Regulation of force and shortening velocity by calcium and myosin phosphorylation in chemically skinned smooth muscle

The phosphatase inhibitor okadaic acid (OA) was used to study the relationship between [Ca2+], rates of phosphorylation/dephosphorylation and the mechanical properties of smooth muscle fibres. Force/velocity relationships were determined with the isotonic quick release technique in chemically skinned guinea-pig taenia coli muscles at 22 degrees C. In the maximally thiophosphorylated muscle neither OA (10 mu M) nor Ca2+ (increase from pCa 9.0 to pCa 4.5) influenced the force-velocity relationship. When the degree of activation was altered by varying [Ca2+] in the presence of 0.5 mu M calmodulin, both force and the maximal shortening velocity (V-max) were altered. At pCa 5.75, at which force was about 350/0 of the maximal at pCa 4.5, V-max was 55% of the maximal value. When OA was introduced into fibres at pCa 6.0, force was increased from less than 5% to 100% of the maximal force obtained in pCa 4.5. The relationship between the degree of myosin light chain phosphorylation and force was similar in the two types of activation; varied [OA] at constant [Ca2+] and at varied [Ca2+]. The relation between force and V-max when the degree of activation was altered with OA was almost identical to that obtained with varied [Ca2+]. The results show that Ca2+ and OA do not influence force or V-max in the maximally phosphorylated state and suggest that the level of myosin light chain phosphorylation is the major factor determining V-max. The finding that the relationship between force and V-max was similar when activation was altered with OA and Ca2+ suggests, however, that alterations in the absolute rates of phosphorylation and dephosphorylation at a constant phosphorylation level do not influence the mechanical properties of the skinned smooth muscle fibres.