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- or 
-tropomyosin
1 Department of Physiology, University of Wisconsin Medical School, Madison, Wisconsin 53706; 2 Department of Pediatric Cardiology, Children's Memorial Hospital, Chicago, Illinios 60614; and 3 Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267
In myocardium, protein
kinase A (PKA) is known to phosphorylate troponin I (TnI) and
myosin-binding protein-C (MyBP-C). Here, we used skinned myocardial
preparations from nontransgenic (NTG) mouse hearts expressing 100%
-tropomyosin (-Tm) to examine the effects of phosphorylated TnI
and MyBP-C on Ca2+ sensitivity of force and the rate
constant of force redevelopment (ktr).
Experiments were also done using transgenic (TG) myocardium expressing
~60% 
-Tm to test the idea that the -Tm isoform is required to
observe the mechanical effects of PKA phosphorylation. Compared with
NTG myocardium, TG myocardium exhibited greater Ca2+
sensitivity of force and developed submaximal forces at faster rates.
Treatment with PKA reduced Ca2+ sensitivity of force in NTG
and TG myocardium, had no effect on maximum ktr
in either NTG or TG myocardium, and increased the rates of submaximal
force development in both kinds of myocardium. These results show that
PKA-mediated phosphorylation of myofibrillar proteins significantly
alters the static and dynamic mechanical properties of myocardium, and
these effects occur regardless of the type of Tm expressed.
protein phosphorylation; Ca2+ sensitivity of force; skinned myocardium
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