AJP - Gastrointestinal and Liver Physiology, Vol 264, Issue 2 319-G324, Copyright © 1993 by American Physiological Society
Bile salts and ileal calcium transport in rats: a neglected factor in intestinal calcium absorption
M. S. Hu, L. H. Kayne, P. A. Willsey, A. B. Koteva, N. Jamgotchian and D. B. Lee
Medical Service, Veterans Health Affairs Medical Center, Sepulveda 91343.
Ileum displays little active transcellular calcium (Ca2+) absorption but is
credited with the bulk of Ca2+ absorbed in vivo. We examined the effect of
taurodeoxycholic acid (TDC, 2 mM), a bile salt, on mannitol (MN, a marker
of intercellular solute traffic) and Ca2+ fluxes in rat ileum. In the
absence of electrochemical gradients between the mucosal (M) and serosal
(S) bathing media in an Ussing chamber, net flux (Jnet) was observed in the
S-to-M direction for both MN and Ca2+, i.e., the unidirectional secretory
S-to-M flux (Js-->m) exceeded the absorptive M-to-S flux (Jm-->s).
Mucosal TDC caused simultaneous increase in transepithelial conductance and
Js-->m for both MN and Ca2+. This was followed by even greater increases
in MN and Ca2+ Jm-->s, so that ultimately Jm-->s equaled Js-->m in
each case. In control tissue, Js-->m for Ca2+ appeared to permeate
exclusively through the intercellular MN pathway while part of Jm-->s
for Ca2+ appeared to traverse through a non-MN route. After the TDC-induced
increase in intercellular solute permeability, both Ca2+ fluxes appeared to
traverse through the aqueous MN conduit. During the postprandial state, the
presence of bile salts and the relative abundance of Ca2+ in ileal lumen
can cause bulk Ca2+ absorption through the intercellular pathway.
