The function of the vascular endothelium after storage at room temperature (24 degrees Celsius) for four, eight, and twenty-four hours was investigated with use of an ex vivo canine tibial perfusion model. Function was assessed in terms of changes in perfusion pressure and changes in the concentration of endothelin-1 in the venous effluent of the perfused tibiae. Endothelin-1 is a potent vasoconstrictor that is produced in low concentrations by normal endothelial cells and in increased concentrations by injured vascular endothelial cells.The mean perfusion pressures at flow rates of 1.0 and 1.5 milliliters per minute were significantly higher in the tibiae that had been stored for eight hours than in the tibiae that had been stored for four hours (p < 0.05), and they were significantly higher in the tibiae that had been stored for twenty-four hours than in the tibiae that had been stored for four or eight hours (p < 0.05). The increase in perfusion pressure with increasing duration of storage was associated with an increase in production of endothelin-1. The production of endothelin-1 in the tibiae that had been stored for eight hours (10.6 ± 0.46 picograms per milliliter) was approximately ten times greater than that in the tibiae that had been stored for four hours (1.1 ± 0.29 picograms per milliliter). The tibiae that had been stored for twenty-four hours had 19.1 ± 1.5 picograms of endothelin-1 per milliliter, nearly twice that produced in the tibiae that had been stored for eight hours.Injection of acetylcholine demonstrated muscarinic receptor-mediated vasodilation in the tibiae that had been stored for four hours. In contrast, the tibiae that had been stored for eight and twenty-four hours had no evidence of acetylcholine-induced vasodilation of baseline perfusion vascular smooth-muscle tone. However, there was some preservation of endothelium-dependent vascular smooth-muscle relaxation in the tibiae that had been stored for eight and twenty-four hours, as norepinephrine-induced vascular smooth-muscle contraction was significantly greater in the presence of NG-monomethyl-L-arginine acetate (p < 0.05). Moreover, in the second phase of the study, a bolus injection of calcium ionophore A23187 in tibiae that had been stored for twenty-four hours relaxed vascular smooth muscle.Adrenomedullin, a novel peptide with known vasodilator properties, relaxed vascular smooth muscle in all three groups and also attenuated the pressor response to norepinephrine.In conclusion, the function of the vascular endothelium was impaired after storage at room temperature for four hours. However, the vascular endothelium in the tibiae that had been stored for twenty-four hours maintained some function with regard to the production of nitric oxide. The effect of adrenomedullin as a potent vasodilator was observed in the tibiae that had been stored for four, eight, and twenty-four hours.CLINICAL RELEVANCE: In the clinical setting, some replantation and microvascular free-tissue transfer procedures may involve ischemic periods of more than four hours. On rare occasions, this period may be as long as twenty-four hours in the absence of hypothermic cooling, particularly in instances of operative re-exploration after thrombosis of a site of vascular anastomosis with microvascular bone transfer. An understanding of time-dependent changes in the function of the vascular endothelium of bone with warm ischemia is important. We have demonstrated that adrenomedullin, a relatively newly identified and potent vasodilator peptide, may have value for the mitigation of these ischemic changes.