Nitric oxide inhibition attenuates systemic hypotension produced by protamine

Background: Protamine reversal of heparin anticoagulation often causes systemic hypotension, and in vitro studies suggest that this may be mediated by release of nitric oxide from the endothelium. The present investigations were designed to evaluate the direct myocardial effects of protamine and to determine in vivo whether nitric oxide inhibition can prevent hypotension during protamine infusion. Methods/Results: Protamine sulfate (50 μg/ml) was added to perfusate of eight isolated rabbit heart preparations; in six other preparations, a similar concentration of protamine was added to heparinized (5 U/ml) Krebs perfusate. Left ventricular developed pressure, maximum rate of pressure rise, and heart rate declined significantly (p < 0.01) in hearts exposed to protamine only (65.0% ± 6.6%, 55.5% ± 6.0%, and 87.6% ± 2.5% of baseline, respectively), whereas protamine added to heparinized perfusate caused little change in developed pressure, maximum rate of pressure rise, and heart rate (85.3% ± 5.4%, 84.9% ± 5.5%, and 98.8% ± 1.6%). To study systemic effects of protamine, we measured hemodynamic parameters in 12 heparinized dogs 1150 U/kg). During protamine infusion (1.5 mg/kg intravenously over 30 seconds), mean blood pressure decreased by 46% ± 7% from baseline (p < 0.05), cardiac output decreased by 38% ± 4% (p < 0.05), and systemic vascular resistance decreased by 14% ± 9%. After hemodynamic stabilization, N²-monomethyl-L-arginine (2 mg/kg), a competitive inhibitor of nitric oxide synthesis, was administered to six dogs, and methylene blue (2 mg/kg), an inhibitor of cyclic guanosine monophosphate synthesis, was administered to the remaining six dogs. After treatment with N(g)-monomethyl- L-arginine and methylene blue, the second infusion of protamine sulfate caused no significant change in blood pressure or cardiac output. In an additional six dogs, N(g)-monomethyl-L-arginine pretreatment (5 mg/kg) blocked the effects of the first dose of protamine. The effect of N(g)- monomethyl-L-arginine could be reversed by the addition of (6 mg/kg) L- arginine but not D-arginine. Conclusions: Protamine-heparin complex does not cause direct myocardial depression but does lead to severe hypotension in vivo. The finding that hypotension can be blocked by inhibitors of the nitric oxide pathway confirms previous in vitro studies indicating that the effects of protamine are mediated, in part, by the vascular endothelium. Further, these studies suggest a novel approach to prevention of hemodynamic complications caused by heparin reversal after cardiopulmonary bypass.