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Summary: This study was designed to determine the critical difference between rat milk and rat chow with respect to their effects on jejunal sucrase activity during the fourth postnatal wk. Rats were weaned onto special diets on postnatal day 17, and jejunal sucrase was assayed on day 28. A pelleted diet containing lactose as sole carbohydrate did not cause depression of sucrase activity. Sucrase values (μmoles/hr/mg protein) were 10.49 ± 0.81 (n = 15) for the lactose chow and 6.65 ± 0.29 (n = 16) for the sucrose chow. This indicates that the nature of the dietary carbohydrate does not account for the sucrase differences of weaned and nonweaned animals. Likewise, the physical consistency of the diet is unimportant because sucrase values were just as high on a liquid diet (10.91 ± 0.77 μmoles/hr/mg protein; n = 8) as on the solid diets. However, when the relative proportions of carbohydrate and fat in the diet were varied, there were significant effects on sucrase activity; values on a low carbohydrate diet (4.30 ± 0.33 μmoles/hr/mg protein; n = 8) being less than one-half those on a high carbohydrate diet (10.91 ± 0.77 μmoles/hr/mg protein; n = 8). Speculation: The terminal maturation of jejunal sucrase is dependent on the normal process of weaning. The critical feature of the weanling diet is that it is a high-carbohydrate, low-fat diet. In the absence of this dietary transition, intestinal development remains incomplete.

Three groups of rats were fed, respectively, a chemically defined diet intragastrically (IG), an equivalent diet intravenously (IV) and solid food orally (CH) for 8 days, and their small intestines and colons compared. All received equal calories. The small intestine was divided into equal proximal (A), middle (B), and distal (C) segments for measurements. Mucosal weight per cm in segments A and B of IG were, respectively, 65 and 38% higher than in IV (P less than 0.01), but 27 and 33% lower than in CH (P less than 0.01). However, in the distal segment, C, mucosal weight in IG was similar to IV and CH was 79% higher (P less than 0.01). DNA and protein followed the same pattern. Segment A sucrase activities were similar in CH and IG and were much higher than in IV (P less than 0.01). Sucrase in IG dropped very rapidly distally so that it became much lower than in CH (P less than 0.05) and similar to IV. Mucosal weight, DNA, and protein in the colon were not significantly different in IG and IV, which were both significantly lower than in CH (P less than 0.01). The results indicate that a chemically defined diet maintains intestinal mass well in the proximal small intestine, but the effect diminishes rapidly in a distal direction so that distal small intestine and colon become atrophied and similar to those in intravenous feeding.

Enzymes capable of hydrolyzing sucrose, trehalose, lactose and cellobiose were detected in the cocoon ofBombyx mori. Sucrase activity was found to be highest and was present only in the outermost layer of the cocoon.

In the adult rat, fed ad lib, the activity of jejunal sucrase shows a diurnal rhythm with a peak during the dark period. Onto-genically this enzyme does not appear until approximately the 16th postnatal day; its activity then rises rapidly and reaches adult levels by the 25th day. Our aim was to determine whether the diurnal rhythm is present from the outset or appears only after sucrase has attained adult levels. Rat pups were raised in litters of 9 with a light/dark cycle of 12h. Solid food and water were available to mothers and pups at all times and mothers were normally removed on the 21st postnatal day. Four litters were studied during the 24h period beginning at 8am on the 19th day. One pup from each litter was removed for assay every 3h. Jejunal sucrase showed considerable variation between pups, but there was no discernable pattern with time. In contrast, when the experiment was repeated with pups aged 22 days, there was a distinct peak in sucrase activity during the early part of the dark period. In order to determine whether the rhythm is cued by the process of weaning, two litters were raised on a feeding schedule which ensured that maternal milk was the only food available to the pups. When studied during the 24h period beginning on the 22nd day, the jejunal sucrase activity of these pups exhibited no rhythm. It is concluded that some component of solid food is responsible for the usual appearance of the diurnal rhythm of this enzyme. (supported by NIH Grant No. HD-10042.)

Increased intake of sucrose (S) is followed in adult and suckling rats by an increase of sucrase activity (SA). This study analyzes the increase of SA in suckling rats evoked by the early feeding of S and other sugars. Ten-day-old rats, removed from their mothers, were gavage fed for 4 days a low carbohydrate milk formula (MF), [10.8% fat, 8% protein, 1.4% carbohydrate-w/v], with or without addition(s) of lactose or fructose (FR), both 13%. Feeding the MF with FR led to diarrhea and an increase of SA. Gastric evacuation of MF with S or FR was significantly slower than MF low in carbohydrates or with lactose. When MF with S or FR were placed directly into the ligated jejunoileum in situ, net water absorption was decreased. Ten-day-old rats kept with the mother, and receiving twice daily 0.5 ml of 5% mannitol in water developed diarrhea within one day, and a marked increase of SA which gradually decreased after the 2nd day of mannitol feeding. Serum corticosterone level was highly increased 8 hrs after the initiation of the mannitol feeding or MF with S or FR. Our experiments thus explain the role of adrenals in increase of SA in sucklings. In contrast to adult rats, increase of SA in suckling rats by dietary sugars is mainly due to the stress caused by diarrhea that is evoked by undigested and/or unabsorbed amount of sugars.

Previous studies indicate long chain fats have trophic effects on post-resection intestinal mucosal adaptation. We have previously shown that short term essential fatty acid (EFA) deficiency impairs normal post-resection mucosal hyperplasia, while 16, 16 dimethyl-PGE2 administration stimulates hyperplasia. The effect of increased dietary linoleic acid (LA) on promoting mucosal adaptation in resected, non-EFA deficient animals was evaluated. Ten Sprague-Dauley rats (5 w/o males) were pair-fed isocaloric diets containing 1% or 5% (LA) (w/w). After 2 weeks all animals underwent 70% proximal jejunoileal resection. Animals were then pair-fed for 14 days. Following sacrifice mucosal weight, protein, DNA, and sucrase activity were determined: Protein levels were increased in all segments in 5% LA animals (p

Sucrase activity is absent in the small intestine of fetal rats, but it increases during the weaning period. At the same time, its jejunoileal gradient, known to exist in most adult mammals, appears. Mechanisms leading to the expression of biochemical specialization of individual sections of the small intestine are not clear. Fragments of fetal rat jejunum and ileum (21-day old, CDF strain Charles River Farm) implanted under the kidney capsula of adult rats (Gastroent. 64:292 and J. End. 74:145) were assayed after 4 and 6 weeks for sucrase activity and compared with small intestine developing in situ. Results demonstrate that expression of jejunoileal gradient of sucrase activity is not dependent on direct contact of food; this factor as well as gastric, duodenal and pancreatic secretion might be responsible for its magnitude. (Support: NIH grant #AM HD 14531).