RENAL AND INTESTINAL PI TRANSPORT ADAPTATION TO LOW PHOSPHORUS DIET IN UREMIC RATS

The normal response of the kidney and intestine to a low-phosphorus diet (LPD) is an increased rate of Na+-dependent P(i) transport by their brush border membranes (BBM). Dietary phosphorus restriction is used to reduce P(i) accumulation in chronic renal failure. It is not known, however, if the uremic state alters the adaptive responses to an LPD. The adaptive response of the renal and intestinal BBM vesicles (BBMV) to LPD in acutely uremic (NX) and sham-operated (SH) control rats placed on a normal diet or an LPD was studied. In renal BBMV, the initial Na+ gradient-dependent P(i) transport was lower in NX than in SH rats. Na+-independent uptake was unchanged. Thyroparathyroidectomy did not reverse the reduced P(i) transport in NX rats. Kinetic studies showed a reduction of the apparent V(max) for P(i) in BBMV from NX compared with SH rats (738 +/- 69 and 1,078 +/- 90 pmol/5 s.mg for NX and SH rats, respectively; P< 0.05; N=5) with no change in the apparent K(m). In intestinal BBMV, the initial Na+ gradient-dependent P(i) transport was not different between SH and NX rats. There was also no difference in P(i) transport kinetics between SH and NX rats. The adaptive response to an LPD persisted in renal and intestinal BBMV from NX rats and was comparable to that observed in SH rats: +54% for SH versus +48% for NX rats in kidney BBMV and +70.2% for SH versus +71.8% for NX rats in intestinal BBMV. It was concluded that uremia does not affect the adaptive response of the renal or intestinal BBM to dietary phosphorus restriction.