AJP - Renal Physiology, Vol 245, Issue 6 743-F754, Copyright © 1983 by American Physiological Society
Flow-dependent water permeability of the rabbit descending limb of Henle's loop
T. Miwa and M. Imai
Because completely opposite results have been reported on the water
permeability of the rabbit descending limbs of Henle's loop (DLH), we
rigorously examined water permeability of the upper portion of the
descending limb of the rabbit long-looped nephron. Even when the
double-cannulation method was used in an attempt to reduce the resistance
of tubular outflow, the collected fluid-to-perfusate inulin ratio was equal
to or very close to the bathing fluid-to-perfusate osmolality ratio,
indicating that osmotic equilibration occurred along the tubule by
absorption of water. When perfusion rates were controlled by varying the
height of the fluid reservoir connected to the perfusion pipette, osmotic
(Pf) as well as diffusional (Pdw) water permeability was shown to be
correlated with perfusion rate and/or perfusion pressure. Pf and Pdw at
zero perfusion rate as determined from the values of the intercept of
regression lines were 253 X 10(-3) and 4.54 X 10(-3) cm X s-1,
respectively. The maximal values for Pf and Pdw were 737-1,098 X 10(-3) and
18.3 X 10(-3) cm X s-1, respectively. By changing the resistance to
perfusion at the tubular outflow, it was shown that changes in Pf
paralleled changes in perfusion rate rather than changes in perfusion
pressure. Under stop-flow conditions the luminal fluid volume rapidly
decreased after the osmolality of the bathing fluid was increased,
suggesting that the segment is highly permeable to water even at zero flow
rate. Reflection coefficients for urea and NaCl were 1.01 and 0.82,
respectively. These data support the view that this segment is highly
permeable to water and that increases in osmolality along the DLH in vivo
may be accounted for mainly by abstraction of water rather than addition of
solutes.
