THE VARIABLE COUPLING BETWEEN FORCE AND MYOSIN LIGHT-CHAIN PHOSPHORYLATION IN TRITON SKINNED CHICKEN GIZZARD FIBER-BUNDLES - ROLE OF MYOSINLIGHT-CHAIN PHOSPHATASE

The mechanism responsible for the regulation of smooth muscle tone at low levels of myosin light chain (MLC) phosphorylation is still poorly understood. According to one model, so-called latchbridges, which con tribute to force maintenance at low levels of MLC phosphorylation, are generated by dephosphorylation of attached and phosphorylated crossbr idges. The model predicts that the force generated for a given level o f MLC phosphorylation depends on the activity of the MLC phosphatase. We tested this hypothesis by reducing the activity of the phosphatase by at least 80% in two ways: inhibition with okadaic acid and extracti on. Under both conditions, higher levels of MLC phosphorylation were r equired to support a given level of force, suggesting a decreased flux of attached phosphorylated to attached dephosphorylated crossbridges, as predicted by this model. Although, under both conditions, the rela tionship between force and MLC phosphorylation was shifted to the righ t, the curves did not superimpose as would have been expected if the p hosphatase activity were the only determinant of the coupling between force and phosphorylation. In the extracted fibres, two more proteins, calponin and SM22, were significantly reduced in addition. Therefore, these proteins might be involved in modulating the relationship betwe en force and MLC phosphorylation.