Leukocyte and platelet-derived factors augment canine coronary constriction to serotonin

In open-chest anesthetized dogs acute hypertension causes neutrophil and platelet adhesion to vascular endothelium and selectively potentiates constriction to serotonin in proximal coronary arteries. To examine underlying mechanisms, canine left anterior descending coronary arteries subjected to 15 min hypertension (LAD-HYP) and control left circumflex coronary arteries (CX) perfused at normal pressure were studied in organ chambers. In endothelium-intact LAD-HYP rings, constriction to serotonin was potentiated fourfold compared with control CX rings but was similar in denuded LAD-HYP and CX vessels. Endothelium-dependent relaxation to acetylcholine was not affected by acute hypertension. In LAD-HYP rings 10 microM LY 83583 (which depletes guanosine 3',5'-cyclic monophosphate and inhibits effects of endothelium-derived relaxing factor) augmented constriction to serotonin twofold. LY 83583 did not affect the serotonin response in hypertensive rings whose endothelium was mechanically removed. Blockade of either leukotriene D4 (LTD4) receptors (either with LY 171883 or SKF 102992) or thromboxane A2 (TxA2) receptors (with SQ 29548) partially blunted constriction to serotonin. Combined LTD4- and TxA2-receptor blockade completely normalized serotonin-induced constriction in LAD-HYP rings. In preconstricted LAD-HYP rings, relaxations to serotonin were markedly impaired but were restored by addition of ketanserin. Normalization of relaxation to serotonin in hypertensive vessels by ketanserin is likely due to inhibition of 5-hydroxytryptamine2 (5-HT2) receptors on platelet membranes. In conclusion, augmented constriction to serotonin in canine epicardial vessels exposed to acute hypertension is not due to an impairment of endothelium-dependent relaxation to the amine but to concomitant release of leukotrienes and TxA2 from leukocytes and platelets adhering to damaged endothelium. Activation of 5-HT2 serotonergic receptors on platelet membranes could be a possible trigger mechanism.