Effect of vagal stimulation and parenteral acetylcholine on canine trachealis muscle shortening

Abstract

Canine trachealis smooth muscle shortening (TMS) in response to vagal nerve stimulation is approximately 30%, far less than the 70% predicted from in vitro studies. We hypothesized that in vivo airway smooth muscle activation during vagal stimulation may be submaximal, and in this study we wished to determine TMS during maximal activation. TMS was studied in 12 alpha-chloralose-anesthetized dogs during vagal stimulation, systemic acetylcholine injection, and local acetylcholine injection. Bilateral vagal stimulation produced TMS of 26 +/- 5% (SE) length at functional residual capacity (LFRC). Maximal TMS during systemic injection of acetylcholine was 28 +/- 12% LFRC but may have been limited by delivery of acetylcholine to the muscle because asystole occurred at higher concentrations. TMS was greatest during local injection of acetylcholine (48 +/- 7% LFRC). There was a greater increase in pulmonary resistance and decrease in dynamic compliance during systemic acetylcholine injection than during vagal stimulation. We conclude that bilateral vagal nerve stimulation does not maximally activate trachealis smooth muscle but that the maximal shortening achieved with local injection of acetylcholine is still less than isotonic shortening in vitro. These data suggest that maximal shortening in vivo is limited by the afterload provided by the tracheal cartilaginous rings.