FUNCTIONAL AND AUTORADIOGRAPHIC EVIDENCE FOR ENDOTHELIN-1 RECEPTORS ON HUMAN AND RAT CARDIAC MYOCYTES - COMPARISON WITH SINGLE SMOOTH-MUSCLE CELLS

This study aimed to determine whether receptors for endothelin were present on the cardiac myocyte as well as on vascular smooth muscle cells. Low- and high-resolution autoradiography was performed using 125I-endothelin 1 on intact rat myocardium and samples of human ventricle obtained from explanted hearts at the time of transplant. In addition to specific binding to the smooth muscle of the blood vessel lumen, there was considerable binding associated with cardiac myocytes. To discover whether there was any functional correlate for this binding, muscle cells were isolated enzymatically from human and rat ventricle and from rat femoral artery, and their contractile characteristics were studied. Single cardiac cells were superfused with physiological saline at 32°C, and their length change was displayed continuously on a chart recorder. Endothelin 1 had a pronounced effect on shortening in both rat and human myocytes. The contraction amplitude was approximately doubled in both cases, from 4.1 ± 0.8% cell length to 8.1 ± 1.3% for rat (mean ± SEM, n = 9, p < 0.001), and from 2.1 ± 0.5% to 4.0 ± 0.5% in human (n = 10, p < 0.001). In rat, the magnitude of the effect was comparable to that of the α-adrenoceptor agonist phenylephrine. The maximum contraction amplitude of human cells, produced by raising extracellular calcium to greater than 10 mM, was 11.4 ± 1.1% cell length (n = 9), significantly greater than that produced by endothelin (p < 0.001). The threshold for the effect of endothelin 1 was around 0.3 nM, and maximum effects were attained at 30 nM, compared with 1 and 100 μM, respectively, for the α-adrenoceptor effect. Endothelin 1 had a potent action on single vascular smooth muscle cells. The EC50 for endothelin 1 was 36 ± 4 pM (n = 8) compared with 3.0 ± 1.6 μM (n = 7) for phenylephrine. Maximum contraction was attained by depolarization with 80 mM KCl, and was 26 ± 3% (n = 11) of resting length. The response to endothelin 1 in single smooth muscle cells was quantitatively similar to the response to phenylephrine (85 ± 9% of KCl contracture [n = 12] versus 83 ± 7 % [n = 7] for phenylephrine). We conclude that endothelin 1 binding has a significant functional correlate in the direct effects on cardiac myocyte contraction for both rat and human, but that concentrations required for cardiac effects are greater than those for smooth muscle activation.