Mechanical responses of tracheal tissue in vitro: dependence on the tissue preparation employed and relationship to smooth muscle content.

We examined the relationship between the quantity of smooth muscle in isolated tracheal preparations and their responses to contractile agonists. The responses of two different tracheal preparations, rings and tubes, to carbachol and serotonin were compared both intra-species (Fisher vs. Lewis strain rats) and inter-species (rat vs. guinea-pig). The rank order for carbachol-induced maximal isometric tensions was Fisher > Lewis > guinea-pig and for serotonin Fisher > guinea-pig > Lewis for tracheal rings. The sensitivities to carbachol and serotonin were greater in Fisher than in Lewis rats. Guinea-pig tracheal rings were comparable to Fisher in sensitivity to carbachol, but were more sensitive to serotonin than either Fisher or Lewis rings. In both species, agonist-independent differences were found in the maximal tension of rings taken from different regions of trachea. For whole tracheal tubes under isovolumetric conditions, the rank order for carbachol-induced changes in the intraluminal pressure was guinea-pig > Lewis > or = Fisher. The sensitivity to carbachol was greater in guinea-pig tubes than in rat. The quantity of tracheal smooth muscle estimated from myosin was greater in guinea-pigs than in either Fisher or Lewis rats. In addition, the area of cartilage determined by morphometry in guinea-pig trachea was greater than that in the rat. We conclude that a concordance between the magnitude of contraction and the amount of tracheal smooth muscle is obtained only in whole tracheal tubes and not in tracheal rings. Several factors could contribute to the observed discrepancies in tracheal rings, including regional differences in efficacy and sensitivity to contractile agonists and the thickness of cartilage.