EFFECTS OF WEANING ON PHOSPHATE-TRANSPORT MATURATION IN THE RAT-KIDNEY - CLEARANCE AND BRUSH-BORDER MEMBRANE STUDIES

Penal phosphate transport matures via an increase in the carrier affin ity for phosphate during the 3rd post-natal week in the rat. This stud y examines whether weaning, which normally takes place during this per iod, plays a role in phosphate transport maturation. Early weaning (EW ) and prevention of weaning (PW) both increased the fractional excreti on of phosphate (EW 26.5 +/- 4.9, PW 26.7 +/- 2.2, controls 11.3 +/- 2 .8, P < 0.001 and P < 0.05, n = 6 in each group). EW and PW also decre ased the uptake of phosphate into brush border membrane vesicles (BBMV ) isolated from the renal cortex of 21-day-old rats. Glucose transport in BBMV was not affected. The kinetics of phosphate uptake, measured in the presence of a sodium gradient, showed lower V-max (4,112 +/- 36 2 pmol/mg protein per 10 s) in EW BBMV than in controls (6,030 +/- 200 , n = 5, P < 0.001), but the affinity of the carrier for phosphate (1/ K-m) did not change. The decrease in Vmax may be due to the enhanced p hosphate supply. The affinity of the carrier was lower in PW rats (K-m = 0.31 +/- 0.04 mM) than in controls (0.18 +/- 0.04, n = 5, P < 0.01) but the V-max remained unchanged. The low affinity may indicate that normal maturation of tubular transport, in which carrier affinity incr eases, is altered. The plasma concentrations of corticosterone, parath yroid hormone, insulin and triiodothyronine and their changes during E W and PW are also reported. We conclude that changing the weaning proc ess alters the maturation pattern of phosphate renal transport. Furthe r studies should establish whether this is due to hormonal changes.