Peptidase modulation of vasoactive intestinal peptide pulmonary relaxation in tracheal superfused guinea pig lungs

The effects of enzyme inhibitors on vasoactive intestinal peptide (VIP)-induced decreases in airway opening pressure (PaO) and VIP-like immunoreactivity (VIP-LI) recovery were studied in isolated tracheal superfused guinea pig lungs. In the absence of inhibitors, VIP 0.38 (95% CI 0.33-0.54) nmol/kg animal, resulted in a 50% decrease in PaO and 33% of a 1 nmol/kg VIP dose was recovered as intact VIP. In the presence of two combinations of enzyme inhibitors, SCH 32615 (S, 10 microM) and aprotinin (A, 500 tyrpsin inhibitor units [TIU]/kg) or S and soybean trypsin inhibitor (T, 500 TIU/kg), VIP caused a significantly greater decrease in PaO and greater quantities of VIP were recovered from lung effluent (both P < 0.001). The addition of captopril, (3 microM), leupeptin (4 microM), or bestatin (1 microM) failed to further increase pulmonary relaxation or recovery of VIP-LI. When given singly, A, T, and S did not augment the effects or recovery of VIP. The efficacy of S (a specific inhibitor of neutral endopeptidase [NEP]) and A and T (serine protease inhibitors) thus implicated NEP and at least one serine protease as primary modulators of VIP activity in the guinea pig lung. We sought to corroborate this finding by characterizing the predominant amino acid sites at which VIP is hydrolized in the lung. When [mono(125I)iodo-Tyr10]VIP was offered to the lung, in the presence and absence of the active inhibitors, cleavage products consistent with activity by NEP and a tryptic enzyme were recovered. These data demonstrate that NEP and a peptidase with an inhibitor profile and cleavage pattern compatible with a tryptic enzyme inactivate VIP in a physiologically competitive manner.