Phosphorus utilisation and sodium-dependent phosphate co-transporters gene expression in growing pigs fed low available phosphorus diets
Type II sodium-dependent phosphate co-transporters (NaPi) encoded by the SLC34A gene family are involved in renal and intestinal phosphate (Pi) absorption to maintain plasma Pi content. However, the effects of reduced dietary phosphorus (P) content on the expression of these genes and P utilisation in growing pigs are poorly defined. Thus, an experiment was conducted with fifty-four growing pigs (19.5 ± 1.11 kg BW) to examine the effects of reduced dietary available phosphorus (aP) content on P utilisation and NaPi gene expression. Pigs housed in groups of three per pen were randomly allotted to three experimental diets containing 0.23% (control), 0.17% and 0.11% aP to give 6 observations per diet and had ad libitum access to feed and water. All diets contained 0.3% TiO2 as an indigestible marker. After each wk, one pig per pen was housed in a metabolic crate for twenty-four hours to collect fecal and urine sample and then sacrificed to obtain jejunal and kidney sample. Fecal and urine sample were sub-sampled and analysed for P content. Expression of NaPi co-transporter genes (NaPi-IIb in jejunum, NaPi-IIa and NaPi-IIc in kidney) was analysed using quantitative real-time polymerase chain reaction (qRT-PCR). The expression of the NaPi-IIb gene in the jejunum was enhanced up to 250% (P < 0.01) in first wk of pigs fed diet with 0.11% level of aP compared to the other dietary treatments. During wk1, wk2 and wk3, the digestibility of P was lower (P < 0.05) in pigs fed diet with 0.11% level of aP (28.23, 27.14 and 23.33%, respectively) compared to control (39.07, 37.71 and 41.51%, respectively). Expression of NaPi-IIa and NaPi-IIc gene in kidney was enhanced (P < 0.05) by up to 160% and 180% respectively in wk 2 in pigs fed 0.11% aP diet. Urinary P was lower (P < 0.05) in pigs fed 0.11% aP diet (26.03 mg/L) compared to control (85.3 mg/L) in wk 3. In conclusion, dietary P content affected NaPi gene expression and P utilisation. Enhanced expression of NaPi-IIb in jejunum was seen in earlier period of reduced aP diets followed by higher expression of NaPi-IIa and NaPi-IIc in kidney later on. NaPi-IIb was not directly associated with the jejunal P uptake suggesting either post-transcriptional regulation or very low amount of aP for intestine to be able to pick up.
Keywords: Sodium-dependent phosphate co-tranporters, Pigs, Available phosphorus.