Phospholemman (PLM) is a small sarcolemmal protein that modulates the activities of Na+/K+-ATPase and the Na+/Ca2+ exchanger (NCX), thus contributing to the maintenance of intracellular Na+ and Ca2+ homeostasis. We characterized the expression and subcellular localization of PLM, NCX, and the Na+/K+-ATPase alpha 1-subunit during perinatal development. Western blotting demonstrates that PLM (15 kDa), NCX (120 kDa), and Na+/K+-ATPase alpha-1 (similar to 100 kDa) proteins are all more than 2-fold higher in ventricular membrane fractions from newborn rabbit hearts (1-4-day old) compared to adult hearts. Our immunocytochemistry data demonstrate that PLM, NCX, and Na+/K+-ATPase are all expressed at the sarcolemma of newborn ventricular myocytes. Taken together, our data indicate that PLM, NCX, and Na+/K+-ATPase alpha-1 proteins have similar developmental expression patterns in rabbit ventricular myocardium. Thus, PLM may have an important regulatory role in maintaining cardiac Na+ and Ca2+ homeostasis during perinatal maturation. (c) 2007 Elsevier Inc. All rights reserved.