Function of the Vascular Endothelium after Hypothermic Storage at Four Degrees Celsius in a Canine Tibial Perfusion Model. The Role of Adrenomedullin in Reperfusion Injury

The function of the vascular endothelium after cold storage at 4 degrees Celsius for one, three, five, and seven days was investigated in a canine tibial perfusion model. Function was assessed in terms of changes in perfusion pressure, changes in the concentration of endothelin in the venous effluent from the perfused tibiae, adrenomedullin-induced vascular smooth-muscle relaxation, and norepinephrine-induced pressor responses in the presence of acetylcholine, NG-monomethyl-L-arginine acetate (an inhibitor of nitric oxide synthesis), or indomethacin (an inhibitor of prostaglandin synthesis) in phase 1 of the study. In phase 2 of the study, the effect of the infusion of tetraethylammonium (a potassium-channel blocker that inhibits the activity of endothelium-derived hyperpolarized factor) was analyzed. The baseline perfusion pressures increased in a time-dependent manner (p < 0.05). In tibiae that had been stored for one or three days, the production of endothelin-1 was less than one picogram per milliliter, but it markedly increased to a mean (and standard error of the mean) of 8.7 ± 3.2 and 10.8 ± 4.3 picograms per milliliter in tibiae that had been stored for five and seven days, respectively (p < 0.05). Acetylcholine attenuated the norepinephrine-induced pressor response in all groups (storage at 4 degrees Celsius for one, three, five, or seven days) compared with the response in the control tibiae (p < 0.05). Perfusion of acetylcholine in the tibiae that had been stored for three days significantly attenuated the pressor response to norepinephrine compared with that in the tibiae that had been stored for five days (p < 0.05). In the presence of NG-monomethyl-L-arginine acetate, the norepinephrine-induced pressor response significantly increased only in the tibiae that had been stored for one day (p < 0.05). In the presence of indomethacin, the norepinephrine-induced pressor response significantly decreased in the tibiae that had been stored at 4 degrees Celsius for one, three, or five days (p < 0.05). Infusion of adrenomedullin relaxed vascular smooth muscle in the tibiae that had been stored for one, three, five, or seven days (p < 0.05). In phase 2 of the study, perfusion of tetraethylammonium in the presence of acetylcholine increased the norepinephrine-induced pressor response in the tibiae that had been stored at 4 degrees Celsius for seven days to a mean of 168 ± 20 per cent, whereas perfusion with acetylcholine alone attenuated the norepinephrine-induced pressor response to a mean of 54.6 ± 3.7 per cent. CLINICAL RELEVANCE: The use of vascularized bone allografts in musculoskeletal reconstructive operations theoretically has many biological and biomechanical advantages. However, unless prolonged preservation is possible, the practical clinical applications of vascularized bone allografts may be extremely limited. The intraosseous endothelium and vascular smooth-muscle cells are especially susceptible to ischemic injury. Maintenance of the viability of the vascular endothelium and the smooth-muscle cells may be the most critical factor that determines whether the revascularization of whole-bone segments is successful. Identification of the optimum conditions for the storage of vascularized autogenous bone grafts or allografts that preserve the integrity of the intraosseous endothelium and the smooth muscle is a prerequisite for the successful transfer of vascularized bone allografts.