AJP - Gastrointestinal and Liver Physiology, Vol 255, Issue 2 151-G157, Copyright © 1988 by American Physiological Society
Regulatory signals for intestinal amino acid transporters and peptidases
R. P. Ferraris, W. W. Kwan and J. Diamond
Department of Physiology, University of California Medical School, Los Angeles 90024.
Dietary protein ultimately regulates many processes involved in protein
digestion, but it is often unclear whether proteins themselves, peptides,
or amino acids (AAs) are the proximate regulatory signal. Hence we compared
several processes involved in protein digestion in mice adapted to one of
three rations, identical except for containing 54% of either casein, a
partial hydrolysate of casein, or a free AA mixture simulating a complete
hydrolysate of casein. We measured brush-border uptakes of seven AAs that
variously serve as substrates for four AA transporters, and brush-border
and cytosolic activities of four peptidases. The three rations yielded
essentially the same AA uptake rates. Peptidase activities tended to be
lower on the AA ration than on the protein ration. In other studies, all
three rations yielded the same rates of brush-border peptide uptake;
protein is only modestly more effective than AAs at inducing synthesis of
pancreatic proteases; and, depending on the animal species, protein is
either much less or much more effective than AAs at stimulating release of
cholecystokinin and hence of pancreatic enzymes. Thus the regulators of
each process involved in protein digestion are not necessarily that
process's substrate. We call attention to other cases in which the
functional significance of regulatory signals remains to be understood.
