| Endothelin-1 (ET-1) modulates vascular smooth muscle contraction, and cardiac contractility in mammals. The effects of ET-1 on contractility in patients with congestive heart failure (CHF) remain controversial. The mechanism for ET-1 effects on contractility involves phosphorylation of several intracellular molecules including the NHE-1 exchanger and sarcolemmal ion channels. I hypothesized the effects of ET-1 on cardiac contractility are calcium dependent phenomena, and this intracellular signaling becomes aberrant in congestive heart failure. Methods. Isolated feline left ventricular myocytes were studied to measure fractional shortening (FS), as well as intracellular calcium transients ([Ca2+]i), action potential duration and pH after exposure to 100 nM ET-1. Extracellular conditions were manipulated by exchanging perfusate with HEPES and bicarbonate based buffer, as well as HOE-642 to block NHE-1 activity. Reverse mode NCX was inhibited by KB-R7943 to determine the contribution of calcium influx as a mechanism for increased contractility. The effects of ET-1 were also studied in human left ventricular myocytes from non-failing, failing, and left ventricular assist device (LVAD) supported myocardium. Direct PKC activation, and mRNA transcription were also studied. Results . ET-1 causes a positive isotropic effect on FS and the [Ca 2+]i, and prolongs the action potential. The increase in contractility is associated with intracellular alkalinization, and is blocked by NHE-1 inhibition in HEPES buffer, but not in bicarbonate buffer. Inhibition of reverse mode NCX abolished the positive isotropic response in both buffer systems. ET-1 causes a negative inotropic effect in failing human myocytes, but reverts to an increase in contractility in LVAD supported myocytes. PKC activation causes a negative inotropic response in failing myocytes, suggesting a role for post-translational modification, rather than mRNA transcription. In conclusion, ET-1 is a potent positive inotropic agent. However, alkalinization is not the sole basis for the calcium dependent increase in contractility, which may be due to action potential prolongation and reverse mode NCX activity. The negative inotropic effect seen in failing myocytes is suggested to be due to post-translational modifications rather than genetic transcription, as restitution of function associated with LVAD support is not linked to parallel changes in mRNA transcription of key ET-1 signaling cascade molecules. |
