Early effects of subtotal nephrectomy and food restriction on urinary calcium in the rat

Urinary calcium excretion (UCa) is normal several weeks following subtotal nephrectomy (Nx) in the rat, despite factors such as magnified solute load per nephron which should promote hypercalciuria. To define the levels of UCa during the first few weeks after Nx and to determine if the development of compensatory mechanisms governing renal conservation of calcium are detectable, we measured UCa in Nx and sham Nx rats with matched food intake (10 g/day) from 10 days before through 20 days following Nx. In the sham Nx animals, UCa rose during the first 5 days following surgery from 27 +/- 8 to 66 +/- 5 microM/day and then plateaued for the remaining time. In Nx rats, UCa also rose during the first 5 postoperative days from 23 +/- 5 to 110 +/- 10 microM/day (Nx vs. sham Nx, p less than 0.05), but then fell over the next 3 days to levels observed simultaneously in sham Nx animals. The rise in UCa after Nx, but not after sham Nx, was associated with a doubling of urine flow rate and increased urinary titratable acid excretion. In parathyroidectomized rats, UCa also rose following Nx; however, maximum UCa was then sustained for at least 4 days. In an additional sham Nx group fed 20 g/day, no increase in UCa occurred following surgery. Thus, hypercalciuria is present following Nx in the rat, in part possibly attributable to increased acid excretion. The transient nature of calciuria reflects an adaptive phenomenon, most likely hyperparathyroidism. Diminished food intake following surgery independently contributes to hypercalciuria, regardless of the status of renal mass.