Circadian rhythm of ornithine decarboxylase activity in small intestine of fasted rats.

The aim of this study was to determine whether the circadian changes in ornithine decarboxylase (ODC) activity of different segments of the small intestine were governed by factors other than food intake. First, the effects of fasting on mucosal ODC activity were examined. The results indicate that mucosal ODC activity in 24 hr and 48 hr fasted rats decreased significantly compared with ad libitum-fed rats. Second, the circadian rhythm of mucosal ODC activity was characterized by measuring mucosal ODC activity in fasted rats at four time points (09:00, 15:00, 21:00, and 03:00 hr; light period: 06:00-18:00 hr). The results from this study indicate that there is a detectable baseline ODC activity in different segments of fasting intestine. In duodenum, mucosal ODC activity was highest at 15:00 hr (light period), a time at which the rat was normally not eating. In jejunum and ileum, mucosal ODC activity increased between 21:00 and 03:00 hr (dark period). The observation that small intestine exhibits a distinct circadian rhythm of ODC activity in fasted rats suggests that not only food but also intrinsic factors can modulate physiologic oscillations in mucosal ODC activity.     

Diurnal rhythm of H+-peptide cotransporter in rat small intestine

In mammals, most physiological, biochemical, and behavioral processes show a circadian rhythm. In the present study, we examined the diurnal rhythm of the H+-peptide cotransporter (PEPT1), which transports small peptides and peptide-like drugs in the small intestine and kidney, using rats maintained in a 12-h photoperiod with free access to chow. The transport of [14C]glycylsarcosine (Gly-Sar), a typical substrate for PEPT1 by in situ intestinal loop and everted intestine, was greater in the dark phase than the light phase. PEPT1 protein and mRNA levels varied significantly, with a maximum at 2000 and minimum at 800. Similar functional and expressional diurnal variations were observed in the intestinal Na+-glucose cotransporter (SGLT1). In contrast, renal PEPT1 and SGLT1 showed little diurnal rhythmicity in protein and mRNA expression. These findings indicate that the intestinal PEPT1 undergoes diurnal regulation in its activity and expression, and this could affect the intestinal absorption of dietary protein.     

Receptors for neurotensin in canine small intestine.

We have previously characterized the neurotensin receptors on the circular smooth muscle (CM) of the canine small intestine (1). In the present studies, using radioligand binding technique, neurotensin receptors were localized on the membranes from deep muscular (DMP) and the submucous plexus while no binding was observed on either the longitudinal smooth muscle or myenteric plexus membranes. The high affinity binding sites (Kd 0.1-0.2 nM) on DMP membranes were similar to those on CM; the low affinity component was of much lower affinity (Kd approximately 40 nM). DMP had 4-6 times higher density of binding sites than the CM. The recognition properties of DMP receptors were similar to those on the CM and reduced sulfhydryl groups were required for the binding activity. The action of neurotensin on the contractility of the canine small intestine, therefore, appears to be through a direct action on the circular smooth muscle and through the prejunctional action on the DMP neurons through distinct receptors. Thiol groups in the neurotensin receptors may be important for the receptor function.     

Release and degradation of neurotensin during perfusion of rat small intestine with lipid

The levels of neurotensin (NT) and its metabolite, the N-terminal octapeptide (NT1-8), identified by HPLC and measured by RIA, were increased in the hepatic-portal circulation of the anesthetized rat during perfusion of the small intestine with a lipid solution, while levels of both peptides remained unchanged in the general circulation. There was no significant arteriovenous difference for NT or NT1-8 during saline perfusion of the small intestine. Plasma collected from the superior mesenteric vein during the infusion of [3H]NT into the superior mesenteric artery showed major peaks of radioactivity with the retention times of NT1-8 and NT1-11 on HPLC. Only 12% of the radioactivity recovered from plasma was intact NT. These studies demonstrate that chromatographically identified NT and its metabolite, NT1-8, are elevated in the portal circulation but not systemic circulation during lipid perfusion and that the small intestine may be both the site of release and metabolism of NT.     

Circadian rhythm of cholesterol biosynthesis: dietary regulation in the liver and small intestine of hamsters.

Sixty three hamsters were divided into two groups which were exposed to the same rigid lighting schedule (06(00)--18(00)) but fed at different time intervals (18(00)--22(00), and 08(00)--12(00), respectively) for five weeks. The cholesterol synthetic activity was then determined in liver, gastrointestinal tract, and kidney by in vivo incorporation of (1--14C)-acetate into cholesterol at different hours of the day. A remarkable circadian rhythm of the activity was found in the liver and small intestine, but not in other organs. Regardless of the lighting schedule, the nadir occurred, in both groups, always at the end of the fasting period and the peak four to six hours after feeding. The study clearly indicated the major role of diet in regulation of such phenomenon.     

Circadian rhythm of lactate dehydrogenase in rat testis

Activity of total lactate dehydrogenase (LDH) and of the isozyme X (LDH X or C4) have been determined at 2 hr intervals during 24 hr cycles in testis of adult rats maintained since birth in a photoperiod of 14 hr light: 10 hr dark. LDH X activity of epididymal sections (caput, corpus and cauda) from the same animals was also determined. Total LDH and LDH X activities in testis exhibited circadian rhythms with different timing. LDH X in the three portions of epididymis showed diurnal variations similar to those in testis. Rats subjected to constant light or constant dark presented marked modifications of LDH X profiles, indicating that the photoperiod plays a synchronizer role. While total soluble proteins did not show variations in testis of rats exposed to the photoperiod, a circadian rhythm was demonstrated in animals maintained in constant light or dark.     

Catabolism of neurotensin in the epithelial layer of porcine small intestine

The mammalian small intestine is both a source and a site of degradation of neurotensin. Metabolites produced by incubation of the peptide with dispersed enterocytes from porcine small intestine were isolated by high-performance liquid chromatography and identified by amino-acid analysis. The principal sites of cleavage were at the Tyr-11-Ile-12 bond, generating neurotensin-(1-11), and at the Pro-10-Tyr-11 bond, generating neurotensin-(1-10). The corresponding COOH-terminal fragments, neurotensin-(11-13) and -(12-13) were metabolized further. Formation of neurotensin-(1-11) and -(1-10) was completely inhibited by phosphoramidon (Ki = 6 nM), an inhibitor of endopeptidase 24.11, but not by captopril, an inhibitor of peptidyl dipeptidase A. Incubation of neurotensin with purified endopeptidase 24.11 from pig stomach also resulted in cleavage of the Tyr-11-Ile-12 and Pro-10-Tyr-11 bonds. A minor pathway of cell-surface-mediated degradation was the phosphoramidon-insensitive cleavage of the Tyr-3-Glu-4 bond, generating neurotensin-(1-3) and neurotensin-(4-13). No evidence for specific binding sites (putative receptors) for neurotensin was found either on the intact enterocyte or on vesicles prepared from the basolateral membranes of the cells. Neurotensin-(1-8), the major circulating metabolite, was not formed when neurotensin(1-13) was incubated with cells, but represented a major metabolite (together with neurotensin-(1-10] when neurotensin-(1-11) was used as substrate. The study has shown that degradation of neurotensin in the epithelial layer of the small intestine is mediated principally through the action of endopeptidase 24.11, but this enzyme is probably not responsible for the production of the neurotensin fragments detected in the circulation.     

Circadian rhythms in disaccharidases of rat small intestine and its relation to food intake.

The activities of maltase and sucrase of the small intestine were low at night and high in the daytime in rats which had been fed from 09.00 h to 15.00 h for 2 weeks. A remarkable rise of enzyme activities was observed at 08.00 h, 1 h before the start of feeding. The rhythmic changes in disaccharidase activities continued for at least 2 days after starvation, but completely disappeared after 5 days of starvation. It was suggested that the disaccharidase rhythms are not a direct consequence of food intake, but that anticipation of food intake acts as a trigger for initiation of the disaccharidase rhythms.     

Circadian rhythm in muscarinic receptor subtypes in rat forebrain

The binding of different ligands to muscarinic receptors in the central nervous system is regulated by several factors. Among these are the administration of drugs, disease, ontogeny or aging. Studies carried out in rat brains have demonstrated changes in the density of the muscarinic receptors at different times of the day. These changes might be related to variations in the circadian rhythms. In this work we have studied the binding of the [3H]-N-methyl-escopolamine, the agonist carbachol and the antagonist pirenzepine to muscarinic receptors in rat forebrains at 10.00, 14.00, 18.00, 22.00, 02.00 and 06.00 hr. We have observed changes in the density of muscarinic receptors but not changes in affinity to the radioligand. The Bmax values obtained by saturation studies were maximum at 14.00 hr and minimum at 02.00 hr (P less than 0.05 Mann-Whitney's test). Inhibition studies in the presence of the non-selective agonist carbachol and the selective antagonist pirenzepine, at the same time-points, did not show statistically significant changes in the Bmax values. These data indicate that changes in the Bmax values are only observed in the total population of muscarinic receptors and are not due to modifications in the subtypes of muscarinic receptors nor to the different affinity states of agonist binding.     

Slow-waves in rat small intestine

Rat small intestine generates rhythmic slow-wave activity. The slow-waves are not eliminated by ouabain application or incubation in potassium free solution. Exposure to low sodium or calcium free solution decreases slow-wave activity. Incubation in sodium and calcium free solution eliminates activity. It is concluded that rat small intestinal slow-waves may not result from the same mechanism as in the cat.     

Circadian rhythm of active amino acid transport in rat liver

Abstract

The circadian change of the encephalic photosensitivity of quail has been demonstrated. In this study the diurnal variation of the retinal photosensitivity was investigated by the electroretinogram (ERG) to explore the phase relation between the retinal and encephalic systems. The b‐wave, a major component of the ERG, was used as a measure of retinal photosensitivity.

1. In the bird maintained in LD12:12 the b‐wave amplitude of the ERG stayed at a high level for the first 10 h of the light period, and decreased abruptly around the time of light‐off. The decreased level continued until midnight. Thenceforth the b‐wave amplitude recovered progressively to the daytime level before the time of next light on.

2. In the bird maintained in LD16:8, the decrease of the retinal sensitivity in the light phase was initiated prior to the time of expected light‐off, and the onset time of decreasing tendency was advanced by 2 h, with dark adaptation for 30 min given immediately before the ERG measurement. Upon continuous light exposure, the b‐wave amplitude always remained at low level.

3. Periodic changes in retinal sensitivity persisted when the environmental dark phase was temporarily extended for 19 to 30 h.

These observations suggest that the diurnal rhythm of the neural retina in quail might be generated endogenously and appears to control the encephalic photosensitive system.     

Circadian rhythm of corticotropin releasing factor-like immunoreactivity in rat hypothalamus

Levels of hypothalamic corticotropin releasing factor-like immunoreactivity (CRF-LI) were measured by radioimmunuoassay (RIA) over a 24 hour light-dark cycle and found to exhibit two peaks. One peak was detected at 1100 hr and a secondary smaller peak was found at 2000 hr. The trough between the two peaks was detected at 1700 hr which coincided with the peak in plasma corticosterone levels. The results are consistent with a decreased level of hypothalamic CRF-LI at 1700 hr reflecting an increased release of peptide followed successively by the release of ACTH and corticosterone.     

Comparison of VIP-induced electrolyte secretion at three levels in rat small intestine.

Duodenal, jejunal and ileal loops were prepared and an iso-osmotic test solution injected, containing 80 mM Na+, 5-mM K+, 1.2 mM Ca2+, 77 mM Cl-, 10 mM HCO3- and 136 mM mannitol. 14CPEG 4000 was used as a non-absorbable marker and 36Cl was added to measure the bidirectional fluxes. During the 60-min in vivo incubation time, the duodenum actively secreted bicarbonate, a virtually zero flux in the jejunum was observed, whereas the ileum absorbed water and chloride and secreted bicarbonate. The response to the perfused doses of 0.15 to 2.4 nmol.100 g-1.h-1 of VIP (vasoactive intestinal peptide) differed qualitatively and quantitatively in the 3 segments: VIP increased bicarbonate secretion and induced chloride secretion in the duodenum, induced chloride secretion in the jejunum without changing bicarbonate minimal influx, induced bicarbonate secretion and suppressed chloride absorption in the ileum. The minimal dose required was lower in the duodenum (0.3 nmol.100 g-1.h-1) than in the jejunum and ileum (1.2 nmol.100 g-1.h-1). The functional heterogeneity of the small intestine was clearly demonstrated after VIP stimulation.     

Circadian regulation of uroguanylin and guanylin in the rat intestine

Uroguanylin (UGN) and guanylin (GN) are theendogenous intestinal ligands for guanylyl cyclase C (GC-C). Weexamined the circadian expression of UGN, GN, and GC-C in the jejunum,ileum, and proximal colon of young adult rats by Northern blotanalyses. These assays revealed that UGN is more abundant in theproximal small intestine, whereas GN and GC-C are more abundant in theproximal colon. mRNA levels showed significant circadian variation forUGN (3- to 18-fold peak/trough difference), GN (2.1- to 2.8-foldpeak/trough difference), and GC-C (3- to 5-fold peak/troughdifference). The maximal abundance occurred in the dark period for allthree mRNAs, although peak UGN and GN expression occurred later in thedark period in the jejunum relative to the ileum and colon. Immunoblotanalyses using monospecific polyclonal antibodies against UGN and GNprohormones confirmed the regional and circadian variation detected byNorthern assays. Thus the expression of these genes is regulated notonly by histological position but also by circadian time.