Endothelin-1 stimulates the Na⁺/Ca²⁺ exchanger reverse mode through intracellular Na⁺ (Na⁺_i) - Dependent and Na⁺_i-independent pathways

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⁺/Ca²⁺ 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.