Renal expression of calcium and phosphorus transporters: Contrasting responses to dietary calcium and microbial phytase in broilers and growing pigs

Enhanced dietary Ca intake linearly increases intestinal Ca absorption in pigs, but not in broilers, suggesting potential differences in whole body Ca homeostasis. To determine the role of kidney in Ca homeostasis in these species, we varied in growing pigs in experiment (Exp) 1, the dietary Ca content 2·0 v. 9·6 g/kg and phytase 0 v. 500 FTU/kg, in broilers, in Exp 2 the dietary Ca/retainable P from 1·3 to 2·8 and phytase 0 v. 1000 FTU/kg, and in Exp 3 dietary Ca/P from 0·50 to 1·75. Increasing dietary Ca reduced renal mRNA expression of Ca-related transporters (TRPV5, TRPV6, CaBP-D28k and NCX1) and tight junctions (CLDN-12 and -16) in pigs, indicating Ca reabsorption was reduced to maintain Ca homeostasis. In broilers (Exp 2), high dietary Ca increased renal TRPV6, CaBP-D28k and CLDN-2 mRNA, indicating an increased capacity for Ca reabsorption. Moreover, the effect of dietary Ca was enhanced by inclusion of dietary phytase in pigs but reduced in broilers. Furthermore, increasing dietary Ca upregulated inorganic phosphate transporter 1 (PiT-1), while phytase downregulated xenotropic and polytropic retrovirus receptor 1 (XPR1) mRNA expression in pigs; in broilers, dietary Ca downregulated renal mRNA expression of Na-dependent phosphate transporter IIa (NaPi-IIa), PiT-1, PiT-2 and XPR1, while phytase downregulated NaPi-IIa but upregulated PiT-2 and XPR1 mRNA expression. In Exp 3, Ca/P effect on transporter mRNA expression was largely consistent with Exp 2. In conclusion of this study, together with previously measured data about Ca and P homeostasis, in pigs the kidneys play a more regulatory role in Ca homeostasis than in broilers where the intestine is more important for regulation.