(Verbatim from the application): The long-term goals of this proposal are to understand the molecular basis for the cardiac action potential (AP) prolongation in cardiac hyperirophy and failure. We have shown that the prolongation of cardiac AP results in part from the downregulation of the underlying outward K+ currents. We have also shown that changes in Ca2 cycling protein can lead to an attenuation of the L-type Ca2+ current (ICa) inactivation and prolongation of cardiac AP. We propose that a decrease in SERCA pumps, coupled with a compensatory increase in Na+-Ca2+ exchanger seen in several models of cardiac failure, would shift the burden of Ca2+ removal from sarcoplasmic reticulum (SR) Ca2+ uptake to Ca2+ extrusion via Na+-Ca2+ exchanger. These changes will result in a decrease in SR Ca2+ load and Ca2+-induced Ca2+ release. We further propose that these chronic changes in intracellular Ca24 (Ca2+i) can result in changes in the ion channel expression, which represents a novel mechanism of crosstalk between changes in Ca2+ cycling protein and cardiac AP profile. Thus, we hypothesize that ion channel expression is lightly regulated by chronic changes in intracellular Ca2+ concentration [ca2+i] in cardiac hypertrophy and failure and as such, there is a strong interdependence between altered Ca2+ cycling protein and the AP profile. We propose to test this hypothesis in a guinea pig and mouse models of cardiac hypertrophy and failure. We will further test the hypothesis directly in two transgenic mouse models 1.) a transgenic mouse model with cardiac-specific overexpression of the SERCA la and 2.) a transgenic mouse with targeted ablation of the SERCA 2 gene. These two transgenic mouse models show significant alteration of the SERCA protein levels as well as robust phenotypes without cardiac hypertrophy or failure.
The specific aims of the present proposal are: 1.) To determine the effects of chronic changes in the Ca2+ handling on the expression of the cardiac ion channels in a guinea-pig and mouse models of pressure-induced cardiac hypertrophy and failure. 2.) Using a transgenic mouse model which overexpress the SERCA1a, we will determine the changes in the Ca2+ handling resulting from increase SERCA pump levels. Equipped with such information, we will determine the function and expression of the ion channels in the transgenic animals compared to the wild-type littermates. 3.) Finally, by using a transgenic knockout model of the SERCA2 gene, we will directly test our hypothesis in a model with reduced Ca2+-induced Ca2+ release using similar techniques. These experiments will be done using the combined techniques of electrophysiologic recordings, Ca2+ measurement, biochemical and molecular biology.
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