Cardiac Troponin I and Tension Generation of Skinned Fibres in the Developing Rat Heart

During development of the myocardium the troponin I (TNI) isoform expression is switched from a cAMP-insensitive, slow skeletal muscle TNI to a cAMP-sensitive, cardiac TNI isoform (cTNI). To study the functional consequence of alterations in cTNI expression in the rat heart we investigated the cAMP-controlled cTNI phosphorylation in comparison with alterations of functional properties of isolated cardiac myofibrils during the first postnatal month. cTNI was identified by Western blot analysis followed by a semi-quantitative assessment. From the third to the 28th postnatal day the relative concentrations of the cardiac isoform of TNI increased 2.9±0.3-fold. In the same period the amount of phosphate incorporated into cTNI in the presence of exogenous cAMP-dependent protein kinase (PKA) and 32 P[γ]-ATP was increased 5.8±0.2-fold (24.2±3.5 v 140.2±7.6 pmolP/mg protein loaded onto the gel) whereas the phosphorylation of C-protein was only increased 1.6±0.2-fold. Ca2+-activated isometric tension generation of skinned heart fibres measured in the range of pCa from 6 to 4.5 was not affected by PKA at day 3. However, isometric tension generation of fibres prepared from 28-day-old rats was suppressed by incubation with PKA which was accompanied by a rightward shift in the force/pCa relation. Under these conditions half-maximal tension development was found at pCa 5.38 v 5.52 (P<0.05) in the absence of PKA. The Ca2+ sensitivity of the contractile apparatus was not affected by PKA-induced phosphorylation of C-protein. These data give direct evidence for the physiological relevance of the onset of cAMP-induced phosphorylation of cTNI for the Ca2+-activated tension generation in cardiac myofibrils during postnatal development.