Endothelin-1 Stimulates the Na + /Ca 2+ Exchanger Reverse Mode Through Intracellular Na + (Na + i )–Dependent and Na + i -Independent Pathways

Abstract

This study aimed to explore the signaling pathways involved in the positive inotropic effect (PIE) of low doses of endothelin-1 (ET-1). Cat papillary muscles were used for force and intracellular Na + concentration (Na + i ) measurements, and isolated cat ventricular myocytes for patch-clamp experiments. ET-1 (5 nmol/L) induced a PIE and an associated increase in Na + i that were abolished by Na + /H + exchanger (NHE) inhibition with HOE642. Reverse-mode Na + /Ca 2+ exchanger (NCX) blockade with KB-R7943 reversed the ET-1–induced PIE. These results suggest that the ET-1–induced PIE is totally attributable to the NHE-mediated Na + i increase. However, an additional direct stimulating effect of ET-1 on NCX after the necessary increase in Na + i could occur. Thus, the ET-1–induced increase in Na + i and contractility was compared with that induced by partial inhibition of the Na + /K + ATPase by lowering extracellular K + (K + o ) . For a given Na + i , ET-1 induced a greater PIE than low K + o . In the presence of HOE642 and after increasing contractility and Na + i by low K + o , ET-1 induced an additional PIE that was reversed by KB-R7943 or the protein kinase C (PKC) inhibitor chelerythrine. ET-1 increased the NCX current and negatively shifted the NCX reversal potential (E NCX ). HOE642 attenuated the increase in NCX outward current and abolished the E NCX shift. These results indicate that whereas the NHE-mediated ET-1–induced increase in Na + i seems to be mandatory to drive NCX in reverse and enhance contractility, Na + i -independent and PKC-dependent NCX stimulation appears to additionally contribute to the PIE. However, it is important to stress that the latter can only occur after the primary participation of the former.