Hyperglycemia alters cardiac function and often leads to diabetic cardiomyopathy as cardiomyocyte apoptosis causes a hypertrophied heart to deteriorate to dilation and failure. Paradoxically, many short-term animal models of hyperglycemia protect against ischemia-induced damage, including apoptosis, by limiting Ca(2+) overload. We have determined that, like nonexcitable cells, both neonatal and adult cardiomyocytes respond to depletion of sarcoplasmic/endoplasmic reticulum Ca(2+) stores with an influx of extracellular Ca(2+) through channels distinct from voltage-gated Ca(2+) channels, a process termed capacitative Ca(2+) entry (CCE). Here, we demonstrate that in neonatal rat cardiomyocytes, hyperglycemia decreased CCE induced by angiotensin II or the Ca(2+)ATPase inhibitor thapsigargin. Hyperglycemia also significantly blunted Ca(2+)-dependent hypertrophic responses by approximately 60%, as well as the Ca(2+)-sensitive nuclear translocation of a chimeric protein bearing the nuclear localization signal of a nuclear factor of activated T-cells transcription factor. The attenuation of CCE by hyperglycemia was prevented by azaserine, an inhibitor of hexosamine biosynthesis, and partially by inhibitors of oxidative stress. This complements previous work showing that increasing hexosamine metabolites in neonatal cardiomyocytes also inhibited CCE. The inhibition of CCE by hyperglycemia thus provides a likely explanation for the transition to diabetic cardiomyopathy as well as to the protection afforded to injury after ischemia/reperfusion in diabetic models.