Background & objective: The transcellular transport of phosphate from the intestinal lumen to the blood requires • Phosphate uptake across the brush border of the enterocyte • Its sojourn through the cytoplasm and • Its exit across the basolateral membrane. The rate-limiting step and the main energy driving part of absorption is first step. Extensive studies are carried out on the luminal phosphate uptake but not on the exit process of phosphate from the cell. The documented theory is that sodium and phosphate cross the brush border bound to a single bifunctional carrier according to the gradient created for sodium by the activity of Na+-K+ ATPase present on the baso-lateral membrane. So the present study is carried out on exit of phosphate from the baso-lateral membrane and its relationship to the functioning of Sodium Potassium ATPase and to sodium gradient. Materials and methods: Everted gut sacs of 6cm of the proximal intestine were prepared from Swiss male albino mice. The sacs were filled with 0. 5ml of serosal fluid and placed in a mucosal medium. After incubation of an hour, the amount of phosphate removed from mucosal medium (mucosal uptake) and serosal gain of phosphate (phosphate release) into the serosal compartment are estimated according to Chen’s method. Sodium substitution study was done on replacing sodium chloride by choline chloride to different degrees and the manipulation of the Sod. Pot. ATPase was done to check its role in phosphate transport with the addition of ouabain in serosal compartment, altering the potassium content of the serosal compartment and by the addition of adenosine. Results: Shows that the uptake of phosphate remains unaffected until 90% of sodium was replaced in the incubation medium. However release of phosphate was impaired significantly at 75% of the normal sodium content (p<0.001). Ouabain inhibits the release of phosphate significantly and this inhibitory effect of ouabain is partly removed on stimulating Sod. Pot. ATP ase with adenosine and with excess potassium inside the serosal compartment. Conclusion: Rather than phosphate uptake, it is the process of phosphate release from the enterocyte that requires sodium-potassium ATPase. A small gradient of sodium is only required for the brush border uptake of phosphate.
