Regulation by calcium of arachidonic acid metabolism in the isolated perfused rabbit heart.

Activation of beta-adrenergic receptors with isoproterenol fails to stimulate prostaglandin synthesis in the isolated rabbit heart perfused with Ca2+-free Krebs-Henseleit buffer. This lack of response could be due to reduced isoproterenol-stimulated liberation of arachidonic acid, reduced cyclooxygenase activity during Ca2+-free perfusion, or both. To test these hypotheses, we have examined the effect of isoproterenol and exogenous arachidonic acid on prostaglandin output in hearts perfused with Ca2+-containing and nominally Ca2+-free Krebs-Henseleit buffer. In hearts prelabelled with [3H]arachidonic acid and perfused with nominally Ca2+-free buffer, the release of radioactivity following isoproterenol was about 90% less than when Ca2+ was included in the perfusion buffer. When exogenous arachidonic acid was administered to hearts perfused with Ca2+-depleted buffer, the output of both 6-keto-prostaglandin F1 alpha and prostaglandin E2 was about threefold greater than when Ca2+ was present in the perfusion medium. In the absence of Ca2+, the dose response curve for arachidonic acid-induced 6-keto-prostaglandin F1 alpha synthesis underwent a parallel shift to the left, with no change in maximal synthesis levels, and a tenfold reduction in the ED50 of arachidonic acid. The cyclooxygenase activities of cell-free homogenates prepared from hearts perfused with and without Ca2+ were not different from each other and were both insensitive to added Ca2+. Reduction of the Na+ concentration of the perfusion medium to 35 mM resulted in increases in arachidonic acid-induced 6-keto-prostaglandin F1 alpha less than those obtained during Ca2+-free perfusion, but greater than that observed during perfusion with normal Na+ (139 mM) and Ca2+ (2.5 mM). Arachidonic acid induced 6-keto-prostaglandin F1 alpha output was inversely correlated with tissue Ca2+ but not Na+ content. These data suggest that in the absence of perfusate Ca2+, the availability of exogenous arachidonic acid to cyclooxygenase is increased, possibly by reduced incorporation into tissue phospholipids.