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K nABSTRACT Background: Pressure overload leads to cardiac hypertrophy, which is often followed by heart failure. We tested the hypothesis that depressed contractility in this process results from an imbalance in Ca2 transport by the sarcoplasmic reticulum (SR) Ca2 ATPase (SERCA) and the sarcolemmal Na/Ca2 exchanger (NCX). Methods and Results: Left ventricular (LV) myocytes (n 79) from 12 normal (N) and 5 hypertrophied (LVH, by aortic banding) feline hearts were studied. Adenoviral gene transfer was used to introduce green fluorescent protein (GFP), SERCA2, and NCX into N and LVH myocytes. Contraction (videomicroscopy) and Ca2 transients (Fluo-3) were measured in steady state and after rest periods of 2 to 120 seconds (rest decay and potentiation). LVH hearts were significantly larger than N (7.1 1.4 versus 4.2 0.2 g/kg). SERCA protein was significantly less abundant in LVH versus N. Steady state contractions and Ca2 transients of LVH-GFP myocytes decayed more slowly and rest decay of contractility was more pronounced compared with N-GFP. Infection of LVH (and N) myocytes with SERCA increased basal contractility and reduced rest decay. Infection of LVH myocytes with NCX almost abolished contraction and in