Apparent dissociation between myosin light chain phosphorylation and maximal velocity of shortening in KCl depolarized swine carotid artery: effect of temperature and KCl concentration.

Stimulation of swine carotid artery medial fibers with 110 mM KCl at 37 degrees C results in increases in myosin light chain (MLC) phosphorylation levels and maximal shortening velocity (Vo) during the period of stress development. During the period of stress maintenance, MLC phosphorylation levels and Vo are not maintained, but fall to suprabasal levels, resulting in a correlation between MLC phosphorylation and Vo and suggesting that MLC phosphorylation regulates Vo. This study identifies other conditions of KCl depolarization of swine carotid medial fibers in which this relationship between MLC phosphorylation and Vo is altered. A decrease in temperature from 37 degrees to 23 degrees C results in a similar magnitude of stress maintenance in response to 110 mM KCl and similar levels of MLC phosphorylation, but a reduction in Vo by approximately 50%. This differential effect of temperature on Vo and MLC phosphorylation results in a downward shift in the slope of the regression line describing the relationship between these two parameters. Decreasing [KCl] to 40 mM in the stimulating solution results again in similar magnitudes of maintained stress. MLC phosphorylation levels are not transient, but maintained at a constant value and Vo is transient at levels approximately 50% of those at 110 mM KCl at 37 degrees C. This results in a complete lack of correlation between MLC phosphorylation and Vo. Thus, stress can be developed to equal magnitudes under differing activation conditions with dissimilar patterns of MLC phosphorylation and Vo. Therefore, there is not a strict relationship between MLC phosphorylation and Vo in all cases.