Inotropic stimulation induces cardiac dysfunction in transgenic mice expressing a troponin T (I79N) mutation linked to familial hypertrophic cardiomyopathy

The cardiac troponin T (TnT) 179N mutation has been linked to familial hypertrophic cardiomyopathy and a high incidence of sudden death, despite causing little or no cardiac hypertrophy. In skinned fibers, 179N increased myofilamental calcium sensitivity (Miller, T,, Szczesna, D,, Housmans, P, Il,, Zhao, J., deFreitas, F., Gomes, A V., Culbreath, L,, McCue, J,, Wang, Y,, Xu, Y.,Kerrick, W. G., and Potter, J. D, (2001) J, Biol. Chen. 276, 3743-3755), To further study the functional consequences of this mutation, we compared the cardiac performance of transgenic mice expressing either human TnT-179N or human wild-type TnT. In isolated hearts, cardiac function was different depending on the Ca2+ concentration of the perfusate; systolic function was significantly increased in Tg-179N hearts at 0.5 and 1 mmol/liter, At higher Ca2+ concentrations, systolic function was not different, but diastolic dysfunction became manifest as increased end-diastolic pressure and time to 90% relaxation, In vivo measurements by echocardiography and Doppler confirmed that base-line systolic function was significantly higher in Tg-179N mice without evidence for diastolic dysfunction. Inotropic stimulation with isoproterenol resulted only in a modest contractile response but caused significant mortality in Tg-179N mice. Doppler studies ruled out aortic outflow obstruction and were consistent with increased chamber stiffness. We conclude that in vivo, the increased myofilament Ca2+ sensitivity due to the 179N mutation enhances base-line contractility but leads to cardiac dysfunction during inotropic stimulation.