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.