[Basic research in cardiology]

Phosphorylation of phospholamban and troponin I of dog hearts was measured depending on the cardiac cycle. Myocardium biopsies were obtained with the help of an ECG-triggered cryobiopsy device. A standardized back phosphorylation technique was performed with (32P)ATP and an excess of exogenically applied c-AMP dependent protein kinase, which was followed by electrophoretic fractionation. This standardized method enables quantitative measuring especially of the c-AMP-stimulated protein phosphorylation. A dichotomous behaviour in the phosphorylation of troponin I and phospholamban was detected in the cardiac cycle. Like c-AMP and the c-AMP depended protein kinase troponin I reach a maximum phosphorylation in the systole. In contrast to this, phospholamban in vivo shows the highest degree of phosphorylation in the diastole and drops to a significant lower value (by approximately 25%) in the systole. The surprising dichotomy support the assumption that the Ca-detensivation of the contractile proteins by phosphorylation of troponin I was a dynamic counterregulation in the presence of a high Ca-level takes effect before the Ca-accumulation which can be stimulated by phospholamban. The results show that covalent modifications of regulatory proteins come to pass in the ms-range and have to be considered in distinctive conditions of myocardial function.