SITE OF SUBSTRATE STIMULATION OF JEJUNAL SUCRASE IN THE RAT

To identify the site stimulation of sucrase by a sucrose diet, changes in sucrase-specific activity of jejunal mucosa were studied after introduction of sucrose diet to carbohydrate-deprived rats. Results were correlated with simultaneous changes in villus gradients of sucrase-specific activity. Simultaneous with the introduction of sucrose diet. [3H]thymidine (100 μCi) was administered intravenously, and rates of cell migration measured during adaptation to the new diet. After a 72-h fast, rats fed sucrose diet for 6, 12, or 18 h showed no change in sucrase-specific activity in either whole mucosa or cillus gradients. However, within 18-24 h after starting a sucrose diet, there was a marked rise in whole mucosal sucrase-specific activity above fasting values (99±14 vs. 38±4 μM glucose/min per g protein, P<0.001) in association with the development of a region of increased activity at the lower villus (154±22 vs. 60±9 μM glucose/min per g protein, P<0.02, but with no change in villus tip activity (56 +5 vs 46±8 μM glucose/min per g protein). Similar changes were seen in animals fed 24 h of sucrose diet after a 72-h carbohydrate-free diet. Fasted animals fed sucrose diet for 36 h had increased sucrase-specific activity at the villus tip (144±11 μM glucose/min per g protein) as well as at the lower villus region, and this pattern persisted at 1 wk of sucrose diet. Maximal activity patterns for isomaltase and maltase paralleled those for sucrase, but the villus gradients for lactase were unaffected by sucrose diet. The region of maximal sucrase-specific activity always coincided with or followed the leading edge of radioactivity as determined by liquid scintillation counting. Therefore, sucrose-mediated changes in sucrase activity of the jejunal mucosa in the rat appear to be initiated at the level of the crypt epithelial cell and are expressed after a latent period of 18-24 h during which these cells mature and migrate toward the villus tip.