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

Abstract

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 (approximately 75% inhibition) by phosphonoformic acid, a competitive inhibitor of mammalian inorganic phosphate transport, as well as by the absence of Na+ (approximately 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.

Contributions of different NaPi cotransporter isoforms to dietary regulation of P transport in the pyloric caeca and intestine of rainbow trout

“…There are modest differences in Pi transport kinetics between intestine, as reported previously (Avila et al, 2000), and PC from the present study. The Km and Vmax values are, respectively, 2.5 and 3.4 times higher in the intestine (1.2·mmol·l -1 , 220·nmol·g -1 ·min -1 ) than in PC (0.474·mmol·l -1 , 64.6·nmol·g -1 ·min -1 ).…”

“…Excess 'unlabeled' Pi reduced tracer Pi uptake bỹ 85% in PC (Fig.·3) and ~95% in the intestine (Avila et al, 2000). PFA reduced both PC and intestinal Pi uptake bỹ 70%.…”

“…Tissue Pi uptake rates were determined in vitro according to Karasov and Diamond (1983) modified by Avila et al (2000). Briefly, each sleeve of PC (1·cm long) was carefully everted and tied to a stainless steel rod of an appropriate diameter.…”

“…The Km and Vmax values are, respectively, 2.5 and 3.4 times higher in the intestine (1.2·mmol·l -1 , 220·nmol·g -1 ·min -1 ) than in PC (0.474·mmol·l -1 , 64.6·nmol·g -1 ·min -1 ). These differences may be due to differences in the transporters (see below) or to developmental stages, as the intestinal Pi transport (Avila et al, 2000) was determined using much smaller fish (11·g·body·mass). The Km of PC, however, is similar to the values reported for mammalian intestine (Danisi and Murer, 1991).…”

Dietary phosphorus requirement of juvenile black seabream, Sparus macrocephalus

“…The difference is probably due to fish species, fish size, diet composition, phosphorus sources and culture system (Schwarz, 1995;Shearer, 1984Shearer, , 1995Shearer, , 2000El-Zibdeh et al, 1995b;Riche and Brown, 1999;Avila et al, 2000;Lall and Vielma, 2001;Satoh et al, 2002). On the other hand, the difference is also due to the response criteria and statistical method/model employed in these studies ( Figs.…”