Renal response to parathyroid hormone in the weanling rat

The renal response to parathyroid hormone and the activity of alkaline phosphatase in kidney cortex were studied in weanling and adult rats. Clearance studies for the determination of glomerular filtration rate and phosphate excretion were started 2 h after sham operation or thyroparathyroidectomy in 24- or 40-day-old rats. Urine was collected under basal conditions and following infusions with 0.5 and 1.0 mumol phosphate per 100 g body wt min-1. For enzyme studies, rats aged 14-40 days were used, and the activity of alkaline phosphatase in renal cortical slices was determined. Glomerular filtration rate was not influenced by thyroparathyroidectomy or phosphate infusions. Plasma phosphate increased during phosphate infusions and was significantly higher in thyroparathyroidectomized rats in all clearance periods. Both the net and the fractional excretions of phosphate were higher in intact than in thyroparathyroidectomized rats and the phosphate-retaining effect of thyroparathyroidectomy was similar in weanling and adult rats. Alkaline phosphatase activity in renal cortical slices increased between days 14 and 24 (1400-3190 IU mg-1 protein, P less than 0.01). During later development, no significant change was seen. It is concluded that the immature kidney of the weanling rats shows a parathyroid hormone-dependent adaptation to acute phosphate loading, and that changes in alkaline phosphatase activity are consistent with a maturation of this system only during early development.