Crcadian Rhythmicity of Glycyl-L-leucine Absorption Mechanism in Rat Small Intestine

Abstract

Adult rats fed ad libitum and on restricted-feeding (food available between 10 : 00 and 16 : 00) regimen showed feeding-cued circadian rhythms in the activity patterns of glycyl-L-leucine and L-leucine absorption by the proximal small intestine. Absorption of the two compounds by the distal small intestine followed these similar, but less overt, rhythmic patterns. The glycyl-L-leucine hydrolase activity of cytosolic fraction of the mucosal homogenates from the proximal intestine also exhibited a similar daily rhythm in the rats on restricted-feeding regimen, but no discernible rhythm in the rats fed ad libitum. Rhythmicity of the hydrolase activity was hardly noticeable in the membrane fraction on either feeding regimen. Restricted-feeding resulted in an enhanced daily average level for those intestinal activities. In the presence of bestatin, rhythmicity of glycyl-L-leucine absorption by the proximal intestine was unimpaired while cytosolic and membrane hydrolysis of the dipeptide was severely inhibited. These results were interpreted as indicating that the glycyl-L-leucine absorption rhythm is associated to a limited extent with daily fluctuations in the activities to hydrolyze the dipeptide and to transport the resulting free amino acids, but to a large extent with change in the activity to transport the dipeptide intact. It is suggested that the peptide absorption rhythm is related to someaspect of the luminal content rather than to its cyclic ingestion alone.