J H Ashton

The distribution of a neutral metalloendopeptidase (NEP), or "enkephalinase," in human lung tissue and cultured cells was compared with that of angiotensin I converting enzyme (ACE). The specific activities of NEP and ACE were measured in homogenates of fetal lung tissue and in isolated airways and pulmonary vessels. NEP activity was highest in airway tissue, and ACE activity was highest in isolated vessels. Human endothelial cells from either umbilical veins or pulmonary arteries had high ACE activity (80 to 90 nmol/h/10(6) cells) but only a trace of NEP activity (0.5 to 0.6 nmol/h/10(6) cells). Fibroblasts cultured from human lungs were low in ACE but richer in NEP than cultured endothelial cells. Fibroblasts from human foreskins or caesarean section skin were the richest source of NEP activity (60 to 80 nmol/h/10(6) cells). Immunohistochemical studies confirmed the biochemical assays. As expected, ACE was localized on the luminal surface of blood vessels, with a distribution similar to that of factor VIII antigen, an endothelial marker. In contrast, NEP was localized within the alveolar septa. Cultured endothelial cells stained only weakly for NEP in contrast to cultured fibroblasts. The location of these 2 enzymes in different cells and the differences in peptide substrate specificity suggests that they act sequentially on circulating peptides or those released within microvascular beds.

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-1-(N2-[(3-methyl-1,2,3,4-tetrahydro-8-quinolinyl (MCI-9038). In group I, cyclic flow variations were abolished by heparin in 12 of 18 dogs and by MCI-9038 in 5 of 7 dogs. In group II, cyclic flow variations were not abolished by heparin in any of seven dogs and were abolished by MCI-9038 in only one of seven dogs. Thus, (a) thrombin appears to be an important mediator of cyclic flow variations in dogs with coronary artery stenosis and endothelial injury and (b) inhibition of thrombin abolishes CFVs after short but not prolonged periods of CFVs.

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 (LAD1) 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, LAD1 (a site of maximal platelet accumulation) cross-sectional area decreased by 52 +/- 10% and 38 +/- 6% in group 1 and 2 animals, respectively (p less than 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: LAD1 cross-sectional area increased by 32 +/- 6% 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.

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.