AJP - Renal Physiology, Vol 252, Issue 4 621-F626, Copyright © 1987 by American Physiological Society
Cholera toxin enhances adenylate cyclase-dependent transport in toad urinary bladder
B. S. Hoch and S. D. Levine
Cholera toxin (CT) irreversibly ADP-ribosylates and activates the
nucleotide-stimulatory (Ns) subunit of adenylate cyclase in many tissues,
thereby eliciting cyclase-dependent functions. Although earlier studies
performed at room temperature could not demonstrate CT-stimulated water
transport in toad urinary bladder, subsequent work in other tissues has
emphasized the need for incubation at 35-37 degrees C to effect
ribosylation and the subsequent physiological effects. We found that
incubating tissues with amphibian culture media, rather than Ringer
solution, maintained tissue viability at this higher temperature and
permitted prolonged incubation with CT. At 37 degrees C, in the presence of
0.1 mM phosphodiesterase inhibitor (1-methyl-3-isobutylxanthine, MIX),
0.2-200 nM mucosal CT caused a dose-dependent but submaximal enhancement of
water flux and urea transport. Elimination of MIX from the bath diminished
subsequent CT-induced stimulation, supporting a role for adenosine
3',5'-cyclic monophosphate (cAMP) as mediator of the CT effect. The
increased water flow was stable for greater than 1 h after removal of CT
from the bath, consistent with irreversible stimulation of the cyclase.
Mucosal CT stimulated transport to a greater degree than serosal CT,
paralleling the pattern seen in the intestine, which is compatible with
passage of the toxin's a subunit across the cell to the serosal membrane
cyclase. Exposure of the tissue's mucosal surface to GM1 ganglioside, (the
natural receptor for the CT b subunit) yielded maximal stimulation of water
flow and near-maximal urea transport, presumably by increasing CT's binding
to the cell membrane.(ABSTRACT TRUNCATED AT 250 WORDS)
