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

In contrast to monogastric species, renal excretion of inorganic phosphate (P-i) in ruminants is low and this could be attributed to an almost complet e tubular P-i reabsorption. However, the functional and regulatory basis fo r 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 an d to compare the data with respective parameters of porcine preparations. N a-dependent Pi uptake into renal cortex BBMV as a function of P-i concentra tion showed typical Michaelis-Menten kinetic and respective Scatchard plot analysis of the specific P-i uptake revealed linearity indicating the predo minant presence of a single type of P-i transporters in the preparations. U nder control conditions V-max values of Na-dependent P-i uptake into BBMV w ere 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 betw een 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 tha n that found in pigs (0.11 mmol.l(-1)). Oligonucleotides derived from rat k idney cortex type IIa Na/P-i cDNA were used for reverse transcriptase-polym erase chain reaction (RT-PCR) in goat, sheep and pig kidney cortex. The pro ducts isolated were 768 bp for sheep and pigs and 765 bp for goats, with th e respective amino acids sequences, representing a segment of approximately 40% in length of the entire transporter, exhibiting an at least 92% sequen ce homology between different species. From the results, involvement of typ e IIa Na/Pi cotransport in tubular P-i reabsorption in small ruminants can be postulated. However, it should not be considered that a potential role o f other P-i transport systems be completely be excluded. Interestingly, nei ther P nor Ca depletion caused significant effects on Na-dependent P-i tran sport capacities and affinities in goats and sheep. From this, parathyroid- hormone independent regulatory pathways of tubular P-i reabsorption can be assumed.