To study asthmatic airway smooth muscle we developed a canine model of ragweed pollen sensitized, airway hyperresponsiveness because of the difficulties in obtaining human tissue. Tracheal and bronchial smooth muscles from sensitized dogs were shown to possess greater ability to shorten and higher maximum shortening velocity (Vo), both of which contribute to the excessive narrowing of airways typical of human asthma. However, maximum force production remained normal, demonstrating the dissociation between the behaviour of shortening and force. Because we found no evidence of inflammation, hypertrophy, or hyperplasia in the sensitized airway smooth muscles, we felt this is a model of early disease and should provide insight into early and perhaps primary pathogenetic mechanisms. Vo is known to be determined by actomyosin ATPase, which in smooth muscle is activated via phosphorylation of the 20-kDa myosin light chain (MLC20) by myosin light chain kinase (MLCK). Therefore, ATPase activity, MLC20 phosphorylation, and MLCK were investigated. Sensitized tracheal and bronchial smooth muscles showed significantly higher ATPase activity, and a higher level of MLC20 phosphorylation, resulting from increased MLCK activity, a consequence of the measured increase in total quantity of MLCK rather than in specific activity. Since MLCK is activated by binding with Ca(2+)-calmodulin complex, intracellular Ca2+ concentration and calmodulin activity were also assessed, but no difference was found between sensitized and control animals. Our study suggests that increased MLCK quantity may be the cause of airway hyperresponsiveness found in sensitized animals, and future investigation should be focused on depicting the reason for the elevated MLCK.