Ca(2+) signal in high-Ca(2+) perimitochondrial microdomains is amplified within the mitochondrial matrix and activates Ca(2+)-dependent dehydrogenases. In steroid-secreting cells, small cytoplasmic Ca(2+) signals may also augment mitochondrial Ca(2+) concentration. The ensuing formation of NADH and NADPH may have an essential role in supporting the increased steroid secretion.