Auxotonic loading and airway smooth muscle shortening

Abstract

Isometric and isotonic contractions represent two extremes in loading regimens that can be imposed on a contracting smooth muscle preparation in vitro. Under physiological conditions, however, it is likely that smooth muscle in hollow organs contracts against auxotonic loads, the magnitude of which is a function of muscle shortening. Therefore, attempts to determine physiologically significant contractile properties of smooth muscle should be made under auxotonic loading conditions. An electronic loader was built to provide four auxotonic loading modes: linear, logarithmic, sigmoidal, and exponential. These shortening-dependent auxotonic loads were imposed on an actively shortening canine tracheal smooth muscle. Results confirmed that (i) active force generated by the muscle is load dependent but that (ii) at the same load borne by the muscle in its steady state, i.e., at the same maximum force generated by the muscle, the maximum shortenings under different loading modes were significantly altered. The shortening was maximal under the exponential loading mode, which provided the smallest load in the early phase of contraction, while it was the least under the logarithmic loading mode, which provided the greatest load in that phase. We concluded that the shortening achieved by an airway smooth muscle was a function not only of the magnitude of the final load imposed on the muscle but, more importantly, of the time course of loading. An explanation was provided by a previous report, which stated that cross-bridge cycling rate of smooth muscle was maximum in the early phase of activation and decreased thereafter, and that about 70% of the total shortening of the muscle was completed in the first 2 s of a light-loaded isotonic contraction with a contraction time of 10 s. A heavier load applied during the early phase of contraction will inhibit the major portion of the shortening, whereas a light load applied during this early stage will allow more shortening. Alteration in the viscoelastic properties of tissues that attach to airway smooth muscle and load it auxotonically could strongly affect the muscle's ability to shorten in vivo.Key words: auxotonic shortening, airway smooth muscle, loading history.