Effects of Zn(2+) on Ca (2+) uptake by mitochondria and endoplasmic reticulum in permeabilized tilapia gill cells.

An intracellular ATP-dependent Ca(2+) pumping mechanism, distinct from mitochondrial Ca(2+) accumulation, was identified within tilapia gill cells. Cell suspensions treated with 0.003% saponin, which selectively permeabilizes the plasma membrane, were used to characterize the Ca(2+) sequentering mechanisms as endoplasmic reticulum and mitochondria and to determine the effect of Zn(2+) on their Ca(2+) storing activity. Of the Ca(2+) taken up by the endoplasmic reticulum, 80% was released by IP3 (10 μmol l(-1)). The Ca(2+) pump of the endoplasmic reticulum was 2.5 times less sensitive to Zn(2+) (IC50=0.05 nmol l(-1)) than was the mitochondrial uptake mechanism (IC50=0.20 nmol l(-1)). The results indicate that Ca(2+) is stored predominantly within the endoplasmic reticulum at 0.1 μmol l(-1) and that this storing capacity is seriously attenuated by namomolar concentrations Zn(2+).