Previous in vivo studies have shown that vasopressin, which releases the endothelium-derived relaxing factor and constricts coronary smooth muscle, produces augmented constriction of coronary microvessels perfused by mature collaterals. We hypothesized that chronic perfusion through collaterals produces endothelial dysfunction in the recipient vasculature. Mature collaterals were stimulated in mongrel dogs by the ameroid constrictor technique. After 3-6 months, rings of conduit vessels (obtuse marginals) were studied in organ chambers, and coronary microvessels (100-220 microns) were studied in a pressurized, no-flow state with a microvessel imaging apparatus. Eleven dogs were used as controls. Large vessels were preconstricted with prostaglandin F2 alpha to 30-70% of the maximum potassium chloride tension, and microvessels were preconstricted to 20-60% of the baseline diameter with the thromboxane mimetic U46619. Relaxations to the receptor-mediated agents acetylcholine and ADP were markedly impaired in collateral-dependent coronary microvessels, whereas relaxations to nitroglycerin were enhanced compared with microvessels from control dogs. Relaxation to the calcium ionophore A23187, which releases the endothelium-derived relaxing factor through nonreceptor-mediated mechanisms, were similar in control and ameroid microvessels. Constriction to vasopressin was augmented in collateral-dependent microvessels compared with controls. Responses to all agonists were similar between control and collateral-dependent large vascular rings. In conclusion, chronic perfusion through collateral vessels selectively impairs receptor-mediated endothelium-dependent relaxations and augments constriction to vasopressin in the coronary microcirculation. These findings may have important implications regarding neurohumoral regulation of perfusion to collateral-dependent myocardium.