Ca2+ can affectV max without changes in myosin light chain phosphorylation in smooth muscle

Abstract

The effects of elevated [Ca²⁺]_o on crossbridge cycling rate, measured as maximum velocity of shortening (V _max) and high energy phosphate usage (Δ∼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°C. In an normal Krebs medium (1.9 mM Ca²⁺) the average rate of Δ∼P during force development is 4xhigher than during force maintenance. In 4.5 mM Ca²⁺-Krebs, the average rate of Δ∼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.V _max increased in the high Ca²⁺ medium, in good agreement with the corresponding rates of Δ∼P, but without any significant change in the degree of MyLCP. Also, in both high and low calcium media,V _max decreased with tetanus duration as did the Δ∼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 inV _max and average rate of Δ∼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.