CARDIAC TROPONIN-I AND TENSION GENERATION OF SKINNED FIBERS IN THE DEVELOPING RAT-HEART

During development of the myocardium the troponin I (TNI) isoform expr ession is switched from a cAMP-insensitive, slow skeletal muscle TNI t o a cAMP-sensitive, cardiac TNI isoform (cTNI). To study the functiona l consequence of alterations in cTNI expression in the rat heart we in vestigated the cAMP-controlled cTNI phosphorylation in comparison with alterations of functional properties of isolated cardiac myofibrils d uring the first postnatal month. cTNI was identified by Western blot a nalysis followed by a semiquantitative assessment. From the third to t he 28th postnatal day the relative concentrations of the cardiac isofo rm 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-de pendent protein kinase (PKA) and P-32[gamma]-ATP was increased 5.8 +/- 0.2-fold (24.2 +/- 3.5 v 140.2 +/- 7.6 pmolP/mg protein loaded onto t he 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 affec ted by PKA at day 3. However, isometric tension generation of fibres p repared from 28-day-old rats was suppressed by incubation with PKA whi ch was accompanied by a rightward shift in the force/pCa relation. Und er 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 th e contractile apparatus was not affected by PKA-induced phosphorylatio n 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 postna tal development.