Neutrophil activation mediates protamine-induced pulmonary hypertension

Protamine reversal of heparin anticoagulation can cause catastrophic pulmonary hypertension, systemic hypotension, and hypoxemia. This reaction is thought to be associated with pulmonary sequestration of activated neutrophils. To examine whether this reaction could be prevented by blocking neutrophil activation with NPC 15669 (N-(9H-(2,7-dimethylfluorenyl-9-methoxy)carbonyl)-l-leucine), an inhibitor of Mac-1 adhesion molecule up-regulation, we gave 12 piglets a heparin bolus (300 IU/kg iv) followed by protamine (3 mg/kg iv over 90 sec) 15 min later. Six of the piglets received NPC 15669 (10 mg/kg iv bolus, then 6 mg/kg/hr iv infusion) 15 min before the heparin bolus. Two minutes after protamine administration, CD18 (β-sub-unit of Mac-1) expression measured by immunofluorescence flow cytometry increased 128 ± 9% (mean ± SEM) over preprotamine levels in control piglets but remained unchanged in NPC piglets (99 ± 2%, P < 0.05). Fifteen minutes after protamine, lung myeloperoxidase activity, an index of neutrophil degranulation, was significantly lower in the NPC group (87.83 ± 10.04 versus 57.23 ± 2.59 μmol/10 mg; P < 0.005). NPC 15669 also prevented postreversal pulmonary artery hypertension (158 ± 30, 150 ± 20, and 140 ± 13 versus 91 ± 7, 91 ± 18, and 85 ± 9% preprotamine at Minutes 2, 5, and 10; P < 0.05). Systemic arterial pressure, cardiac output, and circulating neutrophil counts were not different between groups. These results demonstrate that (1) neutrophil activation occurs following protamine reversal of heparin anticoagulation and (2) inhibition of neutrophil activation prevents pulmonary hypertension after protamine reversal, suggesting an important role for neutrophils in the adverse pulmonary hypertensive reactions related to protamine reversal of heparin.