The Journal of General Physiology, Vol 66, 97-115, Copyright © 1975 by The Rockefeller University Press
Blockage of gallbladder tight junction cation-selective channels by 2,4,6-triaminopyrimidinium (TAP)
JH Moreno
The organic cation 2,4,6-triaminopyrimidinium (TAP) specifically inhibits
Na+ passive permeation (PNa) across gallbladder, small intestine, and
choroid plexus without detectable effect on the Cl(-) permeability,
indicating that Na+ and Cl(-) follow different permeation pathways. In
bullfrog gallbladder, where it was examined in greater detail, the effect
of TAP was shown to be: (a) completely reversible, (b) due only to the
protonated form of 2,4,6-triaminopyrimidine, (c) effective when added to
either one or both sides of the membrane (the rate limiting for the delay
in the response being the diffusion through the unstirred layers), and (d)
exhibiting a typical saturation kinetics, best fitted with the parameters
"Km" = 2.6 mM and maximal effect = 100% inhibition. These data, along with
the fact that the PNa blocking action of chemical analogs of TAP increases
with their ability to donate protons to form hydrogen bonds, suggest that
TAP blocks the cation permeation of the channels by strongly associating,
via hydrogen bonds, with the anionic ligands within the channel.
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