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THIRST AND VOLUME, ELECTROLYTE HOMEOSTASIS

In vivo fractional P_i absorption and NaP_i-II mRNA expression in rainbow trout are upregulated by dietary P restriction

Department of Pharmacology and Physiology, New Jersey Medical School, University of Medicine and Dentistry of New Jersey, Newark, New Jersey 07103

Submitted 10 March 2003 ; accepted in final form 14 June 2003

Mammalian type II sodium-phosphate cotransporter (NaP_i-II) and inorganic phosphate uptake stimulator (P_iUS) genes are upregulated by dietary phosphorus (P) restriction to increase intestinal and renal P transport, but little is known about NaP_i-II and P_iUS regulation in other vertebrates. We studied the 1) the tissue distribution and dietary regulation of NaP_i-II, P_iUS, and sodium-glucose cotransporter (SGLT1) mRNA and NaP_i-II protein in juvenile rainbow trout (Oncorhynchus mykiss) and 2) effects of dietary P on intestinal P_i absorption in vivo. NaP_i-II, P_iUS, and SGLT1 mRNA were found in the proximal and distal intestine, pyloric ceca, and kidney. P_iUS mRNA was also found in the heart, gill, blood, stomach, liver, skin, and muscle. Tissue distribution of NaP_i-II protein correlated with that of NaP_i-II mRNA except in gill ionocytes where NaP_i-II antibodies recognized related epitopes. Chronic consumption of a low-P diet increased NaP_i-II and P_iUS but not SGLT1 mRNA abundance in the intestine and kidney. Unlike mammals, there was no detectable shift in tissue or cellular localization of NaP_i-II protein in response to dietary P restriction. Regulation of NaP_i and P_iUS mRNA expression was observed only in fish grown under optimal aqueous oxygen concentrations. In vivo fractional absorption of P_i by the intestine decreased in fish fed high-P diets. Decreases in absorption were less pronounced in fish previously fed low-P diets, suggesting that diet history modulates acute regulation of P absorption. Regulation of dietary P_i absorption in vivo may involve a specific change in intestinal NaP_i-II and P_iUS gene expression.

fractional absorption; immunocytochemistry; phosphate uptake stimulator; sodium-glucose cotransporter; sodium-phosphate cotransporter

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