The data obtained in this study demonstrate that the concentration of serotonin is markedly elevated (18-to 27-fold) at the site of a coronary arterial stenosis in open-chest, anesthetized dogs with cyclic flow variations. Cyclic flow variations in this experimental preparation were abolished by ketanserin, a 5-hydroxytryptamine antagonist, but serotonin concentration at the site of the coronary stenosis remained elevated. The intra-atrial administration of serotonin (0.16 to 1 mg/min) restored cyclic flow variations after they had been abolished by ketanserin. Taken together, these data suggest that serotonin may be one of the important mediators of cyclic flow variations in this experimental preparation.Circulation 73, No. 3, 572-578, 1986. PREVIOUS STUDIES from our laboratory and others have suggested that platelet aggregation is an important factor in some occlusive coronary disease states.'4 Platelets possess both a2-adrenergic and serotonergic receptors, which are thought to be responsible for the platelet aggregatory effects of catecholamines and serotonin (5-hydroxytryptamine). Separate receptors responsible for regulating the uptake of serotonin into platelets also have been identified.5 6 In concentrically narrowed canine coronary arteries, spontaneous transient coronary thrombosis results in cyclic flow variations (CFVs). This preparation was described by Folts et al.7 and has been used in subsequent studies by our laboratory3'4 and by others2 to examine the mechanism(s) contributing to the development of CFVs in stenotic canine coronary arteries. CFVs are characterized by gradual decreases in blood flow over a period of minutes followed by either spontaneous or induced abrupt increases in flow. CFVs are thought to be produced by platelet aggregation at the stenotic site alternating with thrombus dislodgement and embolization.We have shown that ketanserin In the present study, we tested the hypothesis that serotonin may be one of the important mediators of CFVs in stenotic canine coronary arteries. The tissue concentration of serotonin was determined in both normal canine coronary arterial segments and in stenotic canine coronary arterial segments in which CFVs had occurred. The concentration of serotonin was determined in the same coronary arteries in a separate group of dogs in which CFVs were abolished by ketanserin. Furthermore, the role of serotonin in mediating CFVs was determined by establishing CFVs in another group of dogs, eliminating them with ketanserin, and subsequently infusing serotonin directly into the left atrium to determine whether CFVs could be restored by displacing ketanserin from its receptor site with excess of the agonist.

The goal of the present study was to demonstrate that intracoronary platelet deposition may trigger intense vasoconstriction of large epicardial coronary arteries in vivo and that this is largely mediated by thromboxane A2 and serotonin released by activated platelets. Cyclic flow variations (progressive declines in blood flow followed by sudden restorations of flow) due to recurrent intracoronary platelet activation and thrombus formation were induced by damaging the endothelium and placing a cylindrical constrictor on the left anterior descending coronary artery (LAD) in open-chest, anesthetized dogs. Coronary diameters were measured in vivo by means of ultrasonic crystals sutured on the LAD immediately distal to the constrictor (LADI) and 1 cm below (LAD2) and on the circumflex coronary artery (Cx). Coronary artery diastolic diameters were measured continuously before and during cyclic flow variations and after they were abolished by administration of LY53857, a serotonin-receptor antagonist (group 1, n =7), or SQ29548, a thromboxane-receptor antagonist (group 2, n = 7). During cyclic flow variations, at the nadir of coronary flow, LADI (a site of maximal platelet accumulation) cross-sectional area decreased by 52±10% and 38±6% in group 1 and 2 animals, respectively (p <0.001 compared with values recorded during a brief LAD occlusion obtained by a suture snare), whereas LAD2 (a site of minimal or no platelet accumulation) cross-sectional area did not differ from that recorded during the brief LAD occlusion. SQ29548 abolished cyclic flow variations in seven of seven dogs and LY53857 in six of seven, but they affected the increased coronary vasoconstriction differently: LADI cross-sectional area increased by 32±N6% of the control value in SQ29548-treated animals, whereas it returned to baseline dimension values in the LY53857-treated group as these interventions also abolished the cyclic flow variations. We conclude that a marked coronary vasoconstriction may be triggered by local platelet deposition and that thromboxane A2 and serotonin are mediators of this vasoconstriction. (Circulation 1989;79:154-166) It is currently believed that the pathophysiology of unstable angina is a primary reduction in coronary blood flow.

Cyclic variations in coronary blood flow (CFVs) in dogs with experimental coronary artery stenosis and endothelial injury appear to result primarily from the aggregation of platelets at the site of stenosis followed by dislodgement and distal embolization. Using this canine model, we tested the hypotheses: (a) that thrombin is an important mediator of CFVs in dogs with coronary stenoses and endothelial injury; (b) that inhibition of thrombin with heparin, or MCI-9038, a selective thrombin inhibitor, abolishes CFVs in this model; and (c) that abolition of CFVs by thrombin inhibition is time dependent. CFVs, produced in open-chest dogs by placing a flow-reducing plastic constrictor around the left anterior coronary artery, were monitored for either 30 min (group I) or 3 h (group II) before treatment with either heparin or 4-methyl-14N'-+(3-methyl-1,2,3,4-tetrahydro-8-quinolinyl

Human recombinant tissue-type plasminogen activator (rt-PA) has been shown to be an effective and safe agent for coronary thrombolysis in patients with acute myocardial infarction. However, thrombolysis is associated with a high rate of acute reocclusion after discontinuation of rt-PA. The goals of the present study were to assess whether reocclusion after thrombolysis is caused by intracoronary platelet aggregation and to determine the role of thromboxane A2 (TxA2) No. 3, 678-684, 1988. CORONARY THROMBOLYSIS has recently received great attention for its potential reduction of the1 extent of necrosis after myocardial ischemia. Thrombolysis can be accomplished by infusion of streptoki-

Platelets possess alpha 2-adrenergic and serotonergic (5-hydroxytryptamine) receptors which are thought to mediate the in vitro proaggregatory effects of epinephrine and serotonin, respectively. However, their importance in platelet aggregation in vivo is uncertain. In the present study, we evaluate the ability of yohimbine and ketanserin, relatively selective alpha 2-adrenergic and serotonin antagonists, respectively, to alter cyclic flow reductions in stenosed coronary arteries in open-chest, anesthetized dogs. These cyclic flow reductions, characterized by progressive declines in coronary blood flow interrupted by abrupt and, often spontaneous, restorations of flow, were produced by cylindrical constrictors placed on the left anterior descending coronary artery. A pulsed Doppler flow probe, placed proximal to the constrictor, was used to measure coronary blood flow. Regional myocardial blood flow was measured with 15-micron radiolabeled microspheres before coronary constriction and when coronary blood flow appeared to be at its nadir and zenith during cyclic flow reductions. After the cyclic flow reductions had been observed for 1 hour, yohimbine (1-2 mg/kg), ketanserin (0.25 or 0.5 mg/kg), or saline was given, and coronary blood flow and hemodynamics were monitored for another hour. The frequency of cyclic flow reductions and the mean of the three lowest nadirs of coronary blood flow (mean +/- SE) were compared between the first and second hours. Ketanserin, at doses of 0.25 and 0.50 mg/kg, virtually abolished cyclic flow reductions in all dogs tested. Yohimbine [1 mg/kg ( n = 14)] was partially effective in reducing the frequency (9.6 vs. 5.5 cyclic flow reductions/hr) and severity of cyclic flow reductions (nadirs of coronary blood flow = 6.2 +/- 2.4 vs. 20.9 +/- 6.1% of control). A higher dose of yohimbine [2 mg/kg (n = 7)] was no more effective. The frequency (9.3 +/- 0.9 vs. 9.3 +/- 1.0 CFR/hr) and severity (17.4 +/- 5.4 vs. 12.4 +/- 3.9% of control coronary blood flow) of cyclic flow reductions were not changed by saline. The relatively selective alpha 1-adrenergic antagonist, prazosin (0.01 mg/kg, iv), and the beta-adrenergic antagonist, propranolol (1-2 mg/kg, iv), did not affect the frequency or severity of cyclic flow reductions. Thus, the abilities of yohimbine to inhibit and ketanserin to abolish cyclic flow reductions in stenosed canine coronary arteries suggest that serotonin and, possibly, alpha 2-adrenergic agonists may influence cyclic flow alterations importantly in this model.