Muscarinic excitation-contraction coupling mechanisms in tracheal and bronchial smooth muscles

We investigated the mechanisms underlying muscarinic excitation-contraction coupling in canine airway smooth muscle using organ bath, fura 2 fluorimet ric, and patch-clamp techniques. Cyclopiazonic acid (CPA) augmented the res ponses to submaximal muscarinic stimulation in both tracheal (TSM) and bron chial smooth muscles (BSM), consistent with disruption of the barrier funct ion of the sarcoplasmic reticulum. During maximal stimulation, however, CPA evoked substantial relaxation in TSM but not BSM. CPA reversal of carbacho l tone persisted in the presence of tetraethylammoium or high KCl, suggesti ng that hyperpolarization is not involved; CPA relaxations were absent in t issues preconstricted with KCl alone or by permeabilization with beta -esci n, ruling out a nonspecific effect on the contractile apparatus. Peak contr actions were sensitive to inhibitors of tyrosine kinase (genistein) or Rho kinase (Y-27632). Sustained responses were dependent on Ca2+ influx in TSM but not BSM; this influx was sensitive to Ni2+ but not La3+. In conclusion, there are several mechanisms underlying excitation-contraction coupling in airway smooth muscle, the relative importance of which varies depending on tissue and degree of stimulation.