Sodium-dependent phosphate transport in primary cultures of renal tubule cells from young and adult rats

Epidermal growth factor inhibits Na-Pi cotransport in weaned and suckling rats

In the present study, we determined the effect of epidermal growth factor (EGF; 10 microgram/100 g body wt) on sodium gradient-dependent phosphate transport (Na-Pi cotransport) regulation in suckling (12-day-old) and weaned (24-day-old) rats. Weaned rats had higher proximal tubular brush border membrane vesicle (BBMV) Na-Pi cotransport activity (232 +/- 16 in weaned vs. 130 +/- 9 pmol. 10 s-1. mg protein-1 in suckling rats, P < 0.05). Chronic treatment with EGF induced inhibition of BBMV Na-Pi cotransport in both suckling (130 +/- 9 vs. 104 +/- 7 pmol. 10 s-1. mg protein-1, P < 0. 05) and weaned rats (232 +/- 16 vs. 145 +/- 9 pmol. 10 s-1. mg protein-1, P < 0.005). The inhibitory effect was selective for Na-Pi cotransport as there was no inhibition of Na-glucose cotransport. Weaned rats had a higher abundance of BBMV NaPi-2 protein than suckling rats (increase of 54%, P < 0.001) and a twofold increase in NaPi-2 mRNA. The EGF-induced inhibition of Na-Pi transport was paralleled by decreases in NaPi-2 protein abundance in both weaned (decrease of 26%, P < 0.01) and suckling (decrease of 27%, P < 0.01) animals. In contrast, there were no changes in NaPi-2 mRNA abundance. We conclude that proximal tubule BBMV Na-Pi cotransport activity, NaPi-2 protein abundance, and NaPi-2 mRNA abundance are higher in weaned than in suckling rats. EGF inhibits Na-Pi cotransport activity in BBMV isolated from suckling and weaned rats, and this inhibition is mediated via a decrease in NaPi-2 protein abundance, in the absence of a change in NaPi-2 mRNA.

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.

Induction of calbindin D-28K in Madin-Darby bovine kidney cells by 1,25(OH)2D3

Renal calbindin D-28K is a calcium binding protein, localized to the distal nephron, whose expression is reduced in vitamin D deficiency and increased upon administration of 1,25(OH)2D3, the active form of vitamin D. Investigation into the molecular regulation of renal calbindin D-28K expression has been limited by the lack of an established cell line which expresses calbindin D-28K in a vitamin D responsive manner. In the studies described here, we compared the expression of calbindin D-28K and the vitamin D receptor (VDR) in four renal cell lines: Madin-Darby bovine kidney (MDBK) cells, Madin-Darby canine kidney (MDCK) cells, LLC-PK1 pig kidney cells and opossum kidney (OK) cells. We report that MDBK cells express the highest relative levels of calbindin D-28K and the VDR, and that 1,25(OH)2D3 increases calbindin D-28K expression in these cells. No expression of calbindin D-28K was detected in MDCK, LLC-PK1 or OK cells. Kinetic studies indicated that calbindin D-28K protein increased with time for up to 24 hours after a single dose of 1,25(OH)2D3 (10(-7) M) in MDBK cells. Immunofluorescence studies indicated that in control MDBK cells, the majority of calbindin D-28K was localized in cytosol, with a definite concentration in the peri-nuclear region. In 1,25(OH)2D3 treated cells, calbindin D-28K was enhanced in cytosol and was detected within the nucleus. In contrast to heterogeneous primary culture systems, in which a minority of cells express calbindin D-28K, virtually all MDBK cells expressed calbindin D-28K, even in the absence of 1,25(OH)2D3.(ABSTRACT TRUNCATED AT 250 WORDS)