S100A1 Enhances the L-type Ca2+ Current in Embryonic Mouse and Neonatal Rat Ventricular Cardiomyocytes

Abstract

S100A1 is an EF-hand type Ca2؉ -binding protein with a musclespecific expression pattern. The highest S100A1 protein levels are found in cardiomyocytes, and it is expressed already at day 8 in the heart during embryonic development. Since S100A1 is known to be involved in the regulation of Ca 2؉ homeostasis, we tested whether extracellular S100A1 plays a role in regulating the L-type Ca 2؉ current (I Ca ) in ventricular cardiomyocytes. Murine embryonic (day 16.5 postcoitum) ventricular cardiomyocytes were incubated with S100A1 (0.001-10 M) for different time periods (20 min to 48 h). I Ca density was found to be significantly increased as early as 20 min (from ؊10.8 ؎ 1 pA/pF, n ‫؍‬ 18, to ؊22.9 ؎ 1.4 pA/pF; ؉112.5 ؎ 13%, n ‫؍‬ 9, p < 0.001) after the addition of S100A1 (1 M). S100A1 also enhanced I Ca current density in neonatal rat cardiomyocytes. Fluorescence and capacitance measurements evidenced a fast translocation of rhodamine-coupled S100A1 from the extracellular space into cardiomyocytes. S100A1 treatment did not affect cAMP levels. However, protein kinase inhibitor, a blocker of cAMP-dependent protein kinase A (PKA), abolished the S100A1-induced enhancement of I Ca . Accordingly, measurements of PKA activity yielded a significant increase in S100A1-treated cardiomyocytes. In vitro reconstitution assays further demonstrated that S100A1 enhanced PKA activity. We conclude that the Ca 2؉ -binding protein S100A1 augments transsarcolemmal Ca 2؉ influx via an increase of PKA activity in ventricular cardiomyocytes and hence represents an important regulator of cardiac function.In 1965, Moore (1) described a novel protein "soluble in 100% saturated ammonium sulfate solution" and therefore denoted it S100 protein. Today 21 different S100 proteins belonging to a multigene family of Ca 2ϩ -binding proteins of the EF-hand type are known to be differentially expressed in a large number of cell types (2). S100A1 is found in the cytosol, displays highest expression levels in the heart (3), and is already expressed at day 8 postcoitum in embryonic mouse hearts (4). Although the physiological relevance of S100 proteins in healthy tissue is still unclear, several findings point to a role of S100A1 in the diseased heart, since it is up-regulated during the course of myocardial hypertrophy (5) and markedly down-regulated in the left ventricle of patients with end stage heart failure (6). Recently, S100A1 was identified as a novel regulator of cardiac function based on the observation that overexpression of S100A1 led to a significant increase of contractility in adult cardiomyocytes (7). This was found to be related to an increased sarcoplasmic reticulum Ca 2ϩ -ATPase (8) and ryanodine receptor 2 activity (9) as well as to a decrease in myofilamental Ca 2ϩ sensitivity (8) and might be explained by enhancement of PKA 3 activity. The physiological relevance of S100A1 is further supported by data of Du and co-workers (10), who identified S100A1 as a regulator of cardiac reserve in a S100A1 mouse knock-out model. Accordingly, ...

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