Phosphate transport in brush‐border membranes from control and rachitic pig kidney and small intestine.

Abstract

SUMMARY1. Na-Pi co-transport was analysed using renal cortical and small intestinal brush-border membrane vesicles which were isolated from control (normal, heterozygotes) and rachitic piglets (homozygotes).2. A kinetic analysis of Na-dependent initial linear uptake of Pi was performed using vesicles obtained from control animals. The results suggest similar kinetic properties for the renal and small intestinal co-transport system. (i) A sigmoidal dependence on Na concentration of Pi uptake suggests the involvement of more than one Na ion in the co-transport. (ii) Increasing Na concentration leads to an increase in the apparent affinity of the transport system for Pi and has minimal effect on the apparent Vmax (maximum velocity of uptake). (iii) Increasing pH leads to an increase in Pi transport rate.3. The kinetic characteristics of the Na-Pi co-transport system in vesicles obtained from rachitic animals were similar to those in controls. The apparent Vmax, but not the apparent Km (Michaelis constant) for Na and Pi, is reduced in intestinal and renal brush-border membranes isolated from rachitic animals as compared to control animals. Injection of vitamin D3, three days prior to killing of rachitic litter-mates, increased the Na-Pi uptake rate in the brush-border membrane vesicles isolated from these piglets. 4. It is concluded that intestinal and renal brush-border membranes from piglets contain a similar Na-Pi co-transport system and that in vitamin-D-dependent rickets the number of operating transport units is reduced in both membranes.

Nutrient digestibility response to graded dietary levels of sodium chloride in weanling pigs

“…Absorption of P in the small intestine occurs largely via a transcellular pathway that is initiated by transport of P across the brush border membrane. Transport of P through the apical membrane of the intestinal epithelial cell followed by reabsorption across the renal cell brush border membrane is a secondary active transport mechanism which, in non‐ruminants, is dependent upon Na + (Na + /P i co‐transport) 5, 6. A deficiency of Na + may consequently affect the absorption of P. In this study, APD and TPD increased when the NaCl concentration increased from 0.1 to 0.3%, and then decreased, although APD and TPD in the diets with higher levels of NaCl (0.4–0.6%) were still higher than those in the diet with the lowest NaCl level (0.1%).…”

“…The active transport of P is coupled to cellular Na + uptake by the Na + /P i co‐transporter 5–7. However, only a few studies have been published relating to the effect of NaCl on P i digestibility.…”

Transport of nutrients and electrolytes across the intestinal wall in pigs

“…From functional assays, it was shown in the past for several mammalian species, including the pig that P i uptake across the duodenal brush-border membrane is mediated by a secondary, active Na-dependent P i transport system that is stimulated by calcitriol (Quamme, 1985;Brandis et al, 1987;Caverzasio et al, 1987;Schröder et al, 1998b). In a number of laboratory animals as well as human this P i transport system has been identified as the Na + -P i cotransporter NaPi-IIb, a member of the type II Na + -P i cotransporter family SLC34 (Hilfiker et al, 1998;Murer et al, 2004).…”

Age-dependent regulation of rat intestinal type IIb sodium-phosphate cotransporter by 1,25-(OH)2 vitamin D3

“…Early studies showed that P i transport through the apical membrane of small intestinal epithelial cells is coupled with sodium (4,7,12,21,29,43,47,48). One transporter involved in intestinal P i absorption is the type IIb sodium-coupled P i cotransporter (NaP i -IIb), which has been cloned from rodents and humans (20,25,28,55).…”