Hypoxia-induced release of prostaglandins: mechanisms and sources of production in coronary resistance vessels of the isolated rabbit heart

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Abstract
The mechanism of hypoxia-induced prostacyclin (PGI2) release was studied in isolated, modified Krebs–Henseleit buffer perfused rabbit hearts under constant-flow conditions. The contribution of vascular endothelium and the role of catecholamines and calcium influx through L-type calcium (Ca2+) channels in hypoxic release of PGI2 were investigated. Reduction in the perfusion solution [Formula: see text] (from 476 ± 13 to 127 ± 24 mmHg; 1 mmHg = 133.3 Pa) in the presence of glucose (5 mM) and pyruvate (2 mM) caused a significant release of 6-keto-PGF1α, the stable metabolite of PGI2 (from 3.0 ± 0.4 to 7.3 ± 2.0 pmol ∙ min−1 ∙ g−1, p < 0.05, n = 12), whereas no variation in washout of catecholamines in the perfusate was observed. Electrolysis of the perfusion buffer solution was used to destroy the endothelium. Endothelium impairment by electrolysis almost completely abolished the vasodilation induced by serotonin (5HT) and acetylcholine (ACh), without affecting that caused by papaverine. The basal release of 6-keto-PGF1α was significantly enhanced after electrolysis. However, its release during hypoxia was completely abolished. In a separate group of normal hearts, verapamil (10−7 M) completely blocked the release of 6-keto-PGF1α during hypoxia. Similar results were obtained in 15 mM KCl arrested hearts challenged with hypoxia. Under normoxic conditions, isoproterenol (10−7 M) induced a significant release of 6-keto-PGF1α (from 2.9 ± 0.5 to 5.3 ± 0.8 pmol ∙ min−1 ∙ g−1, p < 0.05, n = 9). To stimulate endogenous catecholamine release, hearts perfused with glucose-free buffer (pyruvate 14.8 mM) were submitted to hypoxia. Under these conditions, hypoxia was accompanied by an enhanced release of noradrenaline (from 1.3 ± 0.5 to 19.7 ± 7.8 pmol ∙ min−1 ∙ g−1). Despite the increased noradrenaline washout in effluent, the hypoxia-induced release of 6-keto-PGF1α, in glucose-free perfused hearts was not enhanced. Our results suggest that endothelial cells are the major site of PGI2 synthesis during hypoxia and that myocardial contractility is a prime factor involved in this process. Verapamil reduction of PGI2 release is most probably through its negative inotropic effect. Although exogenous β-adrenergic stimulation can induce PGI2 release, endogenous catecholamines do not appear to contribute to the hypoxia-induced release of PGI2.Key words: prostacyclin, endothelium, L-type calcium channel blockers, norepinephrine, glucose, hypoxia.

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