Intrinsic Prostaglandin Biosynthesis in Blood Vessels

Regulation of large coronary arteries

The majority of studies on the control of coronary artery vasoactivity have examined changes in coronary blood flow and coronary vascular resistance, indices that primarily reflect regulation of small arterioles and precapillary vessels. With the emergence of coronary artery vasospasm as a significant cause of angina pectoris, myocardial infarction, and sudden death, the control of large coronary artery caliber has assumed more significance. It is clear that resistance coronary vessels and large coronary arteries differ in response to both pharmacologic and physiologic stimuli. Vasodilation of large coronary arteries may occur by direct action of agents on the arterial smooth muscle or by the indirect action of receptor occupation, changes in blood flow, or liberation of endothelial factors. These indirect factors appear to contribute also to responses to agents that constrict coronary smooth muscle directly or through the autonomic nervous system. Furthermore, the mechanisms responsible for control of large coronary vessels in the normal circulation are likely to be profoundly different from those in the presence of diseased vessels. For example, several factors associated with coronary artery disease--elevated plasma cholesterol levels, endothelial disruption, atherosclerosis, vascular stenosis, and aggregated platelets--all have important actions on the control of large coronary arteries.

Inhibitors of prostaglandin synthesis augment beta-adrenergic responsiveness in canine coronary arteries

The effects of inhibition of endogenous prostaglandin synthesis on the release of norepinephrine from sympathetic nerves and on postjunctional adrenergic responsiveness were studied in isolated canine left circumflex coronary arteries. In rings, suspended for isometric tension recording and contracted with prostaglandin F2 alpha, transmural electrical stimulation caused frequency-dependent relaxations, which were blocked by propranolol and augmented by indomethacin. In superfused strips, previously incubated with [3H]norepinephrine, electrical stimulation (2 Hz) increased the overflow of tritiated neurotransmitter; indomethacin did not influence basal or evoked [3H]norepinephrine overflow. Exogenous norepinephrine caused relaxations in rings contracted with prostaglandin F2 alpha, but increases in tension in potassium-depolarized tissues which could be abolished by phentolamine; isoproterenol induced relaxations in both cases. Indomethacin significantly augmented the relaxation in response to exogenous norepinephrine (during contractions with prostaglandin F2 alpha) and reversed norepinephrine-induced contractions (during potassium-depolarization) into relaxation. Other cyclooxygenase inhibitors had comparable effects. In the presence of propranolol, indomethacin did not diminish contractions evoked by norepinephrine in depolarized rings. Relaxations induced by sodium nitroprusside or acetylcholine during contractions caused by prostaglandin F2 alpha or potassium chloride were not affected by indomethacin. The augmentation of beta-adrenergic responsiveness by indomethacin was abolished by exogenous prostacyclin. The prostacyclin synthetase inhibitor tranylcypromine and exogenous prostaglandin E2 depressed beta-adrenergic responsiveness. Indomethacin did not affect the facilitatory action of phosphodiesterase inhibition on beta-adrenergic relaxation. The data suggest that endogenous prostaglandins (most probably prostacyclin and prostaglandin E2) exert a "braking" effect on beta-adrenergic responsiveness in coronary arterial smooth muscle.

Two distinct effects of oxygen on vascular tone in isolated porcine coronary arteries

The relation between PO2 and vessel tone was studied in isolated porcine left descending coronary artery rings. Porcine left descending coronary artery mounted isometrically and equilibrated in Krebs-bicarbonate solution (37 degrees C, pH 7.4, when gassed with 95% oxygen + 5% carbon dioxide) exhibited spontaneous basal tone. Decreasing bath PO2 to 40%, 20%, and 12% elicited sustained increases in basal tension which were reversible, ranging between 10% and 20% of the contraction induced by 40 mM potassium chloride. Further decreases in PO2 to near zero (anoxia) resulted in relaxation to baseline. Cyclooxygenase inhibitors indomethacin (5.5 X 10(-6) M), aspirin (5 X 10(-5) M), and meclofenamate (10(-5) M) decreased vascular tone and totally prevented coronary vasoconstriction induced by lowering bath PO2 to 12% or 40% but did not affect anoxic vasorelaxation. Neither basal tone nor the vasoconstriction induced by decreases in bath PO2 were influenced by the antihistaminergic drug pyribenzamine (10(5) M) or by the alpha-adrenergic blocker phentolamine (10(-6]. Isoproterenol (10(-9) to 10(-8) M) or an elevation of the bath potassium concentration from 5.9 to 11 mM significantly augmented coronary vasoconstriction induced by lowering bath PO2 from 95% to 40%. Elevation of the bath potassium chloride concentration to 40 mM further increased isometric force but inhibited the vasoconstriction in response to decreasing PO2 from 95% to 40%. Anoxia relaxed contractions induced by 40 mM potassium chloride, histamine, or ouabain. The data suggest the existence of two distinct oxygen-sensitive mechanisms in porcine coronary arteries, both of which regulate vascular tone. One is activated at relative high PO2 values (10-40%), and the vasoconstriction induced by this mechanism is mediated by vascular prostaglandin synthesis. The other is expressed at low PO2 values (near zero), and the depression of mechanical activity by this mechanism may be related to limitation of oxidative energy metabolism. The first mechanism can be augmented by beta-adrenoceptor stimulation indicative of an interaction between vascular prostaglandin synthesis and beta-adrenergic mechanisms in the coronary artery wall.