Mechanism of phosphate transport adaptation in rat intestinal and renal brush border membranes

Cholecalciferol modulates plasma phosphate but not plasma vitamin D levels and intestinal phosphate absorption in rainbow trout (Oncorhynchus mykiss)

Since the vitamin D endocrine system modulates phosphorus homeostasis and regulates inorganic phosphate (Pi) uptake by the small intestine in mammals and birds, we determined the effects of dietary cholecalciferol (vitamin D3) on Pi uptake by the small intestine, Pi concentrations in the plasma, Pi concentrations in the intestinal lumen, intestinal weights, liver weights, and concentrations of vitamin D metabolites in the plasma of rainbow trout (Oncorhynchus mykiss) fed phosphorus-sufficient (0.6 g/100 g) diets. Five groups of trout initially weighing 55.8 +/- 0.6 g were fed purified diets containing 0, 300, 2,500, 10,000, and 40,000 IU vitamin D3/kg diet over a 7- to 8-day feeding period. Plasma Pi concentration was higher in trout fed 2,500-40,000 IU/kg diet (8.26 +/- 0.27 mmol/L) than in those fed 0 and 300 IU/kg (6.99 +/- 0.30). Liver weights were 30-50% greater in fish fed 0 IU/kg than in those fed 300-40,000 IU/kg. There were no significant, diet-related differences in plasma levels of 25-hydroxycholecalciferol [25(OH)D3] and 1,25 dihydroxycholecalciferol [1,25(OH)2D3]. Increasing levels of dietary cholecalciferol also did not enhance in vitro Pi uptakes by the intestine (range of means: 0.22-0.29 nmol/mg tissue. min) and Pi concentrations in the intestinal lumen (8.5-13.5 mmol/L). Pi uptake did not differ among tissues incubated in vitamin D3, 25(OH)D3, or 1,25(OH)2D3. These results demonstrate that when fish are fed P-sufficient diets, dietary cholecalciferol increases plasma Pi concentrations but decreases liver weights, alterations which are not accompanied by changes in intestinal weight, Pi uptake by the intestine, Pi concentration in the intestinal lumen, and circulating metabolites of cholecalciferol.

Adaptation to changes in dietary phosphorus intake in health and in renal failure

Phosphate (Pi) homeostasis is maintained by the ability of the kidneys to adjust the tubular reabsorption of Pi to changes in the dietary intake of phosphorus. Renal tubular Pi reabsorption increases with the ingestion of a low-phosphorus diet (LPD) and decreases when a high-phosphorus diet (HPD) is consumed. A similar adaptive mechanism is also operative at the intestinal microvillus. The adaptive changes in Pi reabsorption are independent of parathyroid hormone production and are paralleled by similar changes in the Na+-dependent Pi transport at the brush border membrane (BBM). Type II Na+-Pi cotransporters (NaPi-2) are mainly involved in such regulatory mechanisms. Chronic dietary phosphorus restriction leads to increased Na+-Pi cotransport rate, along with increased NaPi-2 protein and mRNA abundance. In acute dietary phosphorus restriction, transport rate and NaPi-2 protein are also increased, but mRNA abundance remains unchanged. A shuttling mechanism involving translocation of cotransporters from intracellular pools to the BBM is involved in the rapid proximal tubular adaptation. The intestinal adaptation to changes in dietary phosphorus are similar to those described for the renal Pi transport, but the molecular structure of the intestinal Na+-Pi cotransporter is not known. When nephron mass is reduced, phosphate homeostasis is maintained through enhanced Pi excretion by residual nephrons. The adaptation to renal mass reduction is mediated by increased parathyroid hormone (PTH) production and by PTH-independent mechanisms, including increased intrarenal dopamine production. The adaptive changes of Pi transport to dietary phosphorus restriction can counteract the effect of dietary phosphorus reduction often prescribed in patients with renal failure. However, because of the reduced filtered load of Pi, the overall impact on serum Pi concentration is minimal.