Phospholamban ablation has been shown to result in significant increases in cardiac contractile parameters and loss of beta-adrenergic stimulation. To determine whether partial reduction in phospholamban levels is also associated with enhancement of cardiac performance and to further examine the sensitivity of the contractile system to alterations in phospholamban levels, hearts from wild-type, phospholamban-heterozygous, and phospholamban-deficient mice were studied in parallel at the subcellular, cellular, and organ levels. The phospholamban-heterozygous mice expressed reduced cardiac phospholamban mRNA and protein levels (40 +/- 5%) compared with wild type mice. The reduced phospholamban levels were associated with significant decreases in the EC50 of the sarcoplasmic reticulum Ca2+ pump for CA2+ and increases in the contractile parameters of isolated myocytes and beating hearts. The relative phospholamban levels among wild-type, phospholamban-heterozygous, and phospholamban-deficient mouse hearts correlated well with the (1) EC50 of the Ca(2+)-ATPase for Ca2+ in sarcoplasmic reticulum, (2) rates of relaxation and contraction in isolated cardiac myocytes, and (3) rates of relaxation and intact beating hearts. These findings suggest that physiological and pathological changes in the levels of phospholamban will result in parallel changes in sarcoplasmic reticulum function and cardiac contraction.