1 The e ects 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 e ect (PIE) was examined by using phenyl]ethyl]isothiourea methanesulphonate), a selective inhibitor of reverse mode Na + /Ca 2+ exchange. 2 ET-1 at 0.3 pM ± 1 nM increased cell contraction and Ca 2+ transient (CaT) with EC 50 values of 2.9 pM and 1.2 pM, respectively, and the increase in amplitude of CaT was much smaller relative to the PIE: ET-1 at 1 nM 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 myo®brillar Ca 2+ sensitivity. 3 KB-R7943 at 0.1 mM and higher inhibited contraction and CaT induced by 0.1 nM ET-1 and at 0.3 mM it abolished the increase in CaT while inhibiting the PIE by 48.1%. Over concentration range of 0.1 ± 0.3 mM, KB-R7943 neither inhibited baseline contraction and CaT nor the isoprenalineinduced response, although at 1 mM and higher it had a signi®cant inhibitory action on these responses. 4 These results indicate that in rabbit ventricular myocytes both increases in CaT and myo®brillar 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.

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Endothelin-1 (ET-1) increased cell shortening and Ca2+ transients over the concentration of 3 x 10(-11) M to 10(-9) M with EC50 of 8.3 x 10(-11) M in rabbit single ventricular myocytes. Thus ET-1 was approximately 60 times more potent in single myocytes than in papillary muscles (EC50 = 5.1 x 10(-9) M) of the same species. In single myocytes, ET-1 at 10(-8) M elicited an inhibitory response that counteracted the facilitatory response: the concentration-response curve (CRC) for ET-1 was bell shaped. The ET(A)-receptor antagonist BQ-485 shifted CRC for ET-1 to the right in parallel; however, the facilitatory response to 10(-8) M ET-1 was markedly enhanced by BQ-485 and also by the ET(B) antagonist BQ-788. The ET(A)/ET(B) antagonist TAK-044 abolished the ET-1-induced response. These findings indicate that the response to ET-1 of single myocytes is different from that of papillary muscles in concentration dependence, characteristics of the response, and susceptibility to ET-receptor antagonists. Anomalous pharmacological characteristics of ET-1-induced response in rabbit papillary muscles may be due to integrated regulatory mechanisms that may involve also various types of noncardiac cell in ventricular myocardium.

1 The role of Na + /H + exchange in endothelin-1 (ET-1)-induced increases in Ca 2+ transients and cell shortening was studied in rabbit ventricular myocytes loaded with indo-1/AM. Selective inhibitors of Na + /H + exchange HOE642 (4-isopropyl-3-methyl-sulphonylbenzoyl guanidine methanesulphonate) and KB-R9032 (N-(4-isopropyl-2,2-dimethyl-3-oxo-3,4-dihydro-2H-benzo-[1,4]oxazine-6-carbonyl) guanidine methanesulphonate) were used as pharmacological tools for the analysis. 2 ET-1 at 0.1 nM induced an increase in Ca 2+ transients by 45.6%, while it increased cell shortening by 109.6%. For a given increase in cell shortening, the ET-1-induced increase in Ca 2+ transients was much smaller than that induced by isoprenaline (ISO, 10 nM). 3 Pretreatment with HOE642 and KB-R9032 (1 mM) inhibited the increase in cell shortening induced by 0.1 nM ET-1 by 51 and 65.4%, respectively, without a signi®cant alteration of ET-1-induced increase in Ca 2+ transients. HOE642 and KB-R9032 did not a ect baseline levels of cell shortening and peak Ca 2+ transients, and the e ects of ISO (10 nM). 4 These results indicate that activation of Na + /H + exchange by ET-1 may play an important role in the positive inotropic e ect and the ET-1-induced increase in myo®lament Ca 2+ sensitivity in rabbit ventricular myocytes.

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