Myosin light chain phosphorylation inhibits muscle fiber shortening velocity in the presence of vanadate

We have shown that myosin light chain phosphorylation inhibits fiber shortening velocity at high temperatures, 30 degrees C, in the presence of the phosphate analog vanadate. Vanadate inhibits tension by reversing the transition to force-generating states, thus mimicking a prepower stroke state. We have previously shown that at low temperatures vanadate also inhibits velocity, but at high temperatures it does not, with an abrupt transition in inhibition occurring near 25 degrees C (E. Pate, G. Wilson, M. Bhimani, and R. Cooke. Biophys J 66: 1554 - 1562, 1994). Here we show that for fibers activated in the presence of 0.5 mM vanadate, at 30 degrees C, shortening velocity is not inhibited in dephosphorylated fibers but is inhibited by 37 +/- 10% in fibers with phosphorylated myosin light chains. There is no effect of phosphorylation on fiber velocity in the presence of vanadate at 10 degrees C. The K-m for ATP, defined by the maximum velocity of fibers partially inhibited by vanadate at 30 degrees C, is 20 +/- 4 mu M for phosphorylated fibers and 192 +/- 40 mu M for dephosphorylated fibers, showing that phosphorylation also affects the binding of ATP. Fiber stiffness is not affected by phosphorylation. Inhibition of velocity by phosphorylation at 30 degrees C depends on the phosphate analog, with similar to 12% inhibition in fibers activated in the presence of 5 mM BeF3 and no inhibition in the presence of 0.25 mM AlF4. Our results show that myosin phosphorylation can inhibit shortening velocity in fibers with large populations of myosin heads trapped in prepower stroke states, such as occurs during muscle fatigue.