Ouabain inhibits endothelium-dependent relaxations to arachidonic acid in canine coronary arteries

Abstract

Experiments were designed to analyze the effects of ouabain on the actions of exogenous arachidonic acid on endothelial and vascular smooth muscle cells. Rings or strips were prepared from left circumflex canine coronary arteries and suspended for isometric tension recording in organ chambers filled with oxygenerated modified Krebs-Ringer-bicarbonate solution. During contractions evoked by prostaglandin F(2α), arachidonic acid caused relaxations both in the presence and the absence of endothelium. However, removal of the endothelium reduced its inhibitory action. Indomethacin prevented the relaxations in rings without endothelium, but did not affect the response to high doses (10-6 to 10-5 M) of arachidonic acid in preparations with endothelium. The inhibitor of lipoxygenase, nordihydroguaiaretic acid, had no effect on the inhibitory responses to arachidonic acid in rings with or without endothelium. Ouabain abolished both the endothelium-dependent and the direct relaxations to arachidonic acid. Endothelium-dependent relaxations in response to oleic acid, elaidic acid, adenosine diphosphate and thrombin were not affected by ouabain. In the presence of indomethacin, coronary artery strips without endothelium were relaxed by arachidonic acid only when layered (intimal surface against intimal surface) with a longitudinal strip with endothelium. In layered preparations, treatment of the intact longitudinal strip with ouabain before layering prevented the relaxation, whereas pretreatment of the strip without endothelium had no effect. These experiments demonstrate that: 1) arachidonic acid stimulates the release of a relaxing factor from the endothelial cells of canine coronary arteries which differs from prostaglandins; 2) ouabain prevents this effect by acting on endothelial cells; and 3) the selective effect of ouabain on endothelium-dependent relaxations induced by arachidonic acid indicates that it either stimulates the release of a factor different from that produced by other endothelium-dependent relaxing substances and/or causes the release of the same relaxing mediator by different means.