Ca2+ can affect Vmax without changes in myosin light chain phosphorylation in smooth muscle.

The effects of elevated [Ca2+]o on crossbridge cycling rate, measured as maximum velocity of shortening (Vmax) and high energy phosphate usage (delta approximately P), and on the degree of phosphorylation of the 20,000-dalton light chain of myosin (MyLCP) during an isometric tetanus were determined in the rabbit taenia coli at 18 degrees C. In an normal Krebs medium (1.9 mM Ca2+) the average rate of delta approximately P during force development is 4 X higher than during force maintenance. In 4.5 mM Ca2+-Krebs, the average rate of delta approximately P increases by 100% during force development and during force maintenance above that observed in normal Krebs medium, with no significant change in force output. Vmax increased in the high Ca2+ medium, in good agreement with the corresponding rates of delta approximately P, but without any significant change in the degree of MyLCP. Also, in both high and low calcium media, Vmax decreased with tetanus duration as did the delta approximately P; however, the degree of phosphorylation was not directly related to the average rate of energy usage during the two phases of the tetanus. Therefore, in intact smooth muscles Ca2+-dependent and time-dependent changes in Vmax and average rate of delta approximately P can occur without corresponding changes in MyLCP. Modulation of crossbridge cycling rate may be accomplished by a Ca2+-dependent process in addition to MyLCP.