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

Abstract

Stimulation of swine carotid artery medial fibers with 110 mM KCl at 37°C results in increases in myosin light chain (MLC) phosphorylation levels and maximal shortening velocity (V _o) during the period of stress development. During the period of stress maintenance, MLC phosphorylation levels andV _o are not maintained, but fall to suprabasal levels, resulting in a correlation between MLC phosphorylation andV _o and suggesting that MLC phosphorylation regulatesV _o. This study identifies other conditions of KCl depolarization of swine carotid medial fibers in which this relationship between MLC phosphorylation andV _o is altered. A decrease in temperature from 37° to 23°C results in a similar magnitude of stress maintenance in response to 110 mM KCl and similar levels of MLC phosphorylation, but a reduction inV _o by approximately 50%. This differential effect of temperature onV _o 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 miantained stress. MLC phosphorylation levels are not transient, but maintained at a constant value andV _o is transient at levels approximately 50% of those at 110 mM KCl at 37°C. This results in a complete lack of correlation between MLC phosphorylation andV _o. Thus stress can be developed to equal magnitudes under differing activation conditions with dissimilar patterns of MLC phosphorylation andV _o. Therefore, there is not a strict relationship between MLC phosphorylation andV _o in all cases.