Tissue Specificity: Store-Operated Ca Entry in Cardiac Myocytes.

Calcium (Ca) is a key regulator of cardiomyocyte contraction. The Ca channels, pumps, and exchangers responsible for the cyclical cytosolic Ca signals that underlie contraction are well known. In addition to those Ca signaling components responsible for contraction, it has been proposed that cardiomyocytes express channels that promote the influx of Ca from the extracellular milieu to the cytosol in response to depletion of intracellular Ca stores. With non-excitable cells, this store-operated Ca entry (SOCE) is usually easily demonstrated and is essential for prolonging cellular Ca signaling and for refilling depleted Ca stores. The role of SOCE in cardiomyocytes, however, is rather more elusive. While there is published evidence for increased Ca influx into cardiomyocytes following Ca store depletion, it has not been universally observed. Moreover, SOCE appears to be prominent in embryonic cardiomyocytes but declines with postnatal development. In contrast, there is overwhelming evidence that the molecular components of SOCE (e.g., STIM, Orai, and TRPC proteins) are expressed in cardiomyocytes from embryo to adult. Moreover, these proteins have been shown to contribute to disease conditions such as pathological hypertrophy, and reducing their expression can attenuate hypertrophic growth. It is plausible that SOCE might underlie Ca influx into cardiomyocytes and may have important signaling functions perhaps by activating local Ca-sensitive processes. However, the STIM, Orai, and TRPC proteins appear to cooperate with multiple protein partners in signaling complexes. It is therefore possible that some of their signaling activities are not mediated by Ca influx signals, but by protein-protein interactions.