Susan J. Henning

Development of the gastrointestinal tract

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

Kinetics of Circulating Corticosterone in Infant Rats

ABSTRACT: Corticosterone plays an important role in the regulation of postnatal development in the rat. Basal concentrations of plasma corticosterone increase markedly during the 3rd wk of life. To date, however, the physiologic bases of this increase have remained unclear. To understand the determinants of circulating concentrations of corticosterone during this period, the plasma half-life of disappearance at steady state (t½), the apparent volume of distribution, and metabolic clearance rate were determined after injection of a tracer dose of 3H-corticosterone in rats at 12, 16, and 22 days of age. The t½ for total plasma corticosterone decreased with increasing age. The volume of distribution decreased even more steeply and, consequently, the MCR displayed a highly significant decline between 12 and 22 days of age. As plasma concentrations of corticosteroid-binding globulin are known to increase markedly during this period, the t½ of protein-bound corticosterone was measured and that of free corticosterone was computed. At all ages the t½ of bound corticosterone was less than that of free corticosterone. Protein binding of the injected 3H-corticosterone increased significantly with development. Thus, increased binding of corticosterone is associated with decreased t½. The increasing association of corticosterone with corticosteroid-binding globulin during this developmental period is the most likely explanation for the steep decline of volume of distribution and thus of the metabolic clearance rate for corticosterone. The latter provides, for the first time, an understanding of the basis of the developmental increase in plasma concentrations of corticosterone.

Role of diet in the determination of jejunal sucrase activity in the weanling rat.

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.

The distribution of lead in milk and the fate of milk lead in the gastrointestinal tract of suckling rats

ABSTRACT. Milk can be a significant source of lead (Pb) for young mammals, including humans. Certain essential trace elements have previously been shown to be specifically associated with particular milk components and such associations often increase bioavailability. Thus, the first goal of this study was to determine the distribution of Pb in cream, casein, and whey fractions of various milks under various conditions using 203Pb as a tracer. In rat milk almost 90% of the Pb was found to be associated with the casein micelles, regardless of: 1) whether the milk was labeled in vivo or in vitro; b) whether the milk was fresh or frozen; and c) the added concentration of Pb (over the range 0.01–75 μ/ml). The remainder of the Pb was approximately equally distributed between cream and whey. A virtually identical pattern of Pb distribution was observed with bovine milk. Pb added to infant formula also associated predominantly with casein micelles, although the Pb content of this fraction was significantly less than with rat and bovine milks. The second goal of the study was to determine if Pb remained associated with casein as it traversed the gastrointestinal tract of infant rats. For this purpose, rat pups aged 15–16 days were gavaged with 203Pb-labeled rat milk, and lumenal contents from the stomach and small intestine were collected 2 h later. Differential centrifugation of the homogenized lumenal contents showed that in the stomach the Pb was associated primarily with the casein curd. By the time chyme reached the distal small intestine, Pb was found predominantly in a fraction that was not precipitable by high-speed centrifugation (thus, not intact casein micelles), but was nondialyzable. We conclude that Pb in milk is protein bound and remains this way as it traverses the stomach and proximal small intestine of the infant rat.

Role of Milk-Borne Factors in Weaning and Intestinal Development

To determine whether a factor appearing in the milk at the beginning of the 3rd postnatal week elicits the appearance of jejunal sucrase activity in the pups, pups were raised with dams that had been lactating for either a week less or a week more than their natural mothers. Pups were sacrificed at 15, 17, 19, 21 and 23 days of age for assay of jejunal sucrase and for measurement of the proportion of chow in the stomachs. Sucrase activity appeared at the same age in all pups (day 17); however, at days 21 and 23 the enzyme activity was higher in the pups cross-fostered to the longer-lactating dams. Correspondingly, the analysis of stomach contents showed that these pups had almost completed weaning by day 21 whereas the other pups were still taking 40-50% milk. It is concluded that neither the appearance of jejunal sucrase nor the onset of weaning is cued by the appearance of a factor in the milk at the beginning of the 3rd postnatal week. However, the lactational stage of the dam can affect the rate of completion of weaning and, secondarily, the terminal portion of the sucrase rise.

Postnatal development of peptidase enzymes in rat small intestine.

The development and distribution of peptidase activity in mucosal homogenates of rat small intestine has been investigated. Substrates used were glycyl-L-leucine (GL), L-seryl-L-methionine (SM), and L-leucyl-glycyl-glycine (LGG). During the first 2 weeks of life there was high peptidase activity toward GL and SM in the distal regions of the small intestine. In the third postnatal week, activity in the distal small intestine toward GL and SM decreased, while activity in the proximal small intestine increased. In contrast, there was no difference in activity toward LGG along the length of the small intestine, nor was there a developmental change. Activity toward all three substrates was not affected by cortisone acetate treatment. However, the classical effect of glucocorticoids on sucrase activity and body weight was observed. All peptidases studied showed maximal activities at neutral pH, indicating that they were not lysosomal in origin. Activity towards GL and SM was predominantly located in the cytosol. It is suggested that these dipeptidase activities play a role in the terminal steps of protein digestion following pinocytosis and lysosomal hydrolysis.

Nocturnal feeding behavior in the neonatal rat.

Abstract The possibility that neonatal rats (5 days old) exhibit independent feeding rhythms has been examined by studying nipple attachment latencies on anesthetized dams during the day and during the night. Latencies were found to be significantly shorter at night. The results were not affected by the lighting cycle of the dam used for latency testing, nor by the degree of fullness of her mammary glands. We conclude that 5 day-old rats are capable of independent rhythmic behavior and that their feeding preference is intrinsically nocturnal. A corrolary of these findings is that the usual pattern of suckling during the day must be imposed on the pups by the mother.

A noninvasive method for determining patterns of milk intake in the breast-fed infant

A noninvasive adaptation of the test-weighing method has been devised for continuously monitoring milk intake in breast-fed infants. This procedure consists of counting the number of swallows from videotaped sessions of individual feeds and correlating them to the corresponding pre- and postnursing weights of the infants. The method has been tested on four nursing sessions in twins aged 1 1/2 to 2 1/2 months. A high correlation was found between swallow counts and weight gain of the babies. The patterns of milk intake as monitored by these two parameters were similar. This method has the advantage of not disrupting the natural nursing process because test-weighing during the feeding session is not necessary. In addition, it allows for longitudinal studies and uses relatively simple equipment, i.e., a balance and a video camera. It is further suggested that monitoring swallowing in the manner described herein may be a valuable addition to studies on sucking behavior in human infants. Finally, it is noted from the data presented that the pattern of milk transfer in a mother breast-feeding twins is different from that observed in mothers breast-feeding singletons who were monitored by means of a cross-sectional test-weighing procedure.

Postnatal development of cholesterol ester hydrolase activity in the rat adrenal

In certain circumstances the activity of cholesterol ester hydrolase (CEH) activity is believed to be rate-limiting for corticosterone production by the adrenal. The principal aim of the current study was to determine whether the activity of CEH displays a developmental increase in the infant rat which could, in part, account for the marked increase in serum corticosterone which begins at the end of the second postnatal week. The data show that the specific activity of CEH (units/mg cytosolic protein) during development is actually a mirror image of the pattern seen for serum corticosterone, i.e. CEH activities are high when serum corticosterone concentrations are low and then fall when serum corticosterone is rising. Even when total activities of CEH in the adrenal were calculated, there was no increase in parallel with the initial rise of serum corticosterone. At each age studied, stressed pups displayed significant increases of serum corticosterone; however, their CEH activities were no different from those in the non-stressed littermates. It is concluded that the activity of CEH is not the rate-determining factor for the developmental surge of basal concentrations of serum corticosterone nor for stress-induced elevation of corticosterone during the developmental period. A second aim of the current study was to address the more general question of whether steroidogenesis in the developing adrenal is limited by substrate supply. Measurement of the cholesterol content of adrenal mitochondria showed no ontogenic increase, suggesting that substrate supply, from any source, is not rate-limiting for steroidogenesis at these ages.

Duodenal uptake of lead by suckling and weanling rats

The aim of the current study was to determine the site of absorption of Pb by suckling rats. When 203PbCl2 (carrier-free) plus Pb-acetate (50 micrograms/g BW) was intragastrically administered to rat pups aged 10 and 14 days, mucosal uptake of Pb, measured 2 h after intubation, was manyfold greater in the duodenum than in other regions of the small intestine. By 24 days of age, this duodenal uptake was no longer apparent. In suckling pups the duodenal content of Pb became undetectable by 24 h postintubation; in contrast, ileal uptake was minimal at 2 h but increased progressively through 24 h. The ileal uptake component was also age-dependent, having disappeared by 24 days of age. These findings suggest that it is the duodenum where Pb absorption occurs, whereas ileal uptake represents intestinal retention. To assess the role of the milk diet in the elevated Pb absorption by the duodenum of suckling animals, the effect of fasting following intubation was determined. The duodenal uptake of Pb was substantially higher in fasted pups than in suckled littermates. Correspondingly, blood Pb levels were more than 4-fold higher in fasted pups than in suckled pups. We conclude that the enhanced absorption of Pb in the suckling rat is due to duodenal absorption and that this absorptive process is even further enhanced in the absence of milk.