It has been nearly 130 years since Sydney Ringer's astute observations on the indispensability of extracellular Ca 2ϩ to heart muscle contraction (1). At the cellular level, we now know that fluctuations of cytosolic Ca 2ϩ are coordinated by several myocyte proteins in order to functionally couple the cardiac action potential to sarcomeric shortening and mitochondrial energy production. This elegant myocardial Ca 2ϩ cycling demands precise regulation of intracellular Ca 2ϩ , as evidenced by the numerous examples of cardiac dysfunction arising from altered expression or activity of Ca 2ϩ handling proteins (2-5). One such protein, S100A1, has attracted the interest of cardiac scientists because of its myocardial enrichment, known interactions with several other Ca 2ϩ See page 966 handling proteins, including ryanodine receptors (RyR) and sarcoplasmic reticulum (SR) Ca 2ϩ ATPase (SERCA), and the observation that its expression is progressively downregulated in failing myocardium (6). Furthermore, cardiacspecific S100A1 deletion accelerated the progression to heart failure (HF) after myocardial infarction in mice (7), whereas restoration of S100A1 expression via intracoronary adenoviral gene delivery to failing rat hearts normalized contractile performance (8).In this issue of the Journal, Brinks et al. (9) provide the first evidence that S100A1 gene delivery can functionally improve human failing cardiomyocytes. Using ventricular tissue from ischemic HF patients, cardiomyocytes were isolated and infected with adenovirus encoding S100A1 (Ad-S100A1). The authors show that restoring S100A1 expression improved failing cardiomyocyte fractional shortening, relaxation rates, and Ca 2ϩ transients compared with control cells. Moreover, Ad-S100A1 improved both ratedependent and -adrenergic-mediated contractile reserve, while ameliorating markers of abnormal energy metabolism in isolated cardiomyocytes and mitochondria from failing hearts. Despite increases in SR Ca 2ϩ load, elevated S100A1 expression reduced diastolic SR Ca 2ϩ leak and the propensity for aberrant depolarizations.From a mechanistic standpoint, Brinks et al. (9) propose that S100A1 enhances failing cardiomyocyte contraction through improved cytosolic Ca 2ϩ handling. In this regard, it is particularly interesting that the ability of S100A1 to improve Ca 2ϩ transients and pathological shorteningfrequency responses occurred without altering either pathological decreases in SERCA abundance or increases in sarcolemmal Na ϩ /Ca 2ϩ exchanger that have been previously identified as major determinants of these defects in human myocardium (10 -12). Although detailed proof of the molecular mechanism of S100A1's actions is not reported, Brinks et al. speculate that they result from direct interaction of S100A1 with SR and mitochondrial proteins (i.e., RyR, SERCA, and F 1 -ATPase). Recent studies also indicate that S100A1 may enhance contractility and relaxation dynamics by regulating the interaction between the sarcomeric proteins titin and cardiac actin (13).These findin...