Phosphorylation and thiophosphorylation by myosin light chain kinase: Different effects on mechanical properties of chemically skinned ventricular fibers from the pig

Abstract

The influence of myosin light chain phosphorylation (treatment with myosin light chain kinase = MLCK, calmodulin and ATP) and thiophosphorylation (incubation with MLCK, calmodulin and ATPγS) on the maximal shortening velocity ( V max) and Ca 2+ sensitivity of chemically-skinned ventricular fibers from the pig has been studied. V max was determined by the slack-test method and by extrapolation of the force-velocity relation by the isotonic quick release method. V max was 1.53 muscle length/s (L/s) and 1.94 L/s using the force-velocity relation and the slack-test, respectively. Phosphorylation increased the Ca 2+ sensitivity for isometric force development of skinned fibers but had no influence on V max. Thiophosphorylation decreased V max but had no influence on Ca 2+ sensitivity. Phosphorylation pattern of the myosin light chains of the skinned fibers was studied using [γ- 32P]ATP or [γ-P 35S]ATP (250 μCi each) and autoradiography. Incubation of skinned fibers with labeled ATP led to a phosphate incorporation into the 18-kDa myosin light chain (MPLC or regulatory light chain) while incubation with labeled ATPγS led to an incorporation of thiophosphate into the 28-kDa myosin light chain (alkali light chain) and tropomyosin. We suggest that the difference in mechanical behavior between phosphorylated and thiophosphorylated skinned fibers are due to differences in the phosphorylation profiles of myofibrillar regulatory proteins.