Abstract
K +-contracted porcine carotid arterial muscles containing phosphorylated 20-kDa myosin light chains (LC) were exposed to carrier-free [ 32P]orthophosphate in K +-stimulating solution during sustained contraction. The covalently bound LC phosphate was completely replaced by [ 32P]phosphate, indicating that myosin light chain phosphatase and kinase have ready access to the bound phosphate during the sustained contraction. On average, 0.38 mol [ 32P]phosphate was incorporated per mole LC during the sustained K + contraction. This value was about half of the maximal value for [ 32P]phosphate incorporation into LC, 0.74 mol/mol, in muscles contracted with K + for 1 min. Assuming that sustained contraction involves the maximal number of cross-bridges attached to actin, the data suggest that half of the attached cross-bridges contain phosphorylated LC.
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