AJP - Renal Physiology, Vol 249, Issue 5 769-F779, Copyright © 1985 by American Physiological Society
Effects of metabolic acidosis, alkalosis, and dietary hydrogen ion intake on phosphate transport in the proximal convoluted tubule
G. A. Quamme
Early proximal convoluted tubules were perfused in vivo with equilibrium
Ringer buffered at pH 7.65 and 6.5 to characterize phosphate absorption due
to changes in extracellular and intracellular hydrogen ion concentration.
Phosphate absorption is normally greater from intraluminal pH 7.65 compared
with pH 6.5 in thyroparathyroidectomized rats with fractional phosphate
excretion of 0.5%. Metabolic alkalosis produced by bicarbonate infusion in
rats ingesting normal amounts of phosphate (45 mg/day) resulted in an
increase in overall renal phosphate reabsorption (fractional phosphate
excretion 0.3%). The Jmax and Km values were: pH 7.65, 33.67 +/- 13.49 pmol
X min-1 X mm-1, and 1.74 +/- 0.30 mM; pH 6.5, 24.87 +/- 6.22 and 0.50 +/-
0.25, respectively. By contrast, rats on a high dietary phosphate intake
(180 mg/day) demonstrated a large increase in urinary phosphate excretion
(18%) following bicarbonate infusion, which was due to a decrease in
tubular phosphate absorption from both alkaline and acidic luminal pH
values. Acute metabolic acidosis did not significantly alter tubular
transport at either intraluminal pH value. In contrast, rats maintained on
an elevated dietary acid intake for 5 days had a phosphaturia (fractional
excretion 7.1%) and diminished reabsorptive capacity. Dietary acidosis also
decreased tubular phosphate transport in rats previously maintained on
phosphate-restricted diets. These data suggest that acid-base balance may
modulate tubular phosphate transport independent of intraluminal pH and
phosphate concentration. Further, these changes depend on the chronicity of
exposure and act independent but integral to the effects of parathyroid
hormone and the intrinsic adaptation to dietary phosphate availability.
