Dietary regulation of the renal sodium-phosphate (Na+/Pi) transporter during early ontogeny in the rat

Phosphates are necessary for proper skeletal growth and function, as well as for growth and development of cells. Phosphate repletion depends partly on the function of the renal sodium-phosphate (Na+/Pi) transport system that functions to recover filtered urinary phosphate. It has been suggested that in order to meet the higher phosphate requirement of the developing animal, the weanling rat would have a greater adaptive response to chronic phosphate deprivation than the adolescent rat. The current study sought to characterize the adaptive response to dietary phosphate deprivation in terms of Na+/Pi transporter activity, and mRNA and immunoreactive protein levels. Weanling and adolescent rats were pair fed either a low-phosphate diet (LPD) or a control-phosphate diet (CPD) for 1 week. Maximal rates of transport (Vmax) were not different in weanling or adolescent rats on CPD (weanling 2.13 +/- 0.29 nmol/mg protein/10 sec, and adolescent 1.41 +/- 0.036 nmol/mg protein/10 sec, n = 3). K(m) values were not different in either group on CPD (weanling 0.15 +/- 0.08 mM Pi, and adolescent 0.22 +/- 0.13 mM Pi). There were no difference in mRNA abundance (Na+/Pi transporter/1B15 = 0.194 +/- 0.12 for weanling and 0.230 +/- 0.03 for adolescents, n = 3) or immunoreactive protein levels (Na+/Pi transporter/beta-actin = 0.232 +/- 0.01 for weanlings and 0.300 +/- 0.05 for adolescents, n = 3) in the two groups when fed CPD. After chronic Pi deprivation, the weanling rat showed a greater adaptive response than the adolescent as measured by Vmax values (weanling LPD/CPD = 2.01, P < 0.01; adolescent LPD/CPD not different; n = 3), mRNA signal intensity (weanling LPD/CPD = 1.86, P < 0.05; adolescent LPD/CPD not different; n = 3), and protein signal intensity (weanling LPD/CPD = 3.63, P < 0.01, and adolescent LPD/CPD 1.91, P < 0.05; n = 3). K(m) values were not affected by LPD. Immunohistochemical analysis of kidney cortex showed greater apical staining in both groups on LPD, with the increase being noticeably greater in the weanlings. Furthermore, two-way analysis of variance demonstrates a significant adaptive response in the weanling period in regard to maximum transport capacity (Vmax) and immunoreactive protein (Western), suggesting a synergistic effect between the developmental stage and low-phosphate diet. Therefore, it appears that the adaptive response is greater in the more rapidly developing animal (the weanling), and these results suggest a compensatory mechanism to conserve phosphate during periods of rapid growth.