Sk. Singh et al., AN APICAL PERMEABILITY BARRIER TO ND3 NH4+ IN ISOLATED, PERFUSED COLONIC CRYPTS/, Proceedings of the National Academy of Sciences of the United Statesof America, 92(25), 1995, pp. 11573-11577
Fermentation of nonabsorbed nutrients in the colon generates high conc
entrations of NH3/NH4+ in the colonic lumen, NH3 is a small, lipophili
c neutral weak base that readily permeates almost all cell membranes,
whereas its conjugate weak acid NH4+ generally crosses membranes much
more slowly. It is not known how colonocytes maintain intracellular pH
in the unusual acid-base environment of the colon, where permeant aci
d-base products of fermentation exist in high concentration. To addres
s this issue, we hand dissected and perfused single, isolated crypts f
rom rabbit proximal colon, adapting techniques from renal-tubule micro
perfusion, Crypt perfusion permits control of solutions at the apical
(luminal) and basolateral (serosal) surfaces of crypt cells. We assess
ed apical- vs. basolateral-membrane transport of NH3/NH4+ by using flu
orescent dyes and digital imaging to monitor intracellular pH of micro
vacuolated crypt cells as web as luminal pH. We found that, although t
he basolateral membranes have normal NH3/NH4+ permeability properties,
there is no evidence for transport of either NH3 or NH4+ across the a
pical borders of these crypt cells, Disaggregating luminal mucus did n
ot increase the transport of NH3/NH4+ across the apical border, We con
clude that, compared to the basolateral membrane, the apical border of
crypt colonocytes has a very low permeability-area product for NH3/NH
4+. This barrier may represent an important adaptation for the surviva
l of crypt cells in the environment of the colon.