Cytoplasmic Ca2+ at low submicromolar concentration stimulates mitochondrial metabolism in rat luteal cells

The cytoplasmic Ca2+ signal is transferred to the mitochondrial matrix and activates mitochondrial dehydrogenases. The requirement for supramicromolar cytoplasmic [Ca2+] ([Ca2+](i)) in perimitochondrial microdomains in this response has been suggested. We studied the correlation between [Ca2+](i), mitochondrial [Ca2+] ([Ca2+](m)) and mitochondrial formation of reduced nicotinamide adenine dinucleotide (phosphate) [NAD(P)H] in the presence of submicromolar [Ca2+](i) in cultured rat "large" luteal cells. [Ca2+](i) was monitored fluorimetrically with fura-PE3, [Ca2+](m) with rhod-2 and NAD(P)H with autofluorescence. In intact cells. prostaglandin F-2 alpha, which induces both intracellular Ca2+ release and Ca2+ entry, stimulated mitochondrial NAD(P)H formation. Thapsigargin-induced Ca2+ release and subsequent capacitative Ca2+ entry, both resulting in Ca2+ responses not exceeding 150-200 nM, also enhanced the reduction of pyridine nucleotides. As shown in inhibitor studies. the increased steady-state NAD(P)H level was due to activation of Ca2+-dependent dehydrogenases. [Ca2+](m), measured in permeabilized cells, increased moderately, but significantly, following elevation of [Ca2+](i) from 50 to 180 nM, showed a further gradual increase at higher submicromolar [Ca2+](i) values and rose steeply at supramicromolar [Ca2+](i). In summary, our results demonstrate that, in a steroid-producing cell type, net mitochondrial Ca2+ uptake and mitochondrial dehydrogenation can be activated even by low submicromolar increases of [Ca2+](i).