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Abstract

Studies of luminal and contraluminal membranes isolated from epithelial cells have contributed markedly to our understanding of the molecular mechanisms, the driving forces and the sequence of events involved in transcellular transport of solutes. They were extended recently to investigate the properties of phosphate transport systems in renal and intestinal brush border and basal-lateral plasma membrane vesicles. The investigations revealed that both in the renal proximal tubule and in the small intestine the luminal membranes but not the contraluminal membranes contain a sodium-phosphate cotransport system. This suggests that the driving force for the transcellular phosphate movement is derived from the sodium gradient present and maintained across the brush border membrane. The importance of the sodium cotransport system for the active phosphate reabsorption in the proximal tubule is further indicated by the fact that parathyroid hormone (PTH), which decreases the phosphate reabsorption in the renal proximal tubule also reduces the capacity of the sodium-phosphate cotransport system in isolated renal brush border microvilli