Cross-bridge cycling in smooth muscle: a short review

This review is focused on the cross-bridge interaction of the organized con tractile system of smooth muscle fibres. By using chemically skinned prepar ations the different enzymatic reactions of actin-myosin interaction have b een associated with mechanical events. A rigor state has been identified in smooth muscle and the binding of,ATP causes dissociation of rigor cross-br idges at rates slightly slower than those in skeletal muscle, but fast enou gh not to be rate-limiting for cross-bridge turn over in the muscle fibre. The release of inorganic phosphate (P-i) is associated with force generatio n, and this process is not rate-limiting for maximal shortening velocity (V -max) in the fully activated muscle. The binding of ADP to myosin is strong in the smooth muscle contractile system, a property that might be associat ed with the generally slow cross-bridge turn over. Both force and V-max are modulated by the extent of myosin light chain phosphorylation. Low levels of activation are considered to be associated with the recruitment of slowl y circling dephosphorylated cross-bridges which reduces shortening velocity . The attachment of these cross-bridge states in skinned smooth muscle can be regulated by cooperative mechanisms and thin filament associated systems . Smooth muscles exhibit a large diversity in their V-max and the individua l smooth muscle tissue can alter its V-max under physiological conditions. The diversity and the long-term modulation of phenotype are associated with changes in myosin heavy and light chain isoform expression.