AJP - Renal Physiology, Vol 256, Issue 4 577-F582, Copyright © 1989 by American Physiological Society
Proximal bicarbonate absorption independent of Na+-H+ exchange: effect of bicarbonate load
N. Bank, H. S. Aynedjian and B. F. Mutz
Department of Medicine, Montefiore Medical Center, Bronx, New York.
To study proximal tubule bicarbonate absorption that is not due to the
neutral Na+-H+ antiporter, mid to late proximal convolutions of the rat
kidney were microperfused in vivo with a sodium-free choline solution
containing 10(-3) M amiloride. The average sodium concentration resulting
from sodium influx was 12 mM. At such low intraluminal [Na+], 10(-3) M
amiloride should have inhibited the Na+-H+ antiporter by greater than 95%.
When 25 mM HCO3- was in the perfusion fluid, measured total CO2 absorption
was 100 pmol.mm-1.min-1. When luminal [HCO3-] was raised to 50 mM, and
blood [HCO3-] was also raised to approximately 50 mM to avoid a
transepithelial HCO3- concentration gradient, total CO2 absorption
increased to greater than 300 pmol.mm-1.min-1. Thus raising intraluminal
HCO3- concentration caused a marked increase in total CO2 absorption even
though intraluminal [Na+] was low and amiloride was present. Control
perfusions containing 140 mM Na+ yielded total CO2 absorption that was
approximately 100 pmol.mm-1.min-1 higher than with the respective
sodium-free perfusion solutions. In additional experiments, either DCCD or
NEM was added to sodium-free perfusion solutions to inhibit H+-ATPase.
These inhibitors reduced Na+-H+ independent total CO2 absorption markedly.
Our observations suggest that under physiological acid-base conditions,
sodium-independent H+ secretion can account for approximately 50% of total
HCO3- absorption in mid to late proximal convolutions. This mechanism is
stimulated by an increase in ambient HCO(-3) concentration to a degree that
might account for the load-dependency of proximal HCO(-3) absorption in
these segments of the proximal tubule.(ABSTRACT TRUNCATED AT 250 WORDS)
