BRONCHIAL SMOOTH-MUSCLE MECHANICS OF A CANINE MODEL OF ALLERGIC AIRWAY HYPERRESPONSIVENESS

Although we have reported that tracheal smooth muscle from sensitized dogs shows altered mechanical properties, we did not know, because of technical difficulties with the preparation, whether similar changes occur in the properties of sensitized central bronchial smooth muscle (BSM), the site at which the acute asthmatic response is believed to develop. We have now succeeded in developing a cartilage-free BSM preparation that retains optimal mechanical properties. Such strips were obtained from mongrel dogs that had been sensitized to ragweed pollen. Controls were littermates injected with adjuvant alone. Length-tension relationships were obtained for both control and sensitized BSM strips (CBSM and SBSM, respectively). The maximal active stresses were the same (P > 0.05) when normalized to muscle fraction in total tissue cross-sectional area [6.2 +/- 0.6 X 10(4) and 5.9 +/- 0.6 X 10(4) (SE) for SBSM and CBSM, respectively]. This suggests that optimal tension is an insensitive indicator of bronchial hyperresponsiveness and that isotonic studies might be more revealing. The maximal shortening velocity (V(o)) for SBSM at 2 s [0.35 +/- 0.017 (SE) l(o)/s, where l(o) signifies optimal muscle length], in the course of a 10-s contraction, was significantly greater (P < 0.05) than V(o) measured for CBSM (0.27 +/- 0.015 l(o)/s). However, V(o) did not differ at the 8-s point of contraction. The sensitized group demonstrated a statistically significantly greater maximal shortening capacity (0.67 +/- 0.04 l(o)) than the control group (0.51 +/- 0.04 l(o)). At 2 s of contraction, 80% of maximal SBSM shortening had been completed and was significantly greater than for CBSM. These data reflect alterations in the early dynamic mechanical properties of SBSM, which indicate that normally cycling cross-bridges are likely involved mechanistically in the changes observed. The increased shortening velocity of SBSM could be due to increased myosin light chain phosphorylation, and the increased shortening capacity could be the result of increased compliance of the muscle's so-called internal resistor.