THE PARATHYROID GLANDS AND PHOSPHORUS HOMEOSTASIS 12

Abstract

In intact rats fed diets of low P content values close to 1 were found for the ratio of P reabsorbed by the renal tubules to P filtered at the renal glomeruli (T. R. P./G. F. P.). As dietary P was increased progressively from 0.5 g./sq. m./24 hrs. to 8 g./sq.m./24 hrs. the values for the ratio, T.R.P./G.F.P., fell from approx. 1.0 to 0.1. These changes were unaccompanied by major changes in plasma inorganic P concn. but were accompanied by histologic evidence of increasing parathyroid activity. In parathyroidectomized rats values for the ratio, T.R.P./G.F.P., were found to remain high despite increases in dietary P, while plasma inorganic P concn. showed wide fluctuations directly related to dietary P content. In normal human subjects admn. of parathyroid extract regularly caused a diminution in the value of the ratio, T.R.P./G.F.P., and variations in P intake caused changes in the ratio similar to those observed in intact animals. The authors conclude that the relative constancy of serum P concn. in normal individuals is mediated to a major extent by homeostatic variations in parathyroid activity. Their data indicates that parathyroid hormone determines the amt. of P reabsorbed by the renal tubule: relative to the amt. filtered by the renal glomerulus. They suggest therefore that the value for the ratio, T.R.P./G.F.P., can be used as an inverse index of the parathyroid status of an individual at any given time.