AJP - Renal Physiology, Vol 246, Issue 1 27-F31, Copyright © 1984 by American Physiological Society
Nephron sites of action of nicotinamide on phosphate reabsorption
J. A. Haas, T. J. Berndt, A. Haramati and F. G. Knox
The administration of nicotinamide results in urinary phosphate excretions
similar to those obtained with pharmacologic doses of parathyroid hormone
(PTH). Free-flow micropuncture was performed to localize the nephron
site(s) of inhibition of phosphate reabsorption by nicotinamide or PTH in
thyroparathyroidectomized (TPTX) rats stabilized on a normal or low
phosphate diet. In rats fed a normal phosphate diet phosphaturia was
observed following either nicotinamide or PTH treatment. Nicotinamide
inhibited phosphate reabsorption in the loop of Henle (pars recta) but not
in the accessible proximal tubule. PTH inhibited phosphate reabsorption in
both the accessible proximal tubule and the pars recta. In phosphate
deprivation, the phosphaturic response to either nicotinamide or PTH was
blunted. Although phosphate reabsorption was markedly inhibited in the
accessible proximal tubule with both nicotinamide and PTH, subsequent
reabsorption in the loop of Henle and distal tubule blunted the
phosphaturia. We conclude that nicotinamide primarily inhibits phosphate
reabsorption by the pars recta in rats fed a normal phosphate diet, whereas
it inhibits phosphate reabsorption by the proximal convoluted tubule in
rats fed a low phosphate diet. Furthermore, avid reabsorption of phosphate
in the pars recta accounts for the resistance to the phosphaturic effect of
nicotinamide or PTH seen in rats fed a low phosphate diet.
