Carboxypeptidase M-like Enzyme Modulates the Noncholinergic Bronchoconstrictor Response in Guinea Pig

We studied the effects of aerosolized DL-2-mercaptomethyl-3-guanidino-ethylthiopropanoic acid (MGTA) (10−4 M, 90 breaths), a specific inhibitor of carboxypeptidase B-type enzymes, on changes in total pulmonary resistance (Rl) induced by aerosolized capsaicin (10−7 to 10−4 M; 10 breaths at each concentration) and vagus nerve stimulation (5 V, 5 ms, for 20 s at frequencies varying from 2 to 10 Hz) in anesthetized, atropinized, and ventilated guinea pigs. We also studied the effect of aerosolized MGTA on the bronchoconstrictor response to either aerosolized substance P, neurokinin A (10−7 to 10−4 M; 10 breaths at each concentration), and carbachol (10−5 to 2 × 10−4 M; 10 breaths at each concentration) or to i.v. administration of neurokinin A (10−11 to 10−8 mol/kg), bradykinin (10−10 to 10−7 mol/kg), and histamine (10−8 to 10−6 mol/kg). Although aerosolized MGTA caused no change in basal Rl (P > 0.5), it did potentiate the noncholinergic bronchoconstrictor response to capsaicin (n = 5; P < 0.001) as well as to vagus nerve stimulation (n = 5; P = 0.001). In contrast, MGTA did not potentiate the bronchoconstrictor response to either aerosolized substance P, neurokinin A, and carbachol or to i.v. administration of neurokinin A, histamine, and bradykinin. Carboxypeptidase activity cleaving C-terminal arginine or lysine was found in the membrane preparations of trachea and lung from guinea pigs. The membrane-bound carboxypeptidase activity was maximal at pH 7.0 and was enhanced by the presence of CoCl2 (1 mM) in both the tracheal and lung tissue. In the membrane preparation from the trachea, the carboxypeptidase activity was a carboxypeptidase M (CPM)-type enzyme that cleaved the substrate Bz-Gly-Arg 2.0-fold faster than Bz-Gly-Lys, whereas in the lung it was a carboxypeptidase N-type peptidase that cleaved the substrate Bz-Gly-Lys 2.5-fold faster than Bz-Gly-Arg. We conclude that in guinea pigs a membrane-bound CPM-like enzyme exists in the airway and modulates the noncholinergic bronchoconstrictor response to capsaicin and to vagus nerve stimulation. The effect of CPM-like peptidase on noncholinergic response does not appear to be mediated by direct action of the enzyme on substance P, neurokinin A, or bradykinin.