AJP - Gastrointestinal and Liver Physiology, Vol 250, Issue 4 469-G474, Copyright © 1986 by American Physiological Society
Transport of butyric acid in vascularly perfused anuran small intestine: importance of pH and anion transport
D. Hollander, E. M. Gerard and C. A. Boyd
Butyric acid transport was studied in the isolated, vascularly perfused
frog small intestine. At luminal butyric acid concentrations of 5-50 mM,
absorption was a nonlinear function of the luminal concentration, whereas
the relationship of absorption to concentration remained linear at 0-1,000
microM. The most important factor regulating the rate and direction of
butyric acid transport was the pH. We used unidirectional flux analysis to
determine net transport across the epithelium while the pH of the luminal
or vascular compartments was changed. We found a four- to fivefold decrease
in butyric acid transport into the portal circulation as the lumen pH was
increased from 6.0 to 8.0. The pH of the vascular perfusate influenced the
vascular-to-lumen transport of butyric acid in the same proportions. The
second important regulatory factor of butyric acid transport was the
4,4'-diisothiocyananostilbene-2,2'-disulfonic acid (DIDS)-sensitive anion
transport protein. DIDS added to the lumen at 10(-6) M decreased butyric
acid transport by approximately 40% at pH 7.4. DIDS also inhibited butyric
acid transport when added to the vascular perfusate or when transport was
measured in a vascular-to-lumen direction. We suggest that, at the
relatively low pH of the proximal small intestine, butyric acid becomes
protonated and lipophilic and is mainly transported directly through the
cell membrane. At the more alkaline pH of the distal small intestine
butyric acid is in the ionized form and transport by the DIDS-sensitive
anion transport protein may predominate.
