AJP - Renal Physiology, Vol 250, Issue 2 246-F255, Copyright © 1986 by American Physiological Society
Transport of solute in proximal tubules is modified by changes in medium osmolality
J. C. Williams Jr, D. W. Barfuss and J. A. Schafer
To examine the hypothesis that fluid absorption is driven by
transepithelial osmotic differences, we measured such differences in
proximal tubules perfused under oil and attempted to increase the rate of
fluid absorption (Jv) and hence the absorbate-perfusate osmotic differences
by lowering the osmolality of the perfusate. We were able to consistently
increase Jv in this manner only in proximal straight tubules perfused with
a simple perfusate that contained no bicarbonate or amino acids. With the
simple perfusate, a small but significant increase in the
absorbate-perfusate osmolality difference was seen with increased Jv, which
is expected if the volume absorption is driven by a transepithelial osmotic
difference. In addition, lowering the perfusate osmolality from 290 to 160
mosmol/kg H2O increased the rate of solute absorption from 79 +/- 7 to 91
+/- 8 posmol . min-1 . mm-1; this increase was partly accounted for by an
increase in the rate of absorption of glucose from 6.6 +/- 0.9 to 9.5 +/-
1.1 pmol . min-1 . mm-1. In contrast, with the complete perfusate in
proximal straight tubules there was little or no increment in Jv, no change
in transepithelial osmolality differences, and a decrease in the rate of
solute transport with hypoosmolality from 136 +/- 21 to 87 +/- 22 posmol .
min-1 . mm-1. In proximal convoluted tubules, similar results were
obtained, but a time-dependent decline of Jv complicated the interpretation
of the results in the convoluted tubules. It is hypothesized that the
observed changes in solute transport with hypotonic perfusate may be the
result of changes in membrane permeability that are subsequent to cell
swelling.
