Endothelin contraction in pig coronary artery: receptor types and Ca(2+)-mobilization

Endothelin is one of the most potent vasoconstrictors known. It plays an important role in the regulation of vascular tone and in the development of many cardiovascular diseases. This study focuses on the receptor types and the Ca2+ mobilization responsible for endothelin-1 (ET-1) contraction in de-endothelialized pig coronary artery rings. ET-1 contracted the artery rings with an EC50 = 6.5 +/- 1 nM and a maximum contraction which was 98.6 +/- 9% of the contraction produced by 60 mM KCl. BQ123 (5 microM), an ETA antagonist, reversed 78 +/- 3% of the ET-1 contraction (50 nM). IRL1620, a selective ETB agonist, produced 23 +/- 3% of the total ET-1 contraction with an EC50 = 12.7 +/- 2 nM. More than 85% of the contraction due to 100 nM IRL 1620 was inhibited by 200 nMBQ788, an ETB antagonist. Therefore, approximately 80% of the ET-1 contraction in this artery occurred via ETA receptors, and the other 20% was mediated by ETB receptors. To assess the Ca2+ pools utilized during the ET-1 response, ET-1 contraction was also examined in medium containing an L-type Ca2+ channel blocker nitrendipine, and in Ca2+ free medium containing 0.2 mM EGTA. In Ca2+ containing medium the contraction elicited by ET-1 was 98.6 +/- 9% of the KCl contraction, however, in the presence 10 microM nitrendipine the ET-1 induced contraction was 54 +/- 7% of the KCl contraction, and in Ca(2+)-free medium it was 13 +/- 2%. Similarly, the IRL 1620 contractions in Ca2+ containing medium, in the presence of nitrendipine and in Ca(2+)-free medium were 22.4 +/- 3%, 12 +/- 3% and 11 +/- 2% of the KCl response respectively. Thus, both ETA and ETB contractions utilize extracellular Ca2+ pools via L-type Ca2+ channels and other undefined route(s), as well as intracellular Ca2+ pools. In the pig coronary artery smooth muscle, ET-1 contractions occur predominantly via ETA receptors, with ETB receptors using similar Ca2+ mobilization pathways, but the ETB receptors appear to use the intracellular Ca2+ stores to a greater extent.