It is well documented that plasma concentrations of Pi (inorganic phosphorus)
are higher in developing subjects than in adults. In a previous study, we demonstrated
that the Vmax (capacity) of the Na-Pi cotransport mechanism of the renal brush
border membrane vesicles was higher in immature than mature rats.
In this study, we evaluated the role of a growth hormone in the maintenance of a
higher Vmax observed in immature rats.
In mature rats, serum Pi, and the tubular reabsorption of Pi (TRPi) increased in the
growth hormone treated animals. On the other hand, those values were not changed
by growth hormone treatment in immature rats.
In kinetic analysis, the Km (affinity) values were not different between the control
(growth hormone-untreated) and growth hormone-treated renal brush border membrane
vesicles in both immature and mature rats. The Vmax of the immature rats also was
not changed by growth hormone treatment. On the contrary, Vmax increased significantly
in the growth-hormone treated than the control mature rats.
With the above findings, it seems that immature rats reabsorb Pi maximally even
in the control state, and it is likely that a growth hormone is responsible for the phenomenon
