Relationship between renal phosphate reabsorption and renal brush-border membrane transport

Previous studies on thyroparathyroidectomized (TPTX) animals indicate that the fractional excretion (FE) of phosphate (Pi) is considerably lower in the rat compared to both the rabbit and hamster. In the present studies, luminal brush-border membrane (BBM) uptake of Pi was studied in vitro using BBM vesicles from the kidneys of the same animals in which renal excretion of Pi was measured in vivo by clearance techniques. In TPTX animals studied in parallel FE(Pi) was lowest (0.8 ± 0.4%; P<0.001) in the rat, compared to 22 ± 4% in the rabbit and 16 ± 3% in the hamster. Conversely, in BBM vesicles subsequently isolated from the renal cortex of the same animals, Na⁺-dependent Pi transport (pmol.mg protein^-1.30 s^-1) was highest (2,267 ± 155; P<0.01) in the rat compared to 1,007 ± 113 in the rabbit and 1,470 ± 149 in the hamster. The greater capacity of rat BBM for Pi transport may be due principally to a high Vmax compared to the other species. There were no significant differences between species in BBM transport of D-glucose and L-proline. Pooling the data from all species revealed a highly significant positive correlation (r=0.76 ± 0.12; P<0.001) between BBM transport of Pi and tubular reabsorption of Pi. Alkaline phosphatase activity in BBM positively correlated with BBM transport of Pi but did not correlate with BBM uptake of D-glucose and L-proline. We conclude that the rate of Pi transport across renal cortical BBM, determined in vitro, correlates with and contributes to the differences in renal tubular handling of Pi in vivo in various mammalian kidneys.