AJP - Gastrointestinal and Liver Physiology, Vol 250, Issue 4 448-G454, Copyright © 1986 by American Physiological Society
Intestinal sugar transport: does the Na+ gradient provide all the energy?
R. D. Baker
It has not yet been established that the energy released from the Na+
gradient during Na+-sugar cotransport by small intestine is sufficient, by
itself, to drive uphill sugar transport under conditions in which other
sources of energy are available and in which sugar is being vigorously
transported. This work attempts to test the thermodynamic sufficiency of
the Na+ mechanism under such conditions using hamster jejunum in vitro.
From data on tissue uptake of D-galactose and unstirred layer
considerations, the concentration ratio for galactose across the apical
membrane during mucosal-to-serosal galactose transport was estimated to be
at least 50 when concentration in the bulk mucosal solution was 2.2 mM.
This estimate and reasonable assumptions regarding intracellular Na+
activity and apical membrane potential were used to calculate the minimum
physiological coupling ratio (i.e., the ratio of coupled net fluxes of Na+
and galactose) required if all the energy necessary for galactose transport
is transferred from Na+. The minimum physiological coupling ratio required
was roughly 1.4. An estimate based on the ratio of transepithelial chord
permeabilities to galactose, instead of the apical concentration ratio,
would be higher. The measured physiological coupling ratio (0.4) was
considerably less than that required. For reasons discussed, this result
should be interpreted with caution. It can be concluded that the issue of
energetic adequacy is not yet settled.
