Decreased endothelium-dependent pulmonary vasodilator effect of calcitonin gene-related peptide in hypoxic rats contrasts with increased binding sites

Levels of calcitonin gene-related peptide (CGRP), a vasodilator peg tide present in nerves and airway endocrine cells of the rat respiratory tract, are increased in hypoxic lung and decreased in plasma, suggesting impaired CGRP release. We wanted to determine whether there was an adaptive functional response to reduced CGRP levels in hypoxia. Density of binding sites for CGRP were compared with its vascular actions following hypoxia, and with binding following administration of the sensory neurotoxin capsaicin to deplete neural CGRP. Autoradiography of lung sections incubated with (125)l-labelled CGRP and other vasoactive peptides was used to quantify their binding sites, in male Wistar rats exposed to periods of hypoxia (inspiratory oxygen fraction (FI,O-2) = 0.1) ranging 0-10 days (n=5 each), in controls, and in rats treated neonatally with capsaicin. Relaxation to CGRP was compared in pulmonary artery of control and hypoxic rats. CGRP binding was seen in the vascular endothelium and was significantly elevated after 5 days of hypoxia (mean +/- SEM: control 4.6 +/- 0.4 versus hypoxic 16.6 +/- 2.4 amol . mm(-2)). CGRP-induced (5 x 10(-7) M) relaxation of pulmonary artery was reduced, compared with controls, following 8 and 21 days of hypoxia (mean +/- SEM) percentage of relaxation to phenylephrine: 78 +/- 3, 36 +/- 5 and 32 +/- 3, respectively) and was abolished by removal of endothelium. Capsaicin treatment also significantly elevated vascular CGRP binding. Atrial natriuretic peptide (ANP) binding levels were decreased in smooth muscle of all blood vessels after 7 days of hypoxia, but endothelin-1 (ET-1) and vasoactive intestinal peptide (VIP) binding was unchanged. We conclude that the vasodilator effects of CGRP are endothelium-dependent and, whilst they are reduced in hypoxic lung, this is not due to reduction in receptors, thereby implicating alterations in the nitric oxide guanylyl cyclase system. Furthermore, adaptive responses in some peptide binding sites occur in hypoxia, which may be due to changes in endogenous peptide levels.