Pearson, Paul J.

To study the effect of hypoxia on vascular tone in human internal mammary artery (IMA), segments of IMA were suspended in organ chambers and contracted with norepinephrine (at a dose producing 30% of maximal contraction). Exposure of IMA segments with endothelium to hypoxia (partial pressure of oxygen, 38 +/- 4 mmHg) resulted in a transient relaxation (47 +/- 6%) followed by constriction (177 +/- 8%) (n = 14). IMA segments without endothelium exhibited a gradual decrease in tension that almost completely counteracted vasoconstriction. The initial transient endothelium-dependent relaxation could be blocked by indomethacin (10(-6)M) and was associated with a 51% increase in 6-ketoprostaglandin F1 alpha production (n = 22, P < 0.05). The endothelium-dependent contraction to hypoxia could be attenuated by indomethacin (n = 6, P < 0.05) or NG-monomethyl-L-arginine (10(-5)M; n = 9, P < 0.05) and was completely blocked by combination of these agonists (n = 6, P < 0.05). These experiments indicate that on exposure to hypoxia, the human IMA exhibits an initial prostacyclin-mediated relaxation followed by contraction due to the production of a constrictor prostanoid in addition to the inhibition of basal production of endothelium-derived relaxing factor.

Experiments were designed to determine whether endothelial injury contributes to augmented coronary vascular tone seen during myocardial reperfusion. Canine left anterior descending coronary arteries were exposed to ischemia followed by reperfusion (60 minutes each). Rings (3-4 mm) of the reperfused artery and of normal left circumflex (control) coronary artery segments were prepared. Rings were suspended for isometric force measurement in organ chambers containing modified Krebs' Ringer bicarbonate solution (37 degrees C, 95% O2-5% CO2). Endothelium-independent contractions to KCl and prostaglandin F2 alpha were unaltered after reperfusion. Endothelium-dependent relaxations to nitric oxide, sodium nitroprusside, and isoproterenol were comparable in control and reperfused arteries. However, reperfused coronary arteries contracted with prostaglandin F2 alpha lost the ability to express endothelium-dependent relaxations to aggregating platelets. Reperfused arterial rings also exhibited impaired endothelium-dependent relaxations to acetylcholine, the calcium ionophore A23187, and the platelet-derived compounds ADP and serotonin. Quiescent (noncontracted) reperfused arterial rings exhibited larger contractions than controls when exposed to aggregating platelets. In such quiescent rings, the endothelium-dependent increase in tension to hemoglobin was unaltered after reperfusion. Thus, coronary reperfusion impairs the normal endothelium-dependent relaxations to aggregating platelets and vasoactive drugs. This impairment of platelet-mediated coronary relaxation could help explain the increased vascular tone and tendency toward vasospasm commonly observed after reperfusion of the coronary arteries.

BACKGROUND When used to reverse the anticoagulant effect of heparin, protamine sulfate often causes vasodilation that can lead to systemic hypotension. Protamine is rich in the basic amino acid arginine, which is the precursor of endothelial cell synthesis of nitric oxide, and nitric oxide is the active component of endothelium-derived relaxing factor (EDRF). METHODS AND RESULTS To determine whether the hypotensive effect of protamine could be due to stimulated release of EDRF, we studied rings (4-5 mm) of canine coronary, femoral, and renal artery suspended in organ chambers containing physiological salt solution (37 degrees C and 95% O2-5% CO2). Arterial rings with and without endothelium were contracted with prostaglandin F2 alpha (2 x 10(-6) M) and exposed to increasing concentrations of protamine (final organ bath concentration, 40-400 micrograms/ml). In arterial segments without endothelium, protamine caused only a modest decrease in tension. However, protamine induced concentration-dependent relaxation in all arterial segments with endothelium, which was significantly greater than in segments without endothelium (p less than 0.05). The endothelium-dependent relaxation induced by protamine was inhibited by NG-monomethyl-L-arginine (L-NMMA) (10(-5) M), but L-NMMA had no effect on rings without endothelium. The action of L-NMMA could be reversed by L-arginine (10(-4) M) but not D-arginine (10(-4) M). CONCLUSIONS This study demonstrates that protamine stimulates the release of EDRF from arterial endothelium, and that endothelium-dependent vasodilation may be an important cause of systemic hypotension during protamine infusion.

Experiments were designed and performed to determine whether endothelial function remained chronically impaired after coronary artery reperfusion. Canine left anterior descending coronary arteries were exposed to ischemia (60 minutes) followed by reperfusion (12 weeks). Rings (3-4 mm wide) of the reperfused artery and of normal left circumflex (control) coronary artery segments were suspended in organ chambers containing physiological saline solution (37 degrees C, gassed with 95% O2-5% CO2) for isometric force measurement. Endothelium-independent contractions to KCl or prostaglandin F2 alpha and endothelium-independent relaxations to nitric oxide or isoproterenol were comparable in control and chronically reperfused arteries. However, chronically reperfused coronary arteries exhibited impaired endothelium-dependent relaxations to aggregating platelets. In addition, the reperfused coronary arteries exhibited impaired endothelium-dependent relaxations to the platelet-derived compounds adenosine diphosphate, serotonin, and thrombin. However, the endothelium-dependent relaxations to acetylcholine were comparable between control and reperfused arteries. Thus, after 12 weeks of reperfusion, previously occluded coronary arteries exhibited a selective impairment of endothelium-dependent relaxation evoked by aggregating platelets. In vivo, this phenomenon could favor platelet adhesion, aggregation, and platelet-induced contraction of coronary smooth muscle and thus facilitate ischemic events such as vasospasm and coronary thrombosis.