The long-term objectives of this research are to determine the cellular and molecular basis for the changes in cardiac function which come about in compensated hypertrophy (H) and congestive heart failure (HF) resulting from progressive hemodynamic overload. Recent studies have shown that reduced peak systolic Ca2+ and slow decay of the Ca2+ transient are primarily responsible for the reduction in a peak force generation and slowing of contraction and relaxation velocities in H/HF. The working hypothesis of this application is that a coordinate decrease of sarcoplasmic reticulum (SR) Ca2+ uptake and an increase in Na/Ca exchanger activity are responsible for the changes in the Ca2+ transient in H/HF.
The specific aims are: 1) To determine if, and to what extent, reduced SR Ca2+ uptake in H/HF is responsible for slowing the rate of decay of the Ca2+ transient and reducing SR Ca2+ storage and release; and 2) To determine if, and to what extent, increased forward (Ca2+ efflux) and reverse (Ca2+ influx) mode Na/Ca exchange activity contributes to the decay of the Ca2+ transient and SR Ca release in H/HF. Experiments will be performed in single LV myocytes isolated from feline hearts with hypertrophy with and without heart failure induced by slow progressive pressure overload. The extent of left ventricular hypertrophy (LVH) and failure will be determined with echocardiography before sacrifice. SR function will be studied in the absence of Na/Ca exchanger activity in intact H/HF myocytes by using whole cell patch clamp techniques and rapid extracellular solution exchange. Indo-1 will be used to measure cytosolic free Ca2+. SERCA2 mRNA and the Ca2+- dependence of SR Ca2+ uptake will be measured in populations of myocytes from the same hearts. The contribution of Na-Ca exchanger-mediated Ca fluxes to the decay of the Ca2+ transient and to SR Ca2+ release will be measured in intact H/HF myocytes using patch clamp and rapid extracellular solution exchange. Na/Ca exchanger mRNA will be measured in myocytes from the same hearts.
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