Role of corticoids independent of food intake in premature increase of pancreatic enzyme activities following early weaning in rats

Neonatal hypoxia in the rat: effects on exocrine pancreatic development

BACKGROUND

Glucocorticoids profoundly affect pancreatic development during the suckling period. Increases in circulating glucocorticoids during exposure to hypoxia may alter the normal pattern of pancreatic enzyme development.

METHODS

Rats were exposed to hypoxia from birth to 7 days (suckling) or from 28 to 35 days of age (weaned at day 21).

RESULTS

Hypoxia in neonatal rats (0-7 days) led to decreased pancreatic weight, and trypsin, lipase, and amylase activity compared with normoxic controls. In contrast, rats exposed to hypoxia from 28 to 35 days of age had decreased lipase activity but no change in other pancreatic parameters. Two weeks after hypoxia (0-7 days) pups were returned to normoxia, and their body weights remained smaller than the age-matched, previously normoxic controls. Pancreatic enzyme activities were decreased in the group recovering from hypoxia compared with controls. Recovery of enzyme activities was observed 3 weeks after hypoxic rats were returned to normoxia. Normoxic pups were given dexamethasone to simulate the hyperglucocorticoid state in hypoxia at 7-day olds. Dexamethasone administration led to decreased body weight, but increased pancreatic weight and enzyme activity compared with normoxic, age-matched controls.

CONCLUSIONS

Hypoxia in newborn rats delays the maturation of pancreatic exocrine enzymes. The mechanism is not related to increased serum glucocorticoids.

Ontogenic regulation of components of ileal bile acid absorption

The apical sodium-dependent bile acid cotransporter (ASBT) and the ileal bile acid binding protein (IBABP) are two components of ileal bile acid absorption. During the third postnatal week of the rat, there is a dramatic increase in ASBT and IBABP expression. The goals of this study were to examine the role of hormones on the ontogenic expression of ASBT mRNA and the role of weaning for both ASBT and IBABP mRNA. Administration of various doses of dexamethasone during the second postnatal week induced ASBT mRNA levels, and this effect was significantly increased with concomitant thyroxine treatment. Early weaning and weaning prevention were utilized to investigate the influence of dietary factors. ASBT and IBABP mRNA levels were significantly elevated by early weaning and were decreased by weaning prevention compared with littermate controls. Thus, glucocorticoids and thyroxine appear to play a role in the ontogenic expression of ASBT mRNA and weaning appears to participate in both ASBT and IBABP expression.

Milk-borne insulin with trypsin inhibitor in milk induces pancreatic amylase development at the onset of weaning in rats

BACKGROUND

The physiologic significance of milk-borne hormones and growth factors for internal organs of suckling animals is poorly understood. In this study the significance of milk-borne insulin was evaluated, as well as its combination with trypsin inhibitor, and its role in the development of pancreatic digestive capacity at the time of weaning was investigated.

METHODS

Experiments were performed using insulin-deficient milk formula (standard formula), insulin (20 ng/ml) formula, or insulin with trypsin inhibitor (1 U/ml) formula by a rat artificial-rearing technique.

RESULTS

In 17-day-old rats administered standard formula, the plasma insulin level was as low as that in 14-day-old rats. When insulin-trypsin inhibitor formula was administered to rat pups, the plasma insulin level was significantly higher than those in rats given standard or insulin formula. In rats artificially reared on standard formula, the usual developmental increases in pancreatic amylase activity and plasma insulin concentration at the beginning of weaning did not occur. Insulin formula elevated the pups' plasma insulin concentration and amylase activity at the onset of weaning but not to the levels observed in mother-reared rats. In rats reared on insulin-trypsin inhibitor formula, the developmental increases in the plasma insulin concentration and amylase activity observed in mother-reared rats were induced.

CONCLUSIONS

The present study demonstrates the necessity of milk-borne insulin for the development of pancreatic amylase during the weaning period.

Crucial role of milk-borne insulin in the development of pancreatic amylase at the onset of weaning in rats

The development of pancreatic amylase activity was examined in rats fed in regular cages or in special cages, designed so the pups could not reach solid food to prevent weaning. In both groups, the amylase activity in zymogen granules increased in rat pups aged 14 days, peaked at 18 days, and thereafter remained at a 1.6-fold higher level than at 14 days of age. An increase in the plasma concentration of immunoreactive insulin preceded the increase of amylase activity, whereas the plasma concentration of C-peptide, indicating the secretion rate of endogenous insulin, remained unchanged. The administration of insulin at 20 ng/ml (the physiological concentration) in the milk formula caused an increase in the plasma insulin concentration of 17-day-old pups. In addition, increased pancreatic amylase activity was observed in 17-day-old rats raised on milk formula to which insulin was added. We propose that the increase of amylase activity at the beginning of weaning is dependent on the milk-borne insulin and not on the dietary change in rats.

Systemic and luminal influences on the perinatal development of the gut

At birth, the mammalian gastrointestinal tract (GIT) must be able to support a shift from mainly parenteral nutrition in the fetus (via the placenta) to enteral nutrition in the neonate. In the perinatal period the GIT therefore undergoes enhanced growth as well as morphological and functional differentiation, and this maturational programme is influenced by a complex interplay of local, systemic and luminal factors. This review shows how systemic and luminal factors may influence GIT development in the perinatal period of the pig and sheep, two long-gestation species. Adrenocortical hormones play a pivotal role in the prepartum maturation of the GIT in addition to their better known effects on the development of many other tissues and body systems. More particularly, in the fetal pig and sheep, the prenatal development of gastric acid and gastrin secretion, and of GIT hydrolase activities (chymosin, pepsin, amylase, lactase, aminopeptidases) is influenced by cortisol. Additionally, glucocorticoids exert effects throughout the GIT by influencing morphological, cytological, and functional differentiation. Since the GIT epithelial cells comprise a renewing cell population there are also changes in cell kinetics. In addition to systemic factors, the presence of growth factors, hormones and nutrients from swallowed amniotic fluid (fetus) and colostrum (neonate) may influence GIT development. In utero, fetal fluid ingestion has been shown to modulate tissue growth, macromolecule and immunoglobulin transport, enterocyte differentiation, cell turnover and activity of brush-border hydrolases. These effects may be mediated via regulatory peptides (e.g. insulin-like growth factor I, gastrin-releasing peptides, insulin, epidermal growth factor, gastrin). A physiological role of luminally derived growth factors is supported by a number of unique structural and functional adaptations of the GIT in the fetus and neonate (low luminal proteolysis, intestinal macromolecule transport). Thus, in the pig and sheep, both systemic and luminal factors appear to play critical roles in GIT development in the perinatal period.

Postnatal development of the exocrine pancreas in suckling goat kids

A total of 25 preruminant Granadina breed goats were used. They were bottle-fed goat milk ad libitum from postnatal day 3 to 28. Until the age of 3 d, kids were fed colostrum. Body weight, pancreas weight, total protein concentration, enzyme activities in pancreatic tissue and hormone concentrations (cortisol, gastrin, T3 and T4) were determined at 3, 7, 14, 21 and 28 d of age. Our results show that the rates of pancreatic synthesis and secretion of chymotrypsin are well developed at birth in the kid, and may compensate for possible deficiencies in gastric and/or enterocytes intracellular proteolysis. In week 4, there was a marked increase in amylase activity, change that can be attributed to the beginning of the transitional period known as weaning. The significant increase in circulating concentration of cortisol during week 4 suggests the involvement of corticosteroid as a mediator of pancreatic development at weaning. Changes in blood levels of this hormone are believed to be important in the expression of amylase in the neonatal period. However, T3-T4 blood levels remained unchanged from d 3 to 28, suggesting that, in the kid, these hormones appear to have no clear influence upon the postnatal development of the exocrine pancreas.

Precocious cessation of intestinal macromolecular transport and digestive enzymes development by prostaglandin E2 in suckling rats

The effect of repeated oral administration of prostaglandin analogue (dmPGE2) on intestinal macromolecular transport and digestive enzymes development were investigated in the suckling rats. By the administration of dmPGE2 for 7 days, precocious induction of maltase activity, depression of amylase activity and enhancement of trypsin activity in the pancreas occurred. Absorption of bovine IgG was dose dependently depressed by dmPGE2 treatments. The intestinal cessation was also observed in the adrenalectomized pups, but was not influenced by difluoromethyl ornithine administration. These results suggest that oral administration of PGE2 induces precocious maturation of the small intestine and exocrine pancreas and that the intestinal cessation is not directly related to ornithine decarboxylase activity in the suckling rats.

Influence of hydrocortisone acetate on pancreas and mucosal weight, amylase and disaccharidase activities in 14-day-old pigs

1. One litter of 12 pigs was used to evaluate the effects of hydrocortisone acetate injection on organ weight and carbohydrase activities. 2. Pigs were injected with hydrocortisone acetate or an equal volume of saline at 7 days of age and killed at 14 days, and tissues were collected, weighted, and analyzed for carbohydrase activities. 3. Hydrocortisone had no effect (P greater than 0.40) on daily gain, liver weight, spleen weight, or small intestinal length. 4. Hydrocortisone increased pancreatic weight by 29% and total pancreatic alpha-amylase content by 38%. 5. Hydrocortisone increased duodenal mucosal weight by 23%, duodenal lactase activity by 44%, duodenal maltase activity by 163%, and duodenal sucrase activity by 214%.

Pancreatic exocrine enzymes during the neonatal period in postmature rats

Pancreatic content of exocrine enzymes in newborn rat pups shows a sharp decline soon after birth. To investigate if this decline is a preprogrammed and, therefore, inherently controlled phenomenon, or a result of external stimulus, prolonged gestation, or postmaturity (2 extra days in utero) in pregnant dams was induced by daily subcutaneous injection of progesterone from the 20th to 22nd days of gestation. Postmature pups showed the same high levels of lipase, trypsin(ogen), and amylase as control pups at birth. They also exhibited the same decline in these enzymes as control pups by the 2nd day after birth, suggesting that it is a response to external stimulus. Pups prevented from suckling retained the high levels of lipase, amylase, and trypsin(ogen) by the 2nd day. The stimulus, therefore, appeared to be the initiation of suckling. Pups prevented from suckling but given 5% glucose water orally every 4 h starting from birth for 24 h showed a sharp decline in amylase with only slight decreases in lipase and trypsin(ogen) by the 2nd day. The components in the feed, therefore, also seem to be an important determinant for selective enzyme release from the pancreas of the neonates. Electron microscopic studies revealed a sharp decrease in the number of zymogen granules in the continuously-suckled pups as compared to age-matched non-suckled counterparts. The reduction in enzyme content thus is the result of secretion in response to suckling.

Developmental changes of glucocorticoid receptors in the rat pancreas

Glucocorticoids are known to play a role in the maturation of the exocrine pancreas. The exact mechanism of glucocorticoid action in pancreatic ontogeny is, however, not clear. The present study characterized and quantitated the binding of [3H]dexamethasone to cytosol fractions from pancreata of rats at various ages. Trunk blood samples from these rats were also checked for levels of free and bound corticosterone. Specific and saturable bindings for dexamethasone were found in pancreatic cytosol fractions from newborn suckling and adult rats. Competition studies showed a preference for steroids with glucocorticoid activity. Specific binding was relatively low in pancreatic cytosol from newly born and 1-day old pups. A significant rise was seen after day 15. Cytosolic binding capacities were greatest from pancreata obtained from pups at weaning (3rd to 5th weeks). Values then declined toward the adult level. Scatchard analysis revealed a single class of binding sites with a dissociation constant (Kd) of 7.3 (+/- 1.1) X 10(-8) M and number of binding sites equalled to 1.29 (+/- 0.18) X 10(-13) mole/mg of cytosolic protein in adult rat pancreas. Pancreata from 25- and 15-day old rats had Kds of 3.4 (+/- 0.8) X 10(-8) M and 2.7 (+/- 0.7) X 10(-8) M with the number of binding sites equal to 1.77 (+/- 0.21) X 10(-13) mole/mg protein and 1.31 (+/- 0.16) X 10(-13) mole/mg protein respectively. Total plasma corticosterone concentration was low before day 10. It rose significantly by day 15, peaked at day 25, and then declined after weaning. About 5-15% of corticosterone during weaning and about 20-30% before and after weaning were in the free form. The peak level of dexamethasone binding corresponded to an increase in the plasma corticosterone level during weaning. This suggests a close relationship between plasma corticosterone levels and pancreatic glucocorticoid receptors. Both may, therefore, play a role in pancreatic development in the rat.

Streptozotocin effect on the development of the exocrine pancreas in neonate rats

Neonatal rats receiving streptozotocin at birth resulted in an acute but transient hyperglycemia for about ten days. This early transient hyperglycemic condition led to a reduced body weight gain in the suckling period but body weights were normalized by day 42. Streptozotocin pups had higher pancreatic wet weights and pancreatic protein and DNA contents at day 16 as compared to control pups, but not thereafter. At day 16, pancreatic trypsinogen in streptozotocin pups showed a transient increase over control pups but amylase and lipase were similar in the two groups. No difference was observed between the streptozotocin pups and the control group in their pancreatic lipase and trypsinogen concentrations after 16 days. In contrast, pancreatic amylase concentrations were lower in streptozotocin pups from day 18 to day 42 but the difference between the groups was significant only from days 18 to 24. Insulin supplementation of streptozotocin pups for four days in the neonatal period restored the pancreatic amylase concentration to the control level at days 18 and 20. These results indicate that acute streptozotocin administration at birth to neonates affects subsequent exocrine pancreatic development, particularly that of amylase, and that exogenous insulin attenuated the effect.

Acute starvation and suckling rat pancreas. Effect on exocrine pancreas and role of corticosteroids

Starving of rat pups at the 5th or 10th day of life for three days led to lower body and pancreatic weights compared to age-matched controls. When compared to the weights before fasting, only the 5-day-old pups showed increases in pancreatic protein and DNA contents despite food deprivation. Pancreatic acinar responsiveness to carbachol stimulation was not affected. Accumulations of amylase and lipase in the pancreas of the rat pups were increased by fasting, with an accompanied surge in the level of serum corticosterone. In 10-day-old rat pups, injection with aminoglutethimide (AG), a drug which suppresses steroidogenesis, prevented the enzymatic increases and concomitantly suppressed the serum total corticosterone levels at 12 and 24 hr after the onset of fasting. These results strongly support the role of corticosterone in the modulation of the pancreatic adaptation to food deprivation in the suckling rat.