Comparative studies on Na-dependent Pi transport in ovine, caprine and porcine renal cortex

Abstract

In contrast to monogastric species, renal excretion of inorganic phosphate (P_i) in ruminants is low and this could be attributed to an almost complete tubular P_i reabsorption. However, the functional and regulatory basis for this phenomenon has not yet been clarified. Therefore, it was the aim of the present study to characterize the kinetic parameters of the tubular P_i reabsorption system as affected by P or Ca depletion using preparations of renal cortex brush border membrane vesicles (BBMV) from goats and sheep and to compare the data with respective parameters of porcine preparations. Na-dependent P_i uptake into renal cortex BBMV as a function of P_i concentration showed typical Michaelis-Menten kinetic and respective Scatchard plot analysis of the specific P_i uptake revealed linearity indicating the predominant presence of a single type of P_i transporters in the preparations. Under control conditions V_max values of Na-dependent P_i uptake into BBMV were highest in goats and sheep and lowest in pigs (1.98, 1.39 and 0.95 nmol · mg⁻¹ protein · 10 s⁻¹, respectively). K_m values were not different between goats and sheep under all feeding conditions and ranged from between 0.34 mmol · l⁻¹ and 0.55 mmol · l⁻¹ which was three- to five-times higher than that found in pigs (0.11 mmol · l⁻¹). Oligonucleotides derived from rat kidney cortex type IIa Na/P_i cDNA were used for reverse transcriptase-polymerase chain reaction (RT-PCR) in goat, sheep and pig kidney cortex. The products isolated were 768 bp for sheep and pigs and 765 bp for goats, with the respective amino acids sequences, representing a segment of approximately 40% in length of the entire transporter, exhibiting an at least 92% sequence homology between different species. From the results, involvement of type IIa Na/P_i cotransport in tubular P_i reabsorption in small ruminants can be postulated. However, it should not be considered that a potential role of other P_i transport systems be completely be excluded. Interestingly, neither P nor Ca depletion caused significant effects on Na-dependent P_i transport capacities and affinities in goats and sheep. From this, parathyroid-hormone independent regulatory pathways of tubular P_i reabsorption can be assumed.