Sites of prostaglandin synthesis in the bovine heart and isolated bovine coronary microvessels
Metabolically active coronary microvessels (generally < 100 fim in diameter) were isolated from bovine hearts. These preparations were virtually free of contaminating myocytes and consisted of a mixture of arterioles, venules, capillaries, and collagen fibers. The prostaglandin biosynthetic capacity of isolated coronary micro vessels was assessed by quantitating conversion of "C-arachidonic acid and '"C-prostaglandin endoperoxide (PGH») to prostaglandins. Coronary microvessels were found to
... bit low cyclooxygenase activity, i.e., minimal conversion (<1%) of arachidonic acid (AA) to prostagtandins when compared with either the coronary artery (3.6% conversion of AA) or coronary vein (3.3%). Isolated myofibril fractions demonstrated virtually no cyclooxygenase activity (<0.1%). Two enzymatic endoperoxide metabolizing activities were evident in the microvessels, prostacyclin synthetase and PGE isomerase. When compared on a microsomal protein basis, the coronary artery exhibited higher prostacyclin synthetase activity than the microvessels but wag devoid of PGE isomerase activity. In contrast, whereas PGE isomerase activity was detectable in the coronary vein, the activity was lower than that of the micro vessels; further, it could not be demonstrated in the other myocardial fractions. A glutathione-enhanced PGD-forming activity was present in high speed supernatant* derived from the coronary artery and vein preparations, but was not present in the coronary microvessel or isolated myofibril fractions. From these studies we conclude that (1) the major source of prostaglandins released into the coronary circulation is the coronary vasculature, and (2) the resistance elements of the coronary circulation differ from the larger elements (Le., arteries and veins) in the enzyme* of the prostaglandin biosynthetic pathway present and thus in the nature and perhaps quantity of prostaglandins biosynthesized. of prostaglandin synthesis was localized to the coronary vasculature. However, more definitive localization of the prostaglandin biosynthetic pathway in the mammalian myocardium has received limited investigation. The prostaglandin biosynthetic pathway consists of three components: (1) acylhydrolase that liberates arachidonate from membrane phospholipids, -2) cyclooxygenase activity which synthesizes endoperoxides, and (3) prostaglandin endoperoxide metabolizing activities (i.e., PGE-isomerase, PGDisomerase, prostacyclin synthetase, thromboxane synthetase). The type of prostaglandin or prostanoid formed is determined by the activities of the endoperoxide-metabolizing enzymes present and expressed, whereas the quantity of product is governed largely by the first two steps of the pathway. Our experimental approach is to examine systematically each of these steps to develop a more complete definition of this pathway. To facilitate this approach in the mammalian heart, we developed a procedure for the isolation of metabolically active bovine coronary microvessels. Using this preparation, we have examined the nature of the prostaglandin-synthesizing activities (i.e., cyclooxygenase and prostaglandin endoperoxide metabolizing enzymes) in this and four other fractions prepared from the bovine heart: whole myocardium, coronary artery, coronary vein, and isolated myofibrils.