AJP - Renal Physiology, Vol 252, Issue 2 357-F360, Copyright © 1987 by American Physiological Society
Comparison of effects of forskolin, cAMP, and vasopressin on Pf/Pd(w) of toad urinary bladder luminal membrane
S. D. Levine and M. Jacoby
We have reported that Pf/Pd(w) (the ratio of osmotic and diffusional water
permeabilities) for the luminal membrane of toad urinary bladder is
approximately 17 for tissues stimulated with either vasopressin or
8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP). In a recent
abstract, Kachadorian and co-workers have shown that tissues stimulated
with adenosine 3',5'-cyclic monophosphate (cAMP) or forskolin have a lower
Pf than would be anticipated from the frequency of aggregates visualized on
the flat portion of the luminal membrane using freeze-fracture electron
microscopy. We report here measurements of Pf/Pd(w) for the luminal
membrane of tissues receiving these agents: Pf/Pd(w) for submaximally
stimulated tissues was the same, regardless of whether the stimulant was
vasopressin (12.7 +/- 0.3), forskolin (13.7 +/- 0.9), or cAMP (12.0 +/-
1.3). The calculated Pd(w)'s for the series barrier were also identical
(6.8 +/- 0.5, 6.5 +/- 0.3, and 8.2 +/- 1.0 X 10(-4) cm/s respectively). Our
data, taken together with those of Kachadorian et al. are consistent with a
number of possibilities: because our methodology does not permit estimation
of Pf for the series barrier, we cannot rule out the possibility of a
"post-luminal barrier" that is rate-limiting for Pf, but not for Pd(w) in
forskolin- and cAMP-stimulated tissues, Pf and Pd(w) of the luminal surface
aggregates could decrease in parallel, so that luminal membrane Pf/Pd(w)
remains constant, and there could be a diminished frequency of fused
aggregate-rich aggrephores, but not of aggregates that are on the flat
portion of the luminal membrane. Only the latter can be unequivocally
quantitated using freeze-fracture electron microscopy.(ABSTRACT TRUNCATED
AT 250 WORDS)
