Protease inhibitors potentiate smooth muscle relaxation induced by vasoactive intestinal peptide in isolated human bronchi.

To examine the role of endogenous proteases in limiting the bronchodilating effects of vasoactive intestinal peptide (VIP) in human airway, we studied precontracted bronchial rings from five nonsmokers undergoing heart-lung transplantation for pulmonary hypertension, either primary or secondary to congenital heart disease. The protease inhibitors aprotinin, leupeptin, phosphoramidon, and soybean trypsin inhibitors significantly potentiated the bronchodilator response to VIP. Even in the presence of the four protease inhibitors, VIP-induced bronchodilation reversed spontaneously in some tissues. These studies show that degradation by endogenous airway proteases is an important determinant of the bronchodilating potency of VIP in isolated human airway.