Abstract:We have studied the influence of endothelium on BAY K 8644, CGP 28392, potassium, noradrenaline and calcium induced responses in isolated thoracic aorta from rats. In aortic preparations with intact endothelium BAY K 8466 and CGP 28392 (10‐9‐10‐5M) had a concentration dependent contractile effect, but BAY K 8644 was more effective in contracting the vessels than CGP 28932. Removal of endothelium increased the contractile response to both substances without affecting the difference in contractile effect between the two calcium agonists. In high concentrations (10‐4M) BAY K 8644 and CGP 28392 had an equal relaxing effect on potassium precontracted preparations, which was unaffected by removal of endothelium. Removal of endothelium caused a shift to the left of the potassium and noradrenaline concentration response curves without any change in maximal contractile response. The contractile response to calcium in BAY K 8644 and potassium activated vessels was unaffected by removal of endothelium. We conclude that endothelium derived factor(s) depress the contractile response to calcium agonists, potassium, and noradrenaline but has no influence on the relaxant effect of BAY K 8644 and CGP 28392 in rat aorta. The influence of endothelium on the contractile effect on BAY K 8644 and potassium seems not to be due to a direct effect on calcium influx but rather to an influence on the membrane potential of the smooth muscl