Effects of neuropeptides and capsaicin on tracheobronchial blood flow of the pig.

Blood flow changes upon systemic i.v. injections in the pig of various neuropeptides, capsaicin, bradykinin and histamine were directly monitored by a Transonic blood flowmeter in the superior laryngeal, bronchial and femoral arteries and indirectly in the larynx and skin using laser Doppler flowmetry. To minimize influence of compensatory reflexes and indirect effects, the pigs were pre-treated with atropine, guanethidine, chlorisondamine and capsaicin. Substance P (SP), vasoactive intestinal polypeptide (VIP), peptide histidine isoleucine (PHI), calcitonin gene-related peptide (CGRP), capsaicin, bradykinin and histamine all decreased vascular resistance, suggesting vasodilation in the superior laryngeal and bronchial arteries. All peptides and histamine when given i.v. exerted vasodilatory effects independent of autonomic motor nerves and capsaicin-sensitive afferents. SP was the most potent vasodilator agent tested in both tracheal and bronchial circulation, being about 1000-fold more active than histamine. VIP was about 10-fold more potent than PHI in decreasing vascular resistance and had a preferential action on the SLA compared to CGRP. In the femoral artery capsaicin and also SP in the highest dose increased vascular resistance. Capsaicin increased the laser Doppler signal in both laryngeal mucosa and skin, while i.v. peptides caused variable effects. In conclusion, SP and CGRP mimicked capsaicin-induced vasodilation in the tracheobronchial circulation while VIP had a preferential effect on the tracheal circulation.