AN APICAL PERMEABILITY BARRIER TO ND3 NH4+ IN ISOLATED, PERFUSED COLONIC CRYPTS/

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.