CALCIUM-PUMP ACTIVITIES IN THE KIDNEYS OF OREOCHROMIS-MOSSAMBICUS

The mechanism that underlies transcellular Ca2+ reabsorption in the ki dney of the euryhaline teleost Oreochromis mossnmbicus was studied, Pr eparations of membrane vesicles made from the kidneys of freshwater- a nd seawater-adapted fish were more than sevenfold enriched in the baso lateral plasma membrane marker Na+/K+-ATPase. Significant recovery of NADH-cytochrome c reductase enzyme activity and of oxalate-stimulated Ca2+ pump activities in the membrane preparations indicated that the m embrane fraction was of endoplasmic reticular origin, Indeed, thapsiga rgin specifically inhibited Ca2+ pump activity that could be attribute d to oxalate-permeable endoplasmic reticular fragments, Kinetic analys is of thapsigargin-insensitive Ca2+ pump activity indicated the existe nce of homogeneous, high-affinity, ATP-driven Ca2+ pump. No Na+-driven Ca2+ transport mechanism could be demonstrated. Plasma membrane Ca2pump activity was 56 % lower in preparations from seawater-adapted fis h than in preparations from freshwater-adapted fish, suggesting a phys iological role for this Ca2+ pump activity in renal Ca2+ handling by e uryhaline species, with an involvement in the regulation of Ca2+ reabs orption.