SUMMARY1. The ontogenic development of the intestinal Na+-glucose co-transporter was measured in lambs as a function of diet. Transport activity was assayed in brushborder membrane vesicles and the expression of transport protein in the brushborder membrane determined by Western analysis.2. Na+-dependent D-glucose transport increased to a maximum (300-700 pmol mg' s') within the first 2 weeks of birth and then declined to negligible amounts (< 10 pmol mg-' s-1) over the next 8 weeks. There was no further change over the next 2-3 years. Early changes were associated with modifications in both the maximum velocity Vma. for transport and expression of carrier protein in the brush-border plasma membrane.3. Maintaining lambs on a milk replacer diet beyond the normal weaning period prevented the normal decline in the expression of Na+-glucose co-transport. At 5 weeks the transport rate was 433 + 150 pmol mg-' s-I in lambs maintained on milk replacer, but only 79 + 40 pmol mg-' s-I in normally reared control lambs.4. Infusing the proximal intestine of 2-to 3-year-old sheep with 30 mM-D-glucose for four days increased the rate of transport 40-to 80-fold above that found in control animals perfused with mannitol. A similar but smaller increase was observed in one animal perfused with the non-metabolizable sugar a-methyl-D-glucopyranoside. The induced increase in glucose transport was correlated with the expression of the co-transporter protein in the brush-border plasma membrane.5. It is concluded that the age-related decline in Na+-glucose co-transport in the sheep intestine is directly due to the decrease in D-glucose (and D-galactose) reaching the small intestine after development of the rumen. These results further suggest that luminal sugar substrates for the co-transporter promote both the maintenance and the up-regulation of the brush-border transport protein and it is the intact sugar itself which controls gene expression during enterocyte maturation. MS 8905 S. P. SHIRAZI-BEECHEY AND OTHERS

SUMMARY1. Change in digestive enzyme activities determined biochemically in brushborder membrane vesicles and cytochemically in isolated villi of lamb proximal intestine has been related to diet, intestinal structure and rumen development during the first 10 weeks of postnatal life.2. Lactase activity halved, dipeptidylpeptidase IV activity doubled and aminopeptidase N and alkaline phosphatase activities remained constant during this period of development. Maintaining lambs on a milk replacer diet for 5 weeks after birth had no effect on this pattern of postnatal change in digestive enzyme activities.3. Structural changes accompanying these selective effects on enzyme expression included a halving of villus height and a doubling of villus width. Villus surface area remained unaffected by these changes in height and width of villi. Crypt depth doubled during the first 10 weeks of postnatal life. Maintaining lambs on a milk replacer diet for 5 weeks did not affect this pattern of change in intestinal structure.4. It appears from these results that postnatal decrease in lactase and increase in dipeptidylpeptidase IV activities are not regulated by factors such as diet, rumen development, or changes in intestinal structure. Attention is drawn to differences encountered between these results and a postnatal modification of glucose transport which clearly is dependent on diet.