AJP - Renal Physiology, Vol 246, Issue 1 3-11, Copyright © 1984 by American Physiological Society
Hydrogen ion permeability of the rabbit proximal convoluted tubule
L. L. Hamm, L. R. Pucacco, J. P. Kokko and H. R. Jacobson
Acidification of luminal fluid in the proximal convoluted tubule has been
modeled as a pump-leak system. Using isolated perfused rabbit proximal
convoluted tubules in a HCO-3/CO2-free in vitro environment, we studied "H+
leak" by imposing pH gradients across the tubule and measuring the change
in pH from perfusate to collected fluid. Active acidification was inhibited
by acetazolamide with and without hypothermia. At 21 degrees C a
symmetrical H+ leak with an apparent permeability coefficient of
approximately 0.15 cm X s-1 was found with either a lumen-to-bath or
bath-to-lumen [H+] gradient. At 37 degrees C a much higher apparent
permeability coefficient was found that was dependent on luminal lactate.
Phosphate movement did not affect H+ fluxes significantly. Without luminal
lactate, the apparent permeability coefficient was 0.31 cm X s-1. Although
this permeability coefficient is larger than other ionic permeability
coefficients in this segment, it is not sufficient to account for a
significant H+ leak compared with rates of acidification or bicarbonate
reabsorption. To investigate the role of Na+-H+ exchange in mediating the
observed H+ leak, we perfused tubules with low [Na+] solutions with and
without amiloride (10(-3) M). Neither the lower [Na+] nor the presence of
amiloride diminished the apparent [H+] permeability coefficient. We
conclude that a H+ leak pathway independent of Na+-H+ exchange is present
in the proximal convoluted tubule.
