Dietary phosphorus regulates intestinal transport and plasma concentrations of phosphate in rainbow trout

Intestinal inorganic phosphate transport and its regulation have not been s tudied in fish. In this study, we initially characterized the mechanisms of intestinal inorganic phosphate transport in rainbow trout (Oncorhynchus my kiss) then determined the effects of dietary phosphorus concentrations on i ntestinal inorganic phosphate uptake, plasma inorganic phosphate, and intes tinal luminal inorganic phosphate concentrations. In Il-g trout, the satura ble mechanism of brushborder inorganic phosphate uptake had a K-1 = 1.2 mmo l l(-1) and a V-max = 0.22 nmol mg(-1) min(-1), while the diffusive compone nt had a K-d = 0.012 min(-1). Similar kinetic constants were obtained from 51-g trout, suggesting that development or size had little effect on transp ort. Tracer inorganic phosphate (1.18 mmol l(-1)) uptake was almost complet ely inhibited (>95%) by 20 mmol l(-1) unlabeled inorganic phosphate. Inorga nic phosphate uptake (0.2 mmol l(-1)) was strongly inhibited (similar to 75 % inhibition) by phosphonoformic acid, a competitive inhibitor of mammalian inorganic phosphate transport, as well as by the absence of Na+ (similar t o 90% inhibition). Northern blot and reverse transcription-polymerase chain reaction indicated that the intestinal inorganic phosphate transporter in trout is not related to the cloned Naf inorganic phosphate-II transporter o f winter flounder. Intestinal luminal and plasma inorganic phosphate concen trations each increased with dietary P concentrations. Intestinal inorganic phosphate, but not proline, absorption rates decreased with dietary phosph orus concentrations. As in mammals and birds, a Na-dependent inorganic phos phate carrier that is tightly regulated by diet is present in trout small i ntestine.