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1 Departments of Physiology and Biophysics, and 2 College of Medicine, Section of Cardiology, The University of Illinois at Chicago, Chicago, Illinois 60612; and 3 Department of Microbiology, Biochemistry, and Molecular Biology, The University of Cincinnati, Cincinnati, Ohio 45267
We used
transgenic (TG) mice overexpressing mutant
-tropomyosin
[
-Tm(Asp175Asn)], linked to familial hypertrophic
cardiomyopathy (FHC), to test the hypothesis that this mutation impairs
cardiac function by altering the sensitivity of myofilaments to
Ca2+. Left ventricular (LV) pressure was measured
in anesthetized nontransgenic (NTG) and TG mice. In control conditions,
LV relaxation was 6,970 ± 297 mmHg/s in NTG and 5,624 ± 392 mmHg/s in TG mice (P < 0.05). During
-adrenergic
stimulation, the rate of relaxation increased to 8,411 ± 323 mmHg/s in NTG and to 6,080 ± 413 mmHg/s in TG mice
(P < 0.05). We measured the pCa-force relationship (pCa =
log [Ca2+]) in skinned fiber bundles from
LV papillary muscles of NTG and TG hearts. In control conditions, the
Ca2+ concentration producing 50% maximal force
(pCa50) was 5.77 ± 0.02 in NTG and 5.84 ± 0.01 in TG myofilament bundles (P < 0.05). After protein
kinase A-dependent phosphorylation, the pCa50 was 5.71 ± 0.01 in NTG and 5.77 ± 0.02 in TG myofilament bundles
(P < 0.05). Our results indicate that mutant
-Tm(Asp175Asn) increases myofilament Ca2+-sensitivity,
which results in decreased relaxation rate and blunted response to
-adrenergic stimulation.
calcium; cardiomyopathy; hypertrophy; myocardial contraction
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