors. Specific binding of endothelin to atrial tissue is particularly high. In spontaneously contracting rat atrial cells used in this study, all three isoforms of endothelin (endothelin-1, endothelin-2, and endothelin-3) decreased the rate of beating and caused an increase in inwardly rectifying K+ current in voltage-clamped whole cells. Endothelin-3 was the most potent isoform, and its effects on beating rate and K+ current were present at a concentration as low as 100 pM (Kd, approximately 1 nM). the atrial cells did not have the hyperpolarization-activated current (the pacemaker current), If. In excised inside-out patches, all three isoforms of endothelin activated a population of K+ channels with kinetic properties identical to those of acetylcholine (muscarinic)-activated K+ channels, and this was GTP dependent. Endothelin failed to decrease the beating rate or to elicit the K+ current in pertussis toxin-treated cells. These results indicate that endothelin has a potent negatively chronotropic effect by activation of the inwardly rectifying, muscarinic K+ channel and therefore could be an important regulator of heart function.