Role of Na + /Ca 2+ exchange in endothelin‐1‐induced increases in Ca 2+ transient and contractility in rabbit ventricular myocytes: pharmacological analysis with KB‐R7943

The effects of endothelin‐1 (ET‐1) on intracellular Ca 2+ ion level and cell contraction were simultaneously investigated in rabbit ventricular cardiac myocytes loaded with indo‐1/AM. The role of Na + /Ca 2+ exchange in ET‐1‐induced positive inotropic effect (PIE) was examined by using KB‐R7943 (2‐[2‐[4‐(4‐nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulphonate), a selective inhibitor of reverse mode Na + /Ca 2+ exchange. ET‐1 at 0.3 p M –1 n M increased cell contraction and Ca 2+ transient (CaT) with EC 50 values of 2.9 p M and 1.2 p M , respectively, and the increase in amplitude of CaT was much smaller relative to the PIE: ET‐1 at 1 n M increased peak cell shortening by 237%, while it increased peak CaT by 167%. For a given level of PIE, ET‐1‐induced increase in CaT was much smaller than that induced by elevation of [Ca 2+ ] o and by isoprenaline. Therefore, ET‐1 shifted the relationship between peak CaT and cell shortening to the left relative to the relationship for increase in [Ca 2+ ] o , an indication that ET‐1 increased myofibrillar Ca 2+ sensitivity. KB‐R7943 at 0.1 μ M and higher inhibited contraction and CaT induced by 0.1 n M ET‐1 and at 0.3 μ M it abolished the increase in CaT while inhibiting the PIE by 48.1%. Over concentration range of 0.1–0.3 μ M , KB‐R7943 neither inhibited baseline contraction and CaT nor the isoprenaline‐induced response, although at 1 μ M and higher it had a significant inhibitory action on these responses. These results indicate that in rabbit ventricular myocytes both increases in CaT and myofibrillar Ca 2+ sensitivity contribute to the ET‐induced PIE, and the activation of reverse mode Na + /Ca 2+ exchange may play a crucial role in increase in CaT induced by ET‐1 in rabbit ventricular cardiac myocytes.British Journal of Pharmacology (1999) 126 , 1785–1795; doi: 10.1038/sj.bjp.0702454