The trophic action of gastrointestinal hormones.

A sensitive protein-binding assay has been used to measure plasma concentrations of total corticosterone during postnatal development in the rat. These concentrations were extremely low on days 6--12, showed a significant rise by day 14, and then continued to rise until peaking on day 24. Plasma titers of corticosteroid-binding globulin rose even more dramatically from day 12 onward. Consequently, the percentage of total plasma corticosterone, which was protein-bound, showed a gradual rise from 78% on day 12 to 98% on day 24. Despite this trend, when concentrations of free corticosterone were calculated, they were found to have a developmental profile very similar to that of total corticosterone. Assay of jejunal lactase and sucrase in the same animals that were used for the corticosterone studies showed that the ontogenic rise of both total and free corticosterone preceded the developmental changes in the activities of these enzymes by approximately 2 days. The data suggest that the rise in free corticosterone that begins on day 14 acts as a cue for enzymic changes in both liver and intestine.

https://www.physiology.org/doi/pdf/10.1152/ajpendo.1978.235.5.E451

Plasma concentrations of total and free corticosterone during development in the rat.

The ability to assimilate lactose, a primary constituent of milk, is dependent upon the presence of lactase, a genetically controlled enzyme in the small intestine. In most mammals, levels of lactase are high during infancy, but fall rapidly following weaning; adults are lactase-deficient. In man, some populations, including Orientals, Australian Aborigines, and Negroes, are deficient as adults, while others, including Nilotic Negroes and Europeans, maintain consistently high levels of lactase throughout life regardless of individual lactose intake. Ingestion of even small amounts of lactose by lactase-deficient individuals usually gives rise to a variety of symptoms, including cramps, bloating, and diarrhea. It is hypothesized that prior to the domestication of animals and the development of dairying, the normal condition for all men was adult lactase deficiency, but that with the introduction of lactose into the adult diet in certain cultures, new selective pressures were created that favored the genoty...

Lactase Deficiency: An Example of Dietary Evolution

My aim in this review is to use the rat as a model system for a discussion of the functional development of the gastrointestinal tract. I should make it clear at the outset that since the rat is an altricial species, many of the enzymic changes that occur postnatally in the rat occur prenatally in other mammals, including humans (Grand et al. 1976; Lebenthal et al. 1983). Nevertheless, I believe that these studies in the rat. have posed some questions of broader importance and have pointed to some potential answers. Thus, the review is structured 90 as to emphasize the general lessons which have potential application in many species and in many organ systems. Because of space limitations, the review does not attempt to be comprehensive. To set the stage, it is important to realize that the process of spontaneous weaning in the rat is a precisely timed phenomenon which normally begins at 17 d of age and is complete by day 26 (Henning et al. 1979). Weaning constitutes a significant change in dietary composition which, in turn, necessitates changes in digestive function. The entire gastrointestinal tract of the rat is functionally immature at birth and during the first two postnatal weeks. There are extensive changes in digestive capacity during the third postnatal week, and the adult modes of digestion are fully operative by the end of the fourth week. These developmental changes in gastrointestinal function have been reviewed elsewhere in detail (Henning, 1981 ; Klein & McKenzie, 1983) and only those in the small intestine will be summarized here. In the small intestine of the rat, we find that the neonate has hydrolytic activities which are specific for, and largely restricted to, the components of maternal milk. This is demonstrated very nicely by a consideration of carbohydrate digestion. Milk is relatively low in total carbohydrates, and the carbohydrates present are those not geoerally found in adult diets. The major carbohydrate in the milk of most placental mammals is lactose (Jenness et al. 1964), and high activities of its disaccharidase, lactase (EC 3.2.1.23), are found in the intestinal mucosa of the sucking animals (Deren, 1968; Kretchmer, 1971). In the rat, lactase is detectable on day 18 of gestation, has maximal activity during the first week after birth, and then begins to decline, reaching adult values by the end of the fourth week (Doell & Kretchmer, 1962). Many other species, including the human, have lower lactase activity in the adult than in the newborn (Deren, 1968) and accordingly show an inability to utilize ingested lactose in the postweaning period (Kretchmer, 1971). Intestinal hydrolases that are involved in digestion of carbohydrate components of solid food are absent or low at birth, then appear or increase in activity, or both. Maltase (EC 3.2.1.20) has low activity during the first two postnatal weeks then

/pdf/development-of-the-gastrointestinal-tract-1xtd1v70lx.pdf

Development of the gastrointestinal tract.

Intestinal sucrase and maltase activities were measured in three species of hummingbirds and 11 species of passerine birds from western Mexico. Maximal sucrase activity standardized by nominal area of intestine was 2-118 times higher in hummingbirds than in the nine species of passerines that showed significant sucrase activity. Two species of passerines (Turdus rufopalliatus and Catharus aurantiirostris) lacked significant sucrase activity. Lack of functional sucrase activity has been reported in five species of passerine birds restricted to three closely related families (Muscicapidae, Sturnidae, and Mimidae). Interspecific variation in sucrase activity is probably influenced both by feeding habits and phylogenetic affinities. No allometric relation between sucrose hydrolysis capacity and body mass was found. When the two species with no significant sucrase activity were deleted from the sample, maltose hydrolytic capacity and body mass were allometrically related (exponent = 0.65). Maltase and sucrase ...

Salvatorre Auricchio

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Intestinal disaccharidase activities in adult and suckling rats