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A1222 - Shortening Velocity, Power Output, and Myosin Phosphorylation in Fully and Partially Activated Airway Smooth Muscle
Author Block: L. Luo, L. Wang, P. D. Pare, C. Y. Seow, P. Chitano; University of British Columbia, Vancouver, BC, Canada.
Rationale: It is currently accepted that the intrinsic maximal velocity of shortening, Vmax, in smooth muscle is controlled by the degree of myosin light chain (MLC) phosphorylation. This implies that the maximal rate of myosin cross-bridge cycling, which happens when the muscle shortens against zero load, can be varied by the fraction of phosphorylated cross-bridges. However, if we consider MLC phosphorylation as a switch that turns on the cross-bridges, the muscle should shorten at Vmax when both the external and internal loads are absent, regardless of the degree of MLC phosphorylation. The maximal power output (Pmax) of the muscle, however, should be dependent on the degree of MLC phosphorylation because muscle power is directly related to the number of activated cross-bridges. We therefore hypothesize that MLC phosphorylation correlates with Pmax but not Vmax. Methods: In sheep tracheal smooth muscle strips adapted to their in-situ length, we obtained force-velocity curves at the plateau of contraction induced by electrical field stimulation (EFS) using the method of isotonic quick-release. A reduced voltage was used to obtain partial activation and therefore control the degree of MLC phosphorylation at the ser19 site. Results: In partially activated muscle the EFS-induced stress was 52% of that produced by full activation. We found that Vmax and Pmax at zero external load in partially activated muscle were respectively 69% (0.29 vs 0.42 Lref/s) and 45.3% (1.21 vs 2.67 %Fmax×Lref/s) when compared to those in full activation. Extrapolated force-velocity curves at full and partial activation showed that the velocity reached a maximal point at a load less than zero external load, suggesting that there was an internal load. The converging point indicated an internal load of 4.0% Fmax and a true Vmax of 0.7 Lref/s. When factoring in the internal load, Pmax in partially activated muscle was 51.4% (1.51 vs 2.94 %Fmax×Lref/s) when compared to full activation. The EFS-induced MLC phosphorylation in partially activated muscle was 56% (20.8% vs 36.1% of total MLC) when compared to full activation. Conclusion: We found that the degree of muscle activation produced comparable changes in MLC phosphorylation and Pmax, whereas there was a large discrepancy between MLC phosphorylation and Vmax, whether the Vmax was determined at zero external load or at both zero external and internal load. Our data suggest that the apparent correlation between MLC phosphorylation and Vmax is due to the presence of an internal load in the muscle.