Micropuncture Study of Inorganic Phosphate Excretion

The mechanism by which the mammalian kidney excretes inorganic phosphate has not been fully elucidated. The only micropuncture data available are those of Walker and Hudson (1), and they deal with the mode of phosphate excretion in two amphibian species. These investigators reported that the inorganic phosphate concentration rises progressively along the proximal and distal nephron of the frog and Necturus. In the frog this rise can be accounted for by tubular fluid reabsorption, but in the Necturus the secretion of phosphate could not always be excluded. Evidence obtained by clearance experiments in vertebrates indicates significant species differences. Thus, renal tubular phosphate secretion has been conclusively demonstrated in certain fish (2-4), the alligator (5), and the chicken (6). The mechanism of phosphate excretion in the mammalian kidney, on the other hand, has remained, to some extent, controversial. According to Pitts and Alexander, in the dog (7) there is a maximal tubular reabsorptive capacity for phosphate, the renal transfer of phosphate being achieved by a combination of glomerular filtration and subse-