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

Abstract

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 microM) 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 microM calmodulin, both force and the maximal shortening velocity (Vmax) were altered. At pCa 5.75, at which force was about 35% of the maximal at pCa 4.5, Vmax 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 Vmax 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 Vmax in the maximally phosphorylated state and suggest that the level of myosin light chain phosphorylation is the major factor determining Vmax. The finding that the relationship between force and Vmax 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.

Partial bladder outlet obstruction alters Ca2+sensitivity of force, but not of MLC phosphorylation, in bladder smooth muscle

“…Kanaya (11) found similar levels of force development in a saponinpermeabilized preparation of bladder smooth muscle compared with the intact state. In these studies (11,25) as well as others (17,34), the Ca 2ϩ sensitivity of F708 DETERGENT-SKINNED URINARY BLADDER SMOOTH MUSCLE force (EC 50 ) was found to be ϳ1 M Ca 2ϩ , similar to that found in our study using tissue from control animals.…”

“…A very unexpected result, at least to those who have used smooth muscle skinned fiber preparations, was the high level of stress (force/cross-sectional area) developed by the Triton X-100 detergent-skinned fibers. On average, most detergent-skinned smooth muscle preparations develop ϳ40-80% of their preskinning force or stress (13,17,19). In our study, the Triton X-100 detergent-skinned bladder smooth muscle preparations developed nearly four times the level of stress developed by the intact smooth muscle preparation.…”

Urinary Bladder Contraction and Relaxation: Physiology and Pathophysiology

“…43), showing that an individual smooth muscle can modulate the cross-bridge turnover. Experiments on permeabilized smooth muscle have shown that both [Ca 2ϩ ] and myosin light-chain phosphorylation can alter both V max and force (34,438). One important concept in this context is the "latch" phenomenon (149), where cross-bridges are suggested to attach in nonphosphorylated state and thereby lower velocity and maintain tension at low ATP turnover.…”

The putative involvement of actin‐binding proteins and cytoskeleton proteins in pathological mechanisms of ketamine cystitis—Revealed by a prospective pilot study using proteomic approaches

“…Experiments on permeabilized smooth muscle have shown that both [Ca 2+ ] and MYL phosphorylation can alter both shortening velocity ( V max ) and force . Smooth muscle contraction is activated by the levels of phosphorylation of MYL, which is governed by a Ca 2+ ‐calmodulin‐dependent MYL kinase pathway .…”