ALUNG August 21/2

Jones, Keith A., Robert R. Lorenz, Y. S. Prakash, Gary C. Sieck, and David O. Warner. ATP hydrolysis during contraction of permeabilized airway smooth muscle. Am. J. Physiol. 277 (Lung Cell. Mol. Physiol. 21): L334–L342, 1999.— This study determined whether the time-dependent decline in the rate of ATP hydrolysis by actomyosin ATPase during sustained isometric force can occur in the absence of a time-dependent decline in regulatory myosin light chain (rMLC) phosphorylation in Triton X-100-permeabilized canine tracheal smooth muscle. Maximal activation with 10 μM Ca21 induced sustained increases in isometric force, stiffness, and rMLC phosphorylation; however, the increase in the ATP hydrolysis rate was initially high but then declined to a steady-state level above that of the unstimulated muscle (basal 31.8 6 5.8 nmol·cm23 ·s21; peak 81.4 6 11.3 nmol·cm23 · s21; steady-state 62.2 6 9.1 nmol ·cm23 ·s21). Activation of strips in which the rMLC was irreversibly and maximally thiophosphorylated with adenosine 58-O-(3-thiotriphosphate) also induced sustained increases in isometric force and stiffness but a nonsustained increase in ATP hydrolysis rate. There was no significant difference in the peak or steadystate isometric force, stiffness, or ATP hydrolysis rate or in the steady-state maximum unloaded shortening velocity between strips activated by 10 μM Ca21 or rMLC thiophosphorylation (0.058 6 0.016 and 0.047 6 0.011 muscle lengths/s, respectively). Mechanisms other than changes in rMLC phosphorylation contribute to the time-dependent decline in actomyosin ATPase activity during sustained activation of canine tracheal smooth muscle.