Circadian influences on feeding-induced changes in ACTH and corticosterone secretion in rats.

Food ingestion has a variable influence on pituitary-adrenal hormone secretion depending on the species studied and/or the experimental design. In this study, changes in plasma concentrations of ACTH and corticosterone following 30 min of food ingestion were compared in both 24 h fasted and ad libitum fed rats tested during either the early light cycle or the early dark cycle. In the dark, food ingestion caused significant decreases in both ACTH (to 80% of control) and corticosterone (to 32% of control) in both fasted and ad libitum fed rats. In contrast, in the light, food ingestion by the fasted animal resulted in a doubling of corticosterone concentrations. Such a response was not seen in ad libitum fed animals; however, these animals ate very little during the test. There were no significant changes in ACTH during the light phase. These data indicate that time of day has a significant impact on the responses of the pituitary-adrenal system to food ingestion. This circadian effect may be due to the influence of the endogenous levels of ACTH/corticosterone existing at the time of food ingestion.     

Effects of fasting on aldosterone secretion in ovariectomized rats

The present study was designed to assess the effect of fasting on aldosterone secretion in ovariectomized (Ovx) rats. Ovx rats were divided into fed (allowed access to food ad libitum) and fasted (deprived of food for 24 hours) groups. The trunk blood of fed and fasted rats was collected after decapitation. In the in vitro study, adrenal zona glomerulosa (ZG) cells from fed or fasted rats were incubated with angiotensin II (Ang II, 10(-6) M), adrenocorticotropic hormone (ACTH, 10(-9) M), or forskolin (an activator of adenylyl cyclase, 10(-6) M) at 37 degrees C for 30 min. The levels of aldosterone in medium and plasma extracts were measured by radioimmunoassay. Results showed that the levels of plasma aldosterone in fasted rats were lower than those in fed rats. There were no significant differences in basal and Ang II-stimulated aldosterone secretion between fed and fasted groups. The increment of aldosterone induced by ACTH in fasted group was significantly less than that in fed group. Administration of forskolin led to a significant increase in aldosterone secretion in both fed and fasted groups. Fasted group had a decreased aldosterone secretion in response to forskolin as compared with fed group. In summary, these results suggest that fasting decreases aldosterone secretion in Ovx rats through a mechanism in part involving a reduction of aldosterone production in response to ACTH, a decreased activity of adenylyl cyclase, and/or an inhibition of post-cAMP pathway in ZG cells.     

The effect of gastrin-releasing peptide on the endocrine pancreas

The 27-amino acid peptide gastrin releasing peptide (GRP-(1–27)) was infused at 4 dose levels (0.01, 0.1, 1.0, and 10 nM) into the arterial line of the isolated perfused porcine pancreas. Infusions were performed at 3 different perfusate glucose levels (3.5, 5.0, and 8.0 mM) and at two levels of amino acids (5 and 15 mM). GRP-(1–27) stimulated insulin and pancreatic polypeptide secretion and inhibited somatostatin secretion in a dose-dependent manner. Glucagon secretion was unaffected by infusion of GRP under all circumstances. The effect of GRP-(1–27) on insulin secretion was enhanced with increasing perfusate glucose levels, whereas the effects upon somatostatin and pancreatic polypeptide secretion were independent of perfusate glucose levels. The responses to GRP were unaffected by elevation of the concentration of amino acids in the perfusate. The effects of GRP were unaffected by atropine at 10⁻⁶ M. The localization of GRP within the porcine pancreas, its release during electrical stimulation of the vagus nerve, and its potent effects upon pancreatic endocrine secretion make it conceiveable that the peptide participates in parasympathetic regulation of pancreatic endocrine secretion.     

Functional and anatomical relationships between antral gastrin cells and gastrin-releasing peptide neurons

The present studies were directed to examine the effect of gastrin-releasing peptide (GRP) on beta-adrenergic stimulated gastrin release by cultured rat antral mucosa and to assess the anatomical relationship between gastrin cells and GRP nerves in rat and human antrum. Peptide-containing cells were identified by application of an avidin-biotin-peroxidase immunocytochemical double staining method utilizing antibodies to GRP and gastrin prepared in rabbits. Rat antral mucosa was cultured for 60 min and gastrin released into the culture medium was measured by radioimmunoassay. Inclusion of antibodies to GRP in culture medium did not affect carbachol-stimulated gastrin release, whereas isoproterenol-stimulated gastrin release into the medium was inhibited significantly by addition of GRP antiserum to the culture medium. GRP-containing neurons and axonal fibers were stained immunocytochemically with diaminobenzidine (reddish-brown specific staining) and were located in the lamina propria adjacent to and surrounding the main lobules of antral glands. After double staining utilizing 4-Cl-1-Naphthol as substrate, blue stained gastrin-containing cells were identified in the middle and deeper regions of antral glands in close proximity to GRP neuronal elements. These studies suggest that beta-adrenergic, but not cholinergic, stimulation of gastrin release is mediated, at least in part, through GRP. They also demonstrate intimate anatomical, as well as functional, relationships between gastrin cells and GRP-containing neurons.     

Comparison of the actions of bombesin, gastrin-releasing peptide-27, neuromedin B, and gastrin-releasing peptide-10 in causing release of gastrin and gastric inhibitory peptide in rats

The objective of this study was to compare the gastrin- and gastric inhibitory peptide (GIP)-releasing actions of bombesin, gastrin-releasing peptide (GRP)-27, neuromedin B, and GRP-10 in rats. Both bombesin and GRP-27 are potent stimulants of gastrin and GIP release, whereas neuromedin B and GRP-10 are less effective, on a molar basis.     

Influence of fasting on glycogen depletion in rats during exercise

We recently observed that a 24-h fasted group of rats could run longer than an ad libitum fed control group before becoming exhausted. Because of the demonstrated importance of glycogen levels and free fatty acid availability during endurance exercise, we have investigated several parameters of carbohydrate and lipid metabolism in exercised and nonexercised rats that were either fed ad libitum or fasted for 24 h. A 24-h fast depleted liver glycogen, lowered plasma glucose concentration, decreased muscle glycogen levels, and increased free fatty acid and beta-hydroxybutyrate concentrations in plasma. During exercise the fasted group had lower plasma glucose concentration, higher plasma concentration of free fatty acids and beta-hydroxybutyrate, and a lower muscle glycogen depletion rate than did the ad libitum fed group. Since fasted rats were able to continue running even when plasma glucose had dropped to levels lower than those of fed-exhausted rats, it seems unlikely that blood glucose level, per se, is a factor in causing exhaustion. These results suggest that fasting increases fatty acid utilization during exercise and the resulting "glycogen sparing" effect may result in increased endurance.     

Effect of synthetic porcine gastrin-releasing peptide on plasma levels of immunoreactive cholecystokinin pancreatic polypeptide and gastrin in dogs

This study was conducted to determine if synthetic porcine gastrin-releasing peptide (GRP) stimulates the release of immunoreactive cholecystokinin (CCK), pancreatic polypeptide (PP) and gastrin in dogs. Three doses (0.01, 0.1 and 0.5 μg/kg-hr) of synthetic porcine GRP were administered intravenously to six conscious dogs. Synthetic procine GRP stimulated the release of each hormone in a dose-related manner. The effect of GRP on the response of gastrin was greater than its effect on CCK and PP responses. This study indicates that the biological action of synthetic porcine GRP is similar to the bombesin, an amphibian peptide shown previously to stimulate the release of gastrointestinal peptides.     

High potency of a new bombesin antagonist (RC-3095) in inhibiting serum gastrin levels; comparison of different routes of administration.

This study was performed to evaluate the efficacy and duration of action of a new bombesin antagonist D-Tpi6,Leu13 psi (CH2NH)Leu14-bombesin (6-14) (RC-3095), given by different routes of administration, in suppressing gastrin releasing-peptide (GRP(14-27))-stimulated gastrin release in rats. First, we showed that GRP(14-27) itself was highly active when administered by different routes. GRP(14-27), given to rats at a dose of 25 micrograms/100 g b.w. significantly increased serum gastrin levels 3 and 6 min after intravenous and for more than 30 min after subcutaneous administration or pulmonary inhalation. RC-3095 was then injected subcutaneously, intravenously and also delivered by pulmonary inhalation at a dose of 10 micrograms/100 g b.w. in each case to seven male rats 2, 30, 60 or 120 min prior to i.v. administration of 5 micrograms GRP(14-27). RC-3095 administered 2 min prior to GRP(14-27) decreased the gastrin response to GRP(14-27), measured as area under the curve, by 81% in the intravenously injected group and 64% in the pulmonary inhalation group in the first 6 min. When GRP(14-27), was given 30 min after administration of RC-3095, the gastrin response was decreased by 52% in the subcutaneous group, 49% in the pulmonary inhalation group and 11% in the intravenous group during the first 6 min. RC-3095 delivered subcutaneously or by pulmonary inhalation 1 h before GRP(14-27) was also able to significantly inhibit gastrin release. Analysis of the data revealed that the bioavailability of RC-3095 given by the pulmonary inhalation route was about 69% of the s.c. route.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prior pregastric food stimulation and gastrin-releasing peptide1-27 (GRP) synergize to inhibit sham feeding.

The hypothesis that prior pregastric food stimulation is sufficient to reveal an inhibitory effect of gastrin-releasing peptide1-27 (GRP) on sham feeding was tested in 11 male rats equipped with chronic gastric cannulas. Rats were sham fed a high-carbohydrate solution during a 45-min test session, after 17-h food deprivation. GRP (16 or 32 microg/kg) or saline was injected intraperitoneally either at the onset or 5 or 15 min after the onset of sham feeding. This allowed for a 0-, 5-, or 15-min period of pregastric food stimulation before GRP or saline injections. Sham intake was recorded every 5 min, and behavior was observed every minute. GRP inhibited sham feeding when it was administered after 5 or 15 min of prior pregastric food stimulation, but not when it was administered at test onset. A nonsignificant increase in resting behavior and decrease in feeding behavior were associated with the decrease in sham feeding. No anomalous behaviors were noted. We conclude that a synergy between GRP and prior pregastric, presumably oral, food stimulation is sufficient to inhibit sham feeding.     

Indigestible material attenuated changes in apoptosis in the fasted rat jejunal mucosa

We have previously demonstrated that fasting induced apoptosis and decreased cell proliferation in the rat intestinal mucosa. The aim was to investigate the effect of expanded polystyrene as indigestible material on apoptosis and cell proliferation in rat small intestinal mucosa during fasting. Male SD rats were divided into 3 groups. The first group was fed with chow and water ad libitum. The second group fasted for 72 hrs. The third group was fasted for 24 hrs and was fed expanded polystyrene. Intestinal apoptosis was evaluated by percent fragmented DNA assay, terminal deoxynucleotidyl transferase-mediated dUDP-biotin nick end-labeling (TUNEL) staining, and caspase-3 assay. Cell proliferation was analyzed by 5-bromo-2'-deoxyuridine (5-BrdU) uptake. Truncal vagotomy was performed to evaluate a role of the central nervous system. In the 72-hr fasted rat, mucosal height of the rat jejunum was decreased to 73% of that in rats fed ad libitum, and this decrease was partly restored to 90% in rats fed expanded polystyrene. The fragmented DNA was increased in fasted rats (28.0%) when compared with that in rats fed ad libitum (2.6%). The increase in fragmented DNA in fasted rats was recovered by feeding them expanded polystyrene (8.3%). TUNEL staining confirmed this result. The effect of polystyrene on apoptosis was decreased by truncal vagotomy. Expression of cleaved caspase-3 was increased in fasted rats, which was then decreased by feeding of expanded polystyrene. In contrast to apoptosis, feeding of expanded polystyrene had no reconstructive effect on 5-BrdU uptake in the intestinal epithelium, which was decreased by fasting to 60% of that in rats fed ad libitum. In conclusion, feeding of indigestible material partly restored the decrease in intestinal mucosal length in the fasted rats through the apoptotic pathway without any influence on BrdU uptake. Further exploration focused on the mechanism of this effect of indigestible material is required.     

Effects of exercise on insulin-induced hypoglycemia

The purpose of this study was to determine the effect of exercise on the rate of onset of hypoglycemia induced by infusion of excess insulin (0.8 mU.min-1.100 g-1). Rats were either fasted overnight (FS) or fed ad libitum (FD). FS rats were killed after 5, 10, or 15 min of infusion at rest or after running on the treadmill at 21 m/min and 15% grade. FD rats were killed after 10, 20, or 40 min of infusion at rest or after exercise. Rats were also killed 15 min postexercise for FS and 60 or 120 min postexercise for FD with continued insulin infusion. The progressive decline in blood glucose was not altered by exercise in the FS rats. FD rats showed a significant difference due to exercise only after 40 min (rest 4.2 +/- 0.3 mM, exercise 3.2 +/- 0.2 mM). A significant postexercise repletion of glycogen was observed in red vastus and soleus muscles of FD rats despite the decreasing blood glucose values. These data indicate that exercise accelerates the rate of development of hypoglycemia in FD rats. In the FS rats, where the rate of decline in blood glucose was greater, exercise had no effect on the time course of development of hypoglycemia.     

Insulin tolerance in rats.

Insulin tolerance was studied in young (8- to 10-week-old) Wistar rats: a) after the administration of mounting doses of long-acting insulin (10, 40, 160 and 320 units/kg bw.) to animals fed ad libitum on two different diets); b) after the administration of long-acting insulin (1 and 5 units/kg b.w.) to animals which had fasted for different lengths of time. In rats fed ad libitum on the two diets, graded doses of insulin induced (except for the smallest dose) hypoglycaemia of roughly the same intensity, but varying in duration in correlation to the dose. The administration of insulin to fasting rats showed differences in insulin tolerance which were correlated to the duration of the fast. A significant decrease in insulin tolerance was already found after 6 hours. During the given testing period (72 hours), the lowest insulin tolerance was found after a 12 hours' fast and the highest after 48 hours. Insulin tolerance after 24 and 72 hours' fasting was approximately the same; it was higher than after 12 hours, but lower than after 48 hours. The initial blood sugar level (before administering insulin) was not in any way correlated to insulin tolerance determined at various intervals during fasting.     

Restricted feeding schedules alter the circadian rhythms of serum insulin and gastric inhibitory polypeptide

Insulin and gastric inhibitory polypeptide (GIP) have a circadian rhythm of secretion that is altered by various feeding schedules. We acclimated rats over 3 weeks to one of 6 different feeding schedules. They were then killed at intervals over one feeding cycle. Blood was collected, and their stomachs were weighed. Hormones in the serum were measured by radioimmunoassay. When highest and lowest measured concentrations were compared in ad libitum fed rats, insulin more than doubled (445 +/- 50 to 993 +/- 180 pg/ml) and GIP more than tripled (682 +/- 108 to 1964 +/- 145 pg/ml) during a 24-h period. With restricted schedules, concentrations correlated with the feeding schedule, not the light-dark cycle. Hormone levels rose higher during feeding and fell lower with fasting than in ad lib fed rats. For example, GIP in one study fluctuated from 468 +/- 22 to 6433 +/- 432 pg/ml. In another example, insulin ranged from 30 +/- 5 to 2259 +/- 406 pg/ml during a 24-h period. However, insulin did not always correlate well with stomach weight. Circadian rhythms occurred for insulin with all feeding schedules and for GIP with all schedules except fasted rats. This finding implies an endogenous insulin rhythm, whereas food intake controls GIP secretion. Thus, disruption of normal circadian cycles of feeding may yield misleading information about gut hormone secretion.     

Nippostrongylus brasiliensis: changes in plasma levels of gastrointestinal hormones in the infected rat

Plasma concentrations of gastrointestinal hormones were measured by radioimmunoassay in fasted rats 9 days after infection with a range of doses of Nippostrongylus brasiliensis. Values for infected rats fed ad libitum were compared with those of weight matched, pair fed, uninfected rats to control for the possible effects of dose-dependent reductions in food intake associated with infection. The plasma concentrations of some of the gastrointestinal hormones in infected rats were very different from those of their pair fed partners. The magnitude and direction of the changes varied according to the hormone being examined. Plasma concentrations of gastrin and pancreatic polypeptide were similar in pair fed and infected rats at all doses used. For the other hormones assayed, infection was associated with dose-related changes. The plasma concentrations of cholecystokinin and insulin were slightly but significantly reduced in infected rats. In contrast, secretin, enteroglucagon, and pancreatic glucagon concentrations were markedly increased. At the highest dose given (52 larvae/g body wt), the plasma levels of secretin and enteroglucagon in infected rats were elevated 9 X and 15 X, respectively. A comparison of the changes seen in N. brasiliensis-infected rats with those reported for other helminth infections revealed striking differences. The possible etiology of alterations in plasma gastrointestinal hormone concentrations and their contribution to the pathological changes seen in animals infected with helminths are discussed.     

Bombesin-like peptides stimulate growth hormone secretion mediated by the gastrin-releasing peptide receptor in cattle

This study was designed to determine the effects of bombesin-like peptides (BLPs) on the secretion of growth hormone (GH) and to characterize the receptor subtypes mediating these effects in cattle. Four experiments were conducted: (1) six steers were randomly assigned to receive intravenous (IV) bolus injections of 0, 0.2, 1.0, 12.5 and 50.0μg/kg neuromedin C (NMC); (2) seven pre-weaned calves were IV injected with 1.0μg/kg NMC; (3) six steers were IV injected with 2.5μg/kg bovine gastrin-releasing peptide (GRP), 1.0μg/kg NMC combined with 20.0μg/kg [d-Lys(3)]-GHRP-6 (an antagonist for the GH secretagogue receptor type 1a [GHS-R1a]), 1.0μg/kg NMC combined with 20.0μg/kg N-acetyl-GRP(20-26)-OCH(2)CH(3) (N-GRP-EE, an antagonist for the GRP receptor), 20.0μg/kg N-GRP-EE alone, 1.0μg/kg neuromedin B (NMB); and (4) four rats were IV injected 1.0μg/kg NMC. A serial blood sample was collected before and after injection. Plasma GH levels dose-dependently increased at 5min after NMC injection and the minimal effective dose was 1.0μg/kg. Plasma GH level was elevated by GRP, but not by NMB. The NMC-induced elevation of GH was completely blocked by N-GRP-EE. The administration of NMC elevated GH level in pre-weaned calves but not in rats. Ghrelin level was unaffected by any treatments; and [d-Lys(3)]-GHRP-6 did not block the NMC-induced elevation of GH. The results indicate BLP-induced elevation of GH levels is mediated by the GRP receptor but not through a ghrelin/GHS-R1a pathway in cattle.     

The effect of chronic food and water restriction on open-field behaviour and serum corticosterone levels in rats

In operant conditioning experiments, two methods are commonly used to motivate laboratory rats to perform designated tasks. The first is restricting food so that rats are forced to lose 20% of body weight within one week, followed by maintenance at 80% of the baseline weight for the remainder of the experiment. The second is restricting access to water to 15 min in each 24 h period. These methods are effective in motivating the animals. There is, however, little information available on the effects on performance in tests of behaviour that are not related to operant conditioning. In addition, it is not clear if these commonly used methods of food and water restriction will lead to physiological stress as indicated by an elevation of serum corticosterone. Male rats were either food-restricted to reduce and maintain their weight at 80% of baseline weight, or were restricted to 15 min access to water every 24 h. Activity in the open field was significantly greater in food-restricted rats than in water-restricted or control rats, but freezing behaviour was similar in all experimental groups. Food-restricted rats had a higher mean serum corticosterone level than water-restricted and control rats 37 days after the start of the experimental period. These data suggested that chronically restricting food and maintenance of body weight at 80% of baseline body weight led to significant behavioural changes and physiological stress. In contrast, water restriction did not lead to changes in behaviour or corticosterone levels. A second experiment was conducted to compare the effects of food restriction to 80% of baseline body weight, as described above, with a less stringent protocol in which test rats were initially reduced to 80% of baseline weight, but were then maintained at 80% of an ad libitum fed control rat's weight. Serum corticosterone levels and adrenal gland weights were measured after the initial week of forced weight loss and after maintenance for 21 days. Forced loss of 20% of body weight in the first week led to significantly increased serum corticosterone levels and adrenal gland weights compared to ad libitum fed controls. Serum corticosterone levels and adrenal gland weights in rats maintained at 80% of their initial body weight for 21 days remained higher than ad libitum fed control rats. However, rats maintained at 80% of an ad libitum fed control rat's weight did not differ from control rats in serum corticosterone levels or adrenal gland weights at the end of the 21-day study period. Adjustment of the feeding regimen in this manner eliminated physiological evidence of chronic stress.     

Inhibition of insulin release by galanin and gastrin-releasing peptide in the anaesthetized rat

The present study was designed to determine the effects of intravenously administered galanin or gastrin-releasing peptide (GRP) on glucose- and/or glucose-dependent insulinotropic peptide (GIP)-stimulated insulin release in the anaesthetized rat. Galanin inhibited glucose-stimulated insulin responses in a dose-related manner. Galanin also inhibited insulin release in response to glucose administered with GIP; this effect was due largely to inhibition of the glucose-stimulated component since galanin did not inhibit GIP-stimulated insulin release. Galanin also inhibited insulin responses to ingestion of a mixed meal. GRP inhibited glucose-stimulated insulin responses, and the insulin responses to glucose plus GIP; unlike galanin, GRP inhibited both glucose- and GIP-stimulated insulin release. GRP also inhibited insulin release following ingestion of a mixed meal. The results suggest a possible modulatory role for these neuropeptides in regulation of insulin secretion.