Further studies on the effects of myosin P-light chain phosphorylation on contractile properties of skinned cardiac fibres.

We investigated the influence of myosin P-light chain phosphorylation by Ca2+-calmodulin dependent myosin light chain kinase (MLCK) on the sensitivity of the tension-pCa relation and maximum unloaded shortening velocity (Vmax) of chemically skinned heart fibres of the pig. Submaximum Ca2+ stimulation (pCa 5.5) induced 20 +/- 5% of the isometric tension achieved at maximum Ca2+ activation (pCa 4.3). MLCK-induced myosin P-light chain phosphorylation increased the isometric force development at pCa 5.5 by 40% whereas maximum tension at pCa 4.3 was not affected. Unloaded shortening velocity (Vmax) was not altered by myosin P-light chain phosphorylation either at maximum or at submaximum Ca2+ concentration, being c. 1.2 muscle length/s at pCa 5.5 and 2.2 muscle length/s at pCa 4.3. The MLCK-induced increase of the myosin P-light chain phosphorylation level was evaluated by determination of 32P-incorporation. Two phosphorylatable myosin P-light chains could be demonstrated.