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 semiquantitative 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 32P[gamma]-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. Ca(2+)-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 Ca(2+)-activated tension generation in cardiac myofibrils during postnatal development.
Bartel
S
S
Max Delbrück Centre for Molecular Medicine, Berlin-Buch, UK.
Morano
I
I
Hunger
H D
HD
Katus
H
H
Pask
H T
HT
Karczewski
P
P
Krause
E G
EG
D000818:Animals; D000831:Animals, Newborn; D002118:Calcium / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D006321:Heart / Q000187:drug effects / Q000254:growth & development; D066298:In Vitro Techniques; D009200:Myocardial Contraction / Q000502:physiology*; D009206:Myocardium / Q000378:metabolism*; D010766:Phosphorylation; D051381:Rats; D017208:Rats, Wistar; D014336:Troponin / Q000378:metabolism*; D019210:Troponin I
D014336:Troponin; D019210:Troponin I; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D002118:Calcium