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

Intestinal inorganic phosphate transport and its regulation have not been studied in fish. In this study, we initially characterized the mechanisms of intestinal inorganic phosphate transport in rainbow trout (Oncorhynchus mykiss) then determined the effects of dietary phosphorus concentrations on intestinal inorganic phosphate uptake, plasma inorganic phosphate, and intestinal luminal inorganic phosphate concentrations. In 11-g trout, the saturable mechanism of brushborder inorganic phosphate uptake had a Kt=1.2 mmol l−1 and a Vmax=0.22 nmol mg−1 min−1, while the diffusive component had a Kd=0.012 min−1. Similar kinetic constants were obtained from 51-g trout, suggesting that development or size had little effect on transport. Tracer inorganic phosphate (1.18 mmol l−1) uptake was almost completely inhibited (>95%) by 20 mmol l−1 unlabeled inorganic phosphate. Inorganic phosphate uptake (0.2 mmol l−1) was strongly inhibited (˜75% inhibition) by phosphonoformic acid, a competitive inhibitor of mammalian inorganic phosphate transport, as well as by the absence of Na+ (˜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 Na+ inorganic phosphate-II transporter of winter flounder. Intestinal luminal and plasma inorganic phosphate concentrations each increased with dietary P concentrations. Intestinal inorganic phosphate, but not proline, absorption rates decreased with dietary phosphorus concentrations. As in mammals and birds, a Na-dependent inorganic phosphate carrier that is tightly regulated by diet is present in trout small intestine.