Renal Adaptation to a Low Phosphate Diet in Rats

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Abstract
The major renal adaptive changes in response to selective dietary phosphate restriction are a marked reduction in urinary excretion of phosphate and an increased urinary excretion of calcium; at the cellular level, there is selective increase in renal cortical brush border membrane phosphate uptake and increase in specific activity of alkaline phosphatase. In the present study we examined whether these functional and biochemical adaptive changes could be blocked by drugs known to inhibit protein synthesis. Administration of actinomycin D or cycloheximide to rats switched from a diet with normal phosphate content (0.7%) to a diet with low (0.07%) phosphate content either completely (actinomycin D) or partially (cycloheximide) prevented the expected decrease in urinary excretion of phosphate and increase in the urinary excretion of calcium. The specific activity of alkaline phosphatase measured in crude membrane fraction (washed 100,000 g pellet) from renal cortical homogenate in animals fed a low phosphate diet and treated with actinomycin D or with cycloheximide was significantly lower than in control animals also on a low phosphate diet receiving placebo; but there were no differences between treated and untreated animals in the activities of two other brush border enzymes, γ-glutamyltransferase and leucine aminopeptidase. Actinomycin D administered to rats maintained on a normal phosphate diet throughout the course of the experiment caused an increase in the urinary excretion of phosphate on the last (6th) day of the experiment but did not change urinary excretion of calcium. In acute clearance experiments, infusion of actinomycin D to rats adapted to a low phosphate diet did not increase fractional excretion of phosphate. In separate experiments, using the same dietary protocol as above, brush border membrane fraction (vesicles) was prepared from renal cortex of rats sacrificed at the end of the experiment. In this preparation Na+-dependent 32Pi and d-[3H]glucose uptake and activities of brush border enzymes membrane were determined. Brush border membrane vesicles prepared from rats fed a low phosphate diet showed significantly higher Na+-dependent 32Pi uptake compared with rats fed a normal phosphate diet. This increase in 32Pi uptake was completely prevented when rats on a low phosphate diet were simultaneously treated with actinomycin D. These differences were specific for 32Pi transport as no differences were observed in d-[3H]glucose uptake among the three groups. There was a positive correlation (r = 0.82, P < 0.01) between 32Pi uptake and specific activity of alkaline phosphatase measured in aliquots of the same brush border membranes, whereas no such correlation was observed with two other brush border membrane enzymes γ-glutamyltransferase and leucine aminopeptidase. These observations show that actinomycin D prevents both the functional and cellular renal adaptive changes induced by a low phosphate diet. Taken together, these observations suggest that renal adaptation to a low phosphate diet could be prevented by inhibition of de novo protein synthesis.

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