The Physiological Relevance of the Adaptive Capacity of Intestinal Phosphorus Absorption
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Intestinal phosphorus absorption is a key contributor to the body phosphorus pool, but much is unknown regarding physiological adaptations in intestinal phosphorus absorption that occur in vivo. We sought to measure changes in intestinal phosphorus absorption efficiency and phosphorus balance in adolescent females and in rats in response to several factors, using physiologically relevant assessment approaches including whole-body phosphorus balance techniques and in situ ligated intestinal loop absorption methods.
We first assessed phosphorus balance and net phosphorus absorption in female adolescents from a controlled crossover study with two levels of calcium intake. Despite an increased calcium intake of 600 mg/day, there was no change in phosphorus balance, nor a significant change in net phosphorus absorption.
Next, we measured intestinal phosphorus absorption efficiency with the in situ ligated loop method in healthy Sprague Dawley rats as well as the Cy/+ rat model of progressive kidney disease. We found 10-week-old healthy rats had a small but higher absorption efficiency of phosphorus compared to 20- and 30-week-old rats, while 20-week Cy/+ rats had higher absorption efficiency than 30-week-old. Each of these results corresponded to net phosphorus absorption from balance as well as the concentration of 1,25-dihydroxyvitamin D3. In healthy rats, there was no effect of altering the level of phosphorus in the diet on absorption efficiency. In Cy/+ rats, kidney disease produced a small increase in absorption efficiency, contrary to the predicted decrease that would occur with lower 1,25-dihydroxyvitamin D3 observed in CKD. Gene expression of the major intestinal phosphate transporter, NaPi-2b, largely followed absorption patterns.
The utility of the Cy/+ model is limited to males as females do not begin to show signs of progressive kidney decline until a much older age. Therefore, we sought to test whether ovariectomy would accelerate kidney disease in Cy/+ females, with the aim of establishing a postmenopausal model of progressive kidney disease. Our results show that kidney disease is not accelerated by ovariectomy in this rat strain, as measured by kidney weight and biochemistries including blood urea nitrogen, creatinine, creatinine clearance, and plasma phosphorus and calcium.
Our results utilizing in situ absorption measures as well as net absorption of phosphorus suggest that some of the factors that are understood to influence the intestinal absorption of phosphorus do not have a significant influence in a physiological context.