REQUIREMENT FOR TRANSMEMBRANE SODIUM FLUX IN MAINTENANCE OF CYTOSOLICCALCIUM LEVELS IN RAT SERTOLI CELLS

The prompt rise in cytosolic calcium induced by follicle-stimulating h ormone (FSH) in rat Sertoli cells suggests a role for calcium in FSH s ignal transduction. To evaluate the requirement for sodium in transmem brane calcium fluxes in Sertoli cells, we measured intracellular calci um concentration under sodium-free conditions and during stimulation b y monensin and veratridine, used to elevate cytosolic sodium. Cytosoli c calcium levels were measured by dual-wavelength spectrofluorimetry u sing freshly isolated cells loaded with fura-2 acetoxymethyl ester. Wh ereas, removal of extracellular sodium lowered cytosolic calcium in un stimulated cells from 89 +/- 4 to 75 +/- 8 nM, treatment with monensin and veratridine increased cytosolic calcium to 142 +/- 19 and 126 +/- 13 nM, respectively. Without extracellular calcium, monensin still pr oduced 47% of the rise in cytosolic calcium observed in the presence o f extracellular calcium, indicating approximately equal contributions of calcium from intracellular and extracellular sources. Blockade of v oltage-sensitive or/and voltage-insensitive calcium channels by verapa mil and ruthenium red was unable to completely prevent the monensin-in duced elevation of cytosolic calcium. In addition tetrodotoxin failed to block the FSH-induced rise in cytosolic calcium. These observations , together with the considerable reduction in monensin-induced rise in cytosolic calcium under extracellular sodium-free condition, support the hypothesis that sodium-calcium exchange rather than the specific c alcium or sodium channels regulate basal and monensin-induced transmem brane sodium and calcium fluxes in Sertoli cells.