Renal transport of phosphate: role of alkaline phosphatase

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Abstract
A new aspect in the study of mechanisms involved in the renal transport of phosphate, the role of alkaline phosphatase (ALP), was introduced by this laboratory in 1977. The present experiments were designed to examine the effects of levamisole, a known inhibitor of ALP, first on in vitro phosphotransferase activity of rat brush border ALP and second on in vivo transport of phosphate and other ionic species, using both clearance and micropuncture techniques.The results indicate that levamisole inhibits in vitro ALP of brush borders obtained from kidney cortex of dogs and rats by reducing the turnover of orthophosphate on the enzyme. When administered in vivo this drug inhibits the net reabsorption of phosphate in these two different mammalian species. Tubular reabsorption of phosphate falls from 87.0 ± 2.9 to 72.1 ± 3.5% when levamisole is administered in the dog femoral vein (25 mM) and from 85.1 ± 3.4 to 71.3 ± 3.2% when levamisole is infused in the dog renal artery (2.5 mM). In the intact rat this parameter falls from 96.7 ± 1.4 to 46.8 ± 9.8% during levamisole and it also decreases from 98.9 ± 0.8 to 67.4 ± 6.7% in the thyroparathyroidectomized animal. The effect of levamisole on the net tubular transport of phosphate is closely related (r = 0.967) to the amount of the drug reaching the tubular lumen from glomerular filtration: that fraction of administered levamisole which first reaches the early segments of the proximal tubule, where the bulk of filtered phosphate is normally reabsorbed.The effect of levamisole appears to be specific for phosphate as no change in the net transport of other ionic species could be documented in the dog experiments. Levamisole produces a significant decrement in renal plasma flow. The mechanism of this effect is not yet determined but certainly created a situation leading to underestimation of the levamisole effect on the net tubular transport of phosphate.Microinjections of 32P either diluted in isotonic saline or administered with flavone phosphate (0.2 mM), a substrate of ALP, were performed in early segments of superficial proximal tubules of the rat. Urinary 32 P recovery averaged 14 ± 4% and 34 ± 8% following saline and flavone phosphate, respectively.The effect of levamisole does not appear to be mediated by changes in parathyroid hormone secretion or other extrarenal humoral substances as a depression of phosphate reabsorption is seen when the drug is administered in the renal artery. The rapid and reversible effect of flavone phosphate suggests that this compound specifically interacts with ALP of brush border membranes.