Calcium signaling mechanisms in dedifferentiated cardiac myocytes: comparison with neonatal and adult cardiomyocytes

Our studies focused on calcium sparking and calcium transients in cultured adult rat cardiomyocytes and compared these findings to those in cultured neonatal and freshly isolated adult cardiomyocytes. Using deconvolution fluorescence microscopy and spectrophotometric image capture, sequence acquisitions were examined for calcium spark intensities, calcium concentrations and whether sparks gave rise to cell contraction events. Observations showed that the preparation of dedifferentiated cardiomyocytes resulted in stellate, neonatal-like cells that exhibited some aspects of calcium transient origination and proliferation similar to events seen in both neonatal and adult myocytes. Ryanodine treatment in freshly isolated adult myocytes blocked the calcium waves, indicating that calcium release at the level of the sarcoplasmic reticulum and t-tubule complex was the initiating factor, and this effect of ryanodine treatment was also seen in cultured-dedifferentiated adult myocytes. However, experiments revealed that in both neonatal and cultured adult myocytes, the inositol triphosphate pathway (IP3) was a major mechanism in the control of intracellular calcium concentrations. In neonatal myocytes, the nucleus and regions adjacent to the plasma membrane were major sites of calcium release and flux. We conclude: (1) culturing of adult cardiomyocytes leads them to develop mechanisms of calcium homeostasis similar in some aspects to those seen in neonatal cardiomyocytes; (2) neonatal myocytes rely on both extracellular and nuclear calcium for contractile function; and (3) freshly isolated adult myocytes use sarcoplasmic reticulum calcium stores for the initiation of contractile function. (C) 2001 Harcourt Publishers Ltd.