Troponin I protein kinase C phosphorylation sites and ventricular function.
OBJECTIVE
Cardiac Troponin I (cTnI) phosphorylation by protein kinase C (PKC) results in a reduction of maximal actomyosin ATPase activity, an effect that is more marked at higher levels of calcium (Ca2+) and is likely to reduce active force development. We postulated that there would be greater Ca2+-dependent changes in ventricular function in hearts of cTnI transgenic (TG) mice expressing mutant troponin I lacking PKC sites compared to wild-type (WT).


METHODS
We studied left ventricular function in isolated perfused hearts over a wide range of left ventricular volumes (Frank-Starling relationships) and mechanical restitution at three levels of perfusate Ca2+ (1.5, 2.5, and 3.5 mM). Manganese-enhanced magnetic resonance imaging (MRI) was used to study in-vivo sarcolemmal Ca2+ influx. The phosphorylation status of cTnI was examined by western blot analysis.


RESULTS
Systolic contractile function in TG mice was altered in a calcium-dependent manner such that ventricular contractility was significantly greater in TG mice only at 3.5 mM perfusate Ca2+. The relaxation process and passive mechanical properties were unaltered in TG mice. Mechanical restitution parameters were abnormal in TG mice only at 1.5 mM perfusate Ca2+. In-vivo MRI data demonstrated up to 48% reduction in Mn2+-induced contrast enhancement, indicating reduced sarcolemmal Ca2+ influx. Western blot analysis indicated increased cTnI phosphorylation in TG mice.


CONCLUSIONS
(1) TG mice exhibit calcium-dependent positive inotropy without slowed relaxation and this phenotype is mitigated by concomitant (compensatory) changes of reduced intracellular Ca2+ and increased phosphorylation of remaining cTnI sites. (2) The contractile phenotype in TG mice can be interpreted as an amplification of the normal response to changes in cellular Ca2+ observed in WT mice. Thus, PKC phosphorylation sites on cTnI play a role in attenuating contractile responses to changes in intracellular Ca2+.
MacGowan
Guy A
GA
Cardiovascular Institute, University of Pittsburgh, Pittsburgh, PA 15261, USA.
Evans
Caroline
C
Hu
Tom C-C
TC
Debrah
Dan
D
Mullet
Steven
S
Chen
Hsiao-Huei
HH
McTiernan
Charles F
CF
Stewart
Alexandre F R
AF
Koretsky
Alan P
AP
Shroff
Sanjeev G
SG
D000818:Animals; D015153:Blotting, Western / Q000379:methods; D002118:Calcium / Q000494:pharmacology*; D002306:Cardiac Volume; D025461:Feedback, Physiological; D008279:Magnetic Resonance Imaging; D051379:Mice; D008822:Mice, Transgenic; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D010477:Perfusion; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*; D012508:Sarcolemma / Q000378:metabolism; D013599:Systole; D019210:Troponin I / Q000235:genetics / Q000378:metabolism*
D019210:Troponin I; D011493:Protein Kinase C; D002118:Calcium