AJP - Lung Cellular and Molecular Physiology, Vol 273, Issue 5 997-1001, Copyright © 1997 by American Physiological Society

ARTICLES

Mechanism of capsaicin-induced relaxation in equine tracheal smooth muscle

F. X. Zhu, X. Y. Zhang, M. A. Olszewski and N. E. Robinson
Department of Large Animal Clinical Sciences, Michigan State University, East Lansing 48824-1314, USA.

The effects of capsaicin and neuropeptides were examined in equine tracheal smooth muscle (TSM). Neither capsaicin nor substance P (SP) contracted TSM. Capsaicin (100 microM) elicited relaxation in TSM contracted with methacholine. This relaxation was not mimicked by SP or calcitonin gene-related peptide (CGRP). Relaxation was not attenuated by removal of the epithelium or by pretreatment of tissue with meclofenamate or the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine. Previous exposure of TSM to capsaicin did not eliminate the relaxation responses to subsequent capsaicin. Although vasoactive intestinal peptide (VIP) elicited marked relaxation that was attenuated by alpha-chymotrypsin, alpha-chymotrypsin did not affect the capsaicin-induced relaxation. Capsaicin-induced relaxation was abolished by charybdotoxin, a blocker of large-conductance Ca(2+)-activated K+ channels. These results indicate that capsaicin-induced equine TSM relaxation is not mediated either by neuropeptides such as SP or CGRP released from capsaicin-sensitive sensory nerves or by prostanoids, NO, or VIP. Relaxation is due to the effect of capsaicin on large-conductance Ca(2+)-activated K+ channels. The peptidergic nerves play no important role in the regulation of TSM tone in horse airways.

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