Pb. Bijlsma et al., DIFFERENTIAL IN-VIVO AND IN-VITRO INTESTINAL PERMEABILITY TO LACTULOSE AND MANNITOL IN ANIMALS AND HUMANS - A HYPOTHESIS, Gastroenterology, 108(3), 1995, pp. 687-696
Background/Aims: Clinical interpretation of urinary recovery ratios of
lactulose and mannitol is hampered by incomplete understanding of the
mechanisms of transmucosal passage. The aim of this study was to comp
are in vivo and in vitro probe permeability. Methods: Stripped sheets
of small intestine from rodents and human biopsy specimens were mounte
d in Ussing chambers, and mucosa-to-serosa fluxes of lactulose and man
nitol were determined. Urinary recovery of orally applied probes was m
easured in rodents, cats, and humans. Results: In vitro lactulose/mann
itol flux ratios were close to 0.8 in all species. Urinary recovery ra
tios differed between rodents and cats or humans; low ratios in cats a
nd humans were due to high mannitol recovery. Conclusions: Interspecie
s variation in urinary recovery of mannitol is caused by differences s
pecific for the intact small intestines in vivo. Because hyperosmolali
ty of villus tips in vivo varies, being highest in humans and cats as
a result of vascular countercurrent multiplication, it is hypothesized
that the high urinary recovery of mannitol in these species is caused
by solvent drag through pores that allow the passage of mannitol but
not of lactulose. Therefore, the lactulose/mannitol ratio is primarily
a standard for the normal functioning of villus epithelial cells in m
etabolite absorption and for normal villus blood flow.