EFFECT OF THE DURATION OF ROOM-TEMPERATURE ISCHEMIA ON FUNCTION OF THE VASCULAR ENDOTHELIUM - THE ROLE OF ADRENOMEDULLIN IN REPERFUSION INJURY

The function of the vascular endothelium after storage at room tempera ture (24 degrees Celsius) for four, eight, and twenty-four hours was i nvestigated with use of an ex vivo canine tibial perfusion model. Func tion was assessed in terms of changes in perfusion pressure and change s in the concentration of endothelin-1 in the venous effluent of the p erfused tibiae. Endothelin-1 is a potent vasoconstrictor that is produ ced in low concentrations by normal endothelial cells and in increased concentrations by injured vascular endothelial cells. The mean perfus ion 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 hou rs 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 in creasing duration of storage was associated,vith an increase in produc tion of endothelin-l. The production of endothelin-1 in the tibiae tha t had been stored for eight hours (10.6 +/- 0.16 picograms per millili ter) 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 produce d in the tibiae that had been stored for eight hours. Injection of ace tylcholine demonstrated muscarinic receptor-mediated vasodilation in t he 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 s mooth-muscle tone. However, there was some preservation of endothelium -dependent vascular smooth-muscle relaxation in the tibiae that had be en stored for eight and twenty-four hours, as norepinephrine-induced v ascular smooth-muscle contraction was significantly greater in the pre sence of N-G-monomethyl-L-arginine acetate (p <0.05). Moreover, in the second phase of the study, a bolus injection of calcium ionophore A23 187 in tibiae that had been stored for twenty-four hours relaxed vascu lar smooth muscle. Adrenomedullin, a novel peptide with known vasodila tor properties, relaxed vascular smooth muscle in all three groups and also attenuated the presser 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 so me function with regard to the production of nitric oxide. The effect of adrenomedullin as a potent vasodilator was observed in the tibiae t hat had been stored for four, eight, and twenty-four hours. CLINICAL R ELEVANCE: 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 twen ty-four hours in the absence of hypothermic cooling, particularly in i nstances of operative re-exploration after thrombosis of a site of vas cular anastomosis with microvascular bone transfer. An understanding o f time-dependent changes in the function of the vascular endothelium o f bone with warm ischemia is important. We have demonstrated that adre nomedullin, a relatively newly identified and potent vasodilator pepti de, may have value for the mitigation of these ischemic changes.