AJP - Renal Physiology, Vol 251, Issue 5 777-F783, Copyright © 1986 by American Physiological Society
Calcium and renal adaptation to a phosphate load in the thyroparathyroidectomized rat
L. Cheng, C. Dersch, E. Kraus and B. Sacktor
Infusion of phosphate into thyroparathyroidectomized rats on a normal
phosphorus diet caused a decrease in net phosphate reabsorption, even
though the plasma concentration of phosphate continued to rise. This
response was expressed at the level of the proximal tubule brush-border
membrane and was coincident with a decrease in sodium-dependent phosphate
uptake in membrane vesicles. Kinetic experiments indicated that the
increased phosphate load caused a decrease in the Vmax of the membrane
uptake system with no change in the apparent Km for phosphate. The infusion
of phosphate resulted in a lowered plasma calcium concentration, and it was
previously hypothesized that the inhibition of maximal phosphate
reabsorption was mediated by the hypocalcemia. When the fall in plasma
calcium was prevented by the simultaneous infusions of calcium and
phosphate, the reduction in maximal phosphate reabsorption was blunted;
however, the phosphate infusion-induced inhibition of brush-border membrane
vesicle phosphate uptake was still evident. Thus a major discrepancy was
found to the general concept that renal phosphate reabsorption in vivo
correlated positively with sodium-dependent phosphate uptake activity in
proximal tubule brush-border membrane vesicles. Several possible
explanations to account for this anomaly were discussed. It was also found
that calcium infusion into saline-infused thyroparathyroidectomized rats
slightly increased maximal phosphate reabsorption but did not affect
phosphate uptake in the membrane vesicles.
