Calcium sensitivity of myofilaments in cardiac muscle--effect of myosin phosphorylation.

We investigated the influence of the extent of phosphorylation of the myosin P-light-chain on the calcium sensitivity of skinned heart fibres. Treatment of skinned heart fibres with PCM-phosphatase decreased the phosphorylation level of P-light-chain from 0.3 mol P/mol LC-2 and 0.2 mol P/mol LC-2* to 0.16 mol P/mol LC-2 and 0.14 mol P/mol LC-2*. Isometric tension development decreased by 34% at submaximal Ca2+-concentration (pCa 5.5) after incubation with PCM-phosphatase while tension achieved at maximum Ca2+-concentration (pCa 4.3) was not affected. The effect of desensitization on skinned fibres could be reversed by washing out the phosphatase or by addition of myosin light chain kinase.