Influence of the season on the circadian rhythm of serum lipids in male Wistar rats

Young male Wistar rats were adapted to a 12:12 h light: dark regimen (7 a.m. to 7 p.m., 7 p.m. to 7 a.m.), with free access to food and water. In January, April, July and October they were killed at 3-hour intervals and non-esterified fatty acids, tri-glycerides, total cholesterol and phospholipids were determined in their serum. Serum cholesterol was the least influenced by the season; a rhythm was present in all the seasons, the acrophases came in approximately the same part of the day and the mean adjusted oscillation (mesor) values (in the cosinor test) did not differ the whole year round. Serum triglycerides displayed a rhythm except in the summer; their autumn values were lower and their curves attained the maximum in different parts of the photophase. Non-esterified fatty acids and phospholipids were found to be significantly influenced by the season; the shapes of their curves and their maxima varied during the year. For evaluation of the influence of the seasons on the circadian oscillations of lipid metabolism in the rat, information on the ratio of the formation and utilization/degradation of tissue lipids in the various parts of the year is still lacking.     

Influence of light regimens on circadian changes in the blood glucose and tissue glycogen concentration in the rat

Adult male Wistar rats allowed food and drinking water ad libitum were kept 2- 5 weeks under standard conditions, but with a different artificial light regimens. The standard regimen was 12:12 h light and dark (LD) and the other variants were 12:12 h dark and light (DL), continuous darkness (DD) and continuous light (LL). The blood glucose level and the liver, skeletal muscle, heart and brown and white adipose tissue glycogen concentration were tested during the day at 3-hour intervals. The experiments were carried out during the winter and were evaluated by an analysis of variance and the cosinor test. Circadian variation of the blood glucose level and the liver and both the adipose tissue glycogen concentrations was only slightly affected by changes in the light regimen. Conversely, the oscillations of the skeletal muscle and heart glycogen concentration reacted markedly to the variants of the light regimen, their reaction being manifested in the localization of the acrophases in different parts of the day, especially when comparing the DD and LD regimens.     

Circadian rhythm of blood glucose and tissue glycogen in fed and fasted rats

Blood glucose and tissue glucogen circadian rhythms were determined in male Wistar rats adapted 3 weeks to an artificial lighting regimen of 12 hours' light and 12 hours' darkness. Over a period of 24 hours we examined at 3-hour intervals the blood glucose concentration and the glycogen content of the liver, heart, skeletal muscle (quadriceps femoris) and white (epididymal) and brown (interscapular) adipose tissue of fed rats and rats fasted for 24 hours. The experiments were carried out in the autumn and the results were evaluated statistically by an analysis of variance and the cosinor test. The blood glucose level and the glycogen concentration in all the given tissues, in both fed and starved rats, displayed rhythmic oscillations with a 24- or 12-hour period in the course of the day, with the exception of glycogen in the white adipose tissue of fed rats, in which cosinor analysis failed to demonstrate any rhythm. One day's fasting did not affect the character of circadian rhythm.     

Diurnal variations of plasma lipids, tissue and plasma lipoprotein lipase, and VLDL secretion rates in the rat. A model for studies of VLDL metabolism

Circadian rhythms of plasma lipids and lipoproteins, lipoprotein lipase activities and VLDL secretion rates were studied in fed and food-deprived (12 h) male rats after a light/dark synchronization of 14 days. In ad libitum fed rats, a circadian rhythm of plasma triacylglycerol, blood glucose and liver glycogen was clearly identified. A rhythm was also identified for plasma cholesterol, but not phospholipids. The peak of plasma triacylglycerol occurred 2 h after the beginning of the light period (7.00 a.m.), and the nadir, 2 h after the beginning of the dark period (7.00 p.m.). The differences of plasma triacylglycerol at these two circadian stages were even more pronounced in food-deprived rats and were confined to the very-low-density lipoprotein (VLDL) fraction. Plasma post-heparin and heart and muscle lipoprotein lipase activities were 50-100% higher at 7.00 p.m., the time when plasma triacylglycerol were lowest, as compared to 7.00 a.m. Plasma post-heparin hepatic lipase and adipose tissue lipoprotein lipase activities, in contrast, did not change. VLDL secretion rates were somewhat higher at 7.00 a.m. compared to 7.00 p.m., but this difference was not significant. It is concluded that physiological variation of heart and muscle lipoprotein lipase together with small differences of VLDL secretion rates are responsible for normal range oscillations of plasma VLDL triacylglycerol levels.     

Influence of seasons on circadian rhythm of acid phosphatase and beta-acetyl-glucosaminidase activity in the mouse salivary gland and liver

Influence of seasons on circadian changes in acid phosphatase and beta-acetylglucosaminidase activity was studied in the liver and submandibular gland of sexually mature mice. Seasonal differences in circadian rhythm was found in both examined enzymes in both organs. These changes were independent from each other and each rhythm was differently subjected to seasonal changes. The lowest seasonal influence was observed in acid phosphatase in the salivary gland. Activity of the other enzymes changed in different degrees, having their acrophases at different times of the day and different rhythm intensity. The highest circadian activity changes measured by amplitude and mesor were observed in spring and summer whereas in the autumn and winter their activity had much weaker rhythms or even they disappeared completely. An attempt was made to explain the observed changes by changes in hormonal background and a certain kind of genetic memory pertaining to laboratory animals.     

Circadian Rhythm of the Liver of Male Rats Dosed with Phenobarbital—I. Organ Weight,Cellular Structures,Glycogen Contents and Mitotic Activity

The circadian rhythm of the liver, namely organ weight, cellular structures (by light-microscopy), glycogen content (by periodic acid-Schiff (PAS) reaction) and mitotic activity, was studied in 166 male Sprague-Dawley rats orally treated daily at 0800-0900 with 70 (study 1) or 50 (study 2) mg/kg phenobarbital (PB) for 7days. Thereafter, eight (study 1) or five (study 2) rats each were studied at 4-hr intervals at 1000, 1400, 1800, 2200, 0200, 0600 and 1000 through till the following day. The lighting schedule in the colony was 12:12, light:dark (light from 0600 to 1800). The liver weight was raised in PB-treated rats at all times of the day compared to controls and showed a distinct circadian rhythm with a peak at 1000 and a minimum at 2200 in PB-treated rats and the controls. The circadian rhythm of cellular structures was closely related to the hepatic glycogen content which was in good agreement with the controls, but at 1400 and 1800 the glycogen particles were more distinctly diminished in the enlarged centrilobular hepatocytes of PB-treated rats. The mitotic activity of hepatocytes was markedly increased in rats treated with PB but showed the same circadian rhythm as controls with a peak at 1000.     

Circadian Rhythm of the Liver of Male Rats Dosed with Phenobarbital—I. Organ Weight, Cellular Structures, Glycogen Contents and Mitotic Activity

The circadian rhythm of the liver, namely organ weight, cellular structures (by light-microscopy), glycogen content (by periodic acid-Schiff (PAS) reaction) and mitotic activity, was studied in 166 male Sprague-Dawley rats orally treated daily at 0800-0900 with 70 (study 1) or 50 (study 2) mg/kg phenobarbital (PB) for 7days. Thereafter, eight (study 1) or five (study 2) rats each were studied at 4-hr intervals at 1000, 1400, 1800, 2200, 0200, 0600 and 1000 through till the following day. The lighting schedule in the colony was 12:12, light:dark (light from 0600 to 1800). The liver weight was raised in PB-treated rats at all times of the day compared to controls and showed a distinct circadian rhythm with a peak at 1000 and a minimum at 2200 in PB-treated rats and the controls. The circadian rhythm of cellular structures was closely related to the hepatic glycogen content which was in good agreement with the controls, but at 1400 and 1800 the glycogen particles were more distinctly diminished in the enlarged centrilobular hepatocytes of PB-treated rats. The mitotic activity of hepatocytes was markedly increased in rats treated with PB but showed the same circadian rhythm as controls with a peak at 1000.     

Circadian Rhythm of Liver Parameters (Cellular Structures, Mitotic Activity, Glycogen and Lipids in Liver and Serum) During Three Consecutive Cycles in Phenobarbital-Treated Rats

The circadian rhythm of gastric content, serum alkaline phosphatase (alk.P.), serum lipids, body weight (wt), relative (rel.) liver wt, cellular structures (by light- and electron-microscopy), mitotic activity of hepatocytes, glycogen content, protein and lipids in liver was studied in 180 male Sprague-Dawley rats orally treated at 0830-1030 with 50 mg/kg phenobarbital (PB) for 7 days. Thereafter, five PB-treated males and five controls each were studied at 4-hr intervals at 0600, 1000, 1400, 1800, 2200 and 0200 on 3 consecutive days. The lighting schedule in the colony was 12:12 = light/dark (light from 0600 to 1800). Following the rhythm of gastric emptying, the rel. liver wt showed a clear circadian rhythm with a peak at 0800. The rel. liver wt was raised in PB-treated rats at all times of the day. The circadian rhythm of cellular structures was closely related to the hepatic glycogen content which exhibited a clear rhythm with the peak also at 0800, but lowered values were found in PB-treated rats. The mitotic activity of hepatocytes was significantly increased in PB-treated rats but displayed the same circadian rhythm as controls with peaks at noon and troughs at midnight. The well-known hypertrophy of the smooth endoplasmic reticulum in PB-treated rats was not found at 0600, but was fully developed at 1400 and 2200. PB-treatment increased significantly the liver content of cholesterol, triglycerides and phospholipids. Liver cholesterol showed a clear circadian rhythm with peaks at 1800. No rhythm of liver protein, triglycerides and phospholipids was observed. In serum, levels of cholesterol were significantly elevated, those of triglycerides and alk.P. significantly lowered, while those of phospholipids were not affected by the treatment. The three serum lipids, alk.P. and beta-lipoprotein exhibited a clear circadian rhythm, while serum glucose and non-esterified fatty acids did not.     

Interseasonal variation in the circadian rhythms of locomotor activity and temperature selection in sleepy lizards, <Emphasis Type="Italic">Tiliqua rugosa</Emphasis>

Few studies in non-mammalian vertebrates have examined how various effectors of the circadian system interact. To determine if the daily locomotor and behavioural thermoregulatory rhythms of Tiliqua rugosa are both controlled by the circadian system in different seasons, lizards were tested in laboratory thermal gradients in four seasons and in constant darkness. Circadian rhythmicity for both rhythms was present in each season, being most pronounced in spring and summer and least evident in autumn. Most lizards displayed a unimodal locomotor activity pattern across all seasons. However, some individuals presented a bimodal locomotor activity pattern in spring and summer. Seasonal variations in the phase relationships of both rhythms to the light:dark (LD) cycle were demonstrated. No seasonal differences in the free-running period lengths of either rhythm were detected, raising the possibility that a single circadian pacemaker drives both rhythms in this species. Our present results demonstrate that both rhythms are similarly controlled by the circadian system in each season. Although seasonal variations in the thermal preferences of reptiles both in the field and laboratory have previously been well documented, this study is the first to demonstrate circadian rhythms of temperature selection in a reptile species in each season.     

Circadian Rhythm of Liver Parameters (Cellular Structures,Mitotic Activity,Glycogen and Lipids in Liver and Serum) During Three Consecutive Cycles in Phenobarbital-Treated Rats

The circadian rhythm of gastric content, serum alkaline phosphatase (alk.P.), serum lipids, body weight (wt), relative (rel.) liver wt, cellular structures (by light- and electron-microscopy), mitotic activity of hepatocytes, glycogen content, protein and lipids in liver was studied in 180 male Sprague-Dawley rats orally treated at 0830-1030 with 50 mg/kg phenobarbital (PB) for 7 days. Thereafter, five PB-treated males and five controls each were studied at 4-hr intervals at 0600, 1000, 1400, 1800, 2200 and 0200 on 3 consecutive days. The lighting schedule in the colony was 12:12 = light/dark (light from 0600 to 1800). Following the rhythm of gastric emptying, the rel. liver wt showed a clear circadian rhythm with a peak at 0800. The rel. liver wt was raised in PB-treated rats at all times of the day. The circadian rhythm of cellular structures was closely related to the hepatic glycogen content which exhibited a clear rhythm with the peak also at 0800, but lowered values were found in PB-treated rats. The mitotic activity of hepatocytes was significantly increased in PB-treated rats but displayed the same circadian rhythm as controls with peaks at noon and troughs at midnight. The well-known hypertrophy of the smooth endoplasmic reticulum in PB-treated rats was not found at 0600, but was fully developed at 1400 and 2200. PB-treatment increased significantly the liver content of cholesterol, triglycerides and phospholipids. Liver cholesterol showed a clear circadian rhythm with peaks at 1800. No rhythm of liver protein, triglycerides and phospholipids was observed. In serum, levels of cholesterol were significantly elevated, those of triglycerides and alk.P. significantly lowered, while those of phospholipids were not affected by the treatment. The three serum lipids, alk.P. and beta-lipoprotein exhibited a clear circadian rhythm, while serum glucose and non-esterified fatty acids did not.     

Circadian changes of gluconeogenic enzymes in irradiated rats

The authors studied the effect of whole body irradiation at different times of day on the circadian rhythms of gluconeogenic enzymes. They found that: 1. liver and kidney enzyme activities were highest in the light part of the day and lowest in the middle of the dark part; 2. 12-h circadian rhythm of glucose-6-phosphatase and fructose-1, 6-bisphosphatase activity in the liver and the renal cortex followed a similar course; 3. a lethal whole body dose of 14.4 Gy X-rays did not affect the circadian oscillation curves of the given enzymes, with the exception of fructose-1, 6-bisphosphatase in the liver of irradiated rats, where the rhythm was lost.     

Seasonal and circadian fluctuations of the biochemical indices of the blood in mice

Seasonal (in January, April, July, October) changes of aspartate aminotransferase (AST), alanine aminotransferase (ALT), protein, bilirubin, glucose, cholesterol, creatinine, blood urea nitrogen, Cl-, K+, Na+ content were studied in the blood plasma of mice at different time of day (6 p. m., midnight, 6 a. m., midday). The analysis of the average daily indices has shown that the most expressed variations were the following: AST (spring maximum is 3.7 times higher than autumn minimum), ALT (winter maximum is 2.9 times higher than autumn minimum), creatinine (summer maximum is 2.5 times higher than winter minimum), blood urea nitrogen (summer maximum is 2.5 times higher than autumn minimum). Bilirubin and protein content in spring is insignificant, but it is significantly higher than in other seasons. Cholesterol content is lower in winter. No differences in glucose, Cl-, K+, Na+ content in different months have been revealed. The largest circadian synchronization was observed in winter in AST, glucose, cholesterol, protein, Cl-, K+, Na+ (the level observed at 6 p. m. and at midday is higher than that observed at midnight and 6 a. m.) and in autumn in AST, ALT, glucose, cholesterol, blood urea nitrogen, with the circadian curves inverse as compared to the winter period. In spring practically no circadian synchronization was observed.     

Seasonal changes in haematology and metabolic resources in the tench

Significant variations in the number of white and red blood cells, haematocrit and haemoglobin were found throughout the year in sexually mature male and female tench Tinca tinca . In general, the lowest values were observed during autumn–winter and the highest during summer, with males exhibiting higher values than females. Plasma glucose, cholesterol and triglycerides were lower during the winter than during the summer–autumn seasons in both sexes. Gonado-somatic and hepato-somatic indices were inversely correlated in female tench throughout the year. Seasonal patterns in liver metabolic resources were very similar for both sexes. For males and females, liver glycogen and proteins increased during the autumn, whereas the liver stored lipid during spring. Dorsal muscle mainly deposited glycogen, whereas lipid was mainly stored in the ventral muscle. Relations between seasonal changes in environmental factors, such as feeding and temperature are discussed.     

Seasonal changes in the circadian activity rhythm of light-dark (LD) and completely dark (DD) regimen in the mouse submandibular gland in the presence of light-dark (LD) and completely dark (DD) regimen

Influence of seasons on circadian activity changes and the influence of one and six weeks of DD upon these changes of acid phosphatase (AP) and beta-acetylglucosaminidase (AM) was studied in the submandibular gland of sexually mature male mice. Total enzyme activity was determined in tissue homogenates at four-hour intervals in March, June, October, and February under standard LD12/12 conditions and after one and six weeks of the DD regime. The rhythms were analysed according to cosinor method. Under constant lighting conditions the seasonal differences in the AM circadian activity rhythm were found. AP activity was considerably less influenced by seasonal changes. Both enzyme activity changes were independent of each other and each rhythm was differently influenced by DD. In the case of AM the most pronounced circadian activity changes had the highest amplitude and mesor occurred in summer. The strongest influence of DD upon this enzyme activity rhythm was observed in spring and summer especially after the first week, after six weeks the acrophase returned to the LD group value (spring). In autumn and winter the reaction to DD was different to that of summer and spring. For AP the circadian changes of activity were non-rhythmic in spring, whereas in all other seasons the acrophases occurred almost at the same time in the afternoon. In DD the activity rhythm significantly changed after six weeks. In all seasons, except spring the circadian rhythm of activity was not observed after six weeks of DD. An attempt was made to explain the observed results by the certain kind of genetic memory present in laboratory animals the neurohormonal system of which is influenced by seasonal changes.     

Circadian rhythm of serum and tissue lipids in the rat: the effect of limited access to food

Young male Wistar rats of a conventional breed were kept 4 weeks in a separate room with a light: dark regimen of 12:12 h. Some were fed ad libitum (group C, the control), some were meal-fed from 8.30 to 10.30 a.m. (group A) and the others were meal-fed from 8.30 to 10.30 p.m. (group B). When adapted to the nutritional regimen the animals were decapitated at 3-hour intervals, starting at 8 a.m., and their serum and epididymal tissue non-esterified fatty acid values, their serum and liver triacylglycerol and total cholesterol levels and their serum phospholipid concentration were determined. Limitation of the period for which they had access to food at different times of the day significantly synchronized the circadian oscillations of their serum and tissue lipid levels. The most pronounced differences in the form of the antiphase position of the maxima of the curves in groups A and B, or A and C, were recorded in the oscillations of the serum and white adipose non-esterified fatty acid values, in serum triacylglycerol levels and in liver cholesterol levels. It is reasonable to assume that the synchronizing role of timed meal-feeding will be subject to the influence of seasonal factors.     

Time-restricted feeding entrains daily rhythms of energy metabolism in mice

Energy metabolism, oxygen consumption rate (VO2), and respiratory quotient (RQ) in mice were monitored continuously throughout 12:12-h light-dark cycles before, during, and after time-restricted feeding (RF). Mice fed ad libitum showed robust daily rhythms in both parameters: high during the dark phase and low during the light phase. The daily profile of energy metabolism in mice under daytime-only feeding was reversed at the beginning of the first fasting night. A few days after daytime-only feeding began, RF also reversed the circadian core body temperature rhythm. Moreover, RF for 6 consecutive days shifted the phases of circadian expression patterns of clock genes in liver significantly by 8-10 h. When mice were fed a high-fat (HF) diet ad libitum, the daily rhythm of RQ dampened day by day and disappeared on the sixth day of RF, whereas VO2 showed a robust daily rhythm. Mice fed HF only in the daytime had reversed VO2 and RQ rhythms. Similarly, mice fed HF only in the daytime significantly phase shifted the clock gene expression in liver, whereas ad libitum feeding with HF had no significant effect on the expression phases of liver clock genes. These results suggested that VO2 is a sensitive indicator of entrainment in the mouse liver. Moreover, physiologically, it can be determined without any surgery or constraint. On the basis of these results, we hypothesize that a change in the daily VO2 rhythm, independent of the energy source, might drive phase shifts of circadian oscillators in peripheral tissues, at least in the liver.     

Circadian features of carbohydrate metabolism in domestic fowls exposed to weekly 120 degree-shifts of synchronizer schedule.

Effects of weekly 8 hr advance- or delay-shifts on the circadian rhythm of plasma glucose, liver glycogen and muscle glycogen in male domestic fowls, beginning at about 3 days of age, were examined. Circadian rhythm in the aforesaid indices of carbohydrate metabolism in control birds was also studied. Blood and tissue samples were collected from birds in all the three groups at 4 hr intervals over a single 24 hr time scale both at 6th and 12th week of age. Plasma glucose and glycogen content in the tissues were determined by employing standard techniques. Cosinor rhythmometry was used for analyzing time series data. In general, a statistically significant circadian rhythm was documented for all the three indices in control and advance-schedule birds, irrespective of age. In contrast, in delay-schedule birds, statistically significant circadian rhythm could not be detected, excluding in muscle glycogen at 12th week of age. The poor growth rate in the delay-schedule birds could be imputed to the disappearance of circadian rhythm in the indices of carbohydrate metabolism.     

Rhythmic cortisol secretion in the equine: Analysis and physiological mechanisms

Abstract

Four Thoroughbred mares (no. 1–4) were maintained under constant temperature (24°C) and controlled light (L/D:12/12 with lights on at 06.00 hr) conditions. They were fed and watered ad libitum with fresh feed and water given at 09.00 hr. After a 45‐day pre‐conditioning period, blood samples were obtained by veinipuncture at 4‐hr intervals for 14 days to determine circadian and day‐to‐day variation. The horses exhibited a circadian rhythm with maximum values attained at about 12.00 hr, however, there are periods of days in which no rhythm is distinguishable. Ultradian rhythms with mean periods of 105 to 128 and 24 to 31 min are superimposed upon the circadian rhythm. The individual rhythms are quite variable from horse to horse and within the same horse. During periods of decline in plasma cortisol with metabolic half‐lives of approximately 70 min, secretion of cortisol was very low or had ceased. During periods of increasing plasma concentration, secretion was occurring at a faster rate than degradation. Rapid decreases in plasma concentration (metabolic half‐life of approximately 30 min) was accompanied by a rise in specific activity indicating cortisol with a high specific activity was entering the plasma pool from other storage pools.     

Prolonged melatonin administration in 6-month-old Sprague-Dawley rats: metabolic alterations

The aim of this work was to evaluate the effect of prolonged melatonin administration on chosen metabolic and hormonal variables in male and female Sprague-Dawley rats. Melatonin was administered in tap water (4 microg/ml) daily from the 6th month of age. Rats were fed a standard type of diet ad libitum and were kept in a light regimen L:D--12:12h. The experiment was terminated after 12 weeks of melatonin administration. Melatonin decreased body mass during the whole experiment in females and from the 42nd day of the experiment in males. Relative heart muscle weight in females and absolute/relative thymus weight in males were increased after melatonin administration. Melatonin decreased glycaemia, heart muscle glycogen concentration in females and liver glycogen concentration in both sexes. Serum insulin concentration in males was decreased; serum corticosterone concentration was increased in both males and females. Serum triacylglycerol and heart muscle cholesterol concentration in females were decreased, however in males serum and heart muscle cholesterol concentration was increased. Liver phospholipid concentration in females was decreased and heart muscle phospholipid concentration in males was increased. Melatonin increased malondialdehyde concentration in heart muscle in males and in liver in both sexes. Melatonin induced prominent sex-dependent changes in both carbohydrate and lipid metabolism.     

Abnormalities of lipid metabolism in the vitamin E-deficient monkey

A soybean protein diet was used to induce vitamin E deficiency in rhesus monkeys. The deficient monkeys had reduced triglyceride concentrations in liver and skeletal muscle, but the cholesterol concentration in their skeletal muscle was increased. A constant amount of radioactively labeled (3)H-cholesterol-7alpha-(3)H was fed daily for 48-114 days to control and vitamin E-deficient monkeys to study the relationship between plasma, liver, and skeletal muscle cholesterol. Plasma cholesterol reached constant, maximum specific activity by the 42nd day both in control and in vitamin E-deficient monkeys. In control and previously deficient vitamin E-treated monkeys the specific activity of cholesterol in liver and skeletal muscle was approximately equal to that of plasma. In vitamin E-deficient monkeys the liver cholesterol specific activity was equal to that of plasma cholesterol, but the ratio of skeletal muscle cholesterol specific activity to plasma cholesterol specific activity was reduced. It is concluded from these studies that there is a specific defect(s) in cholesterol metabolism in the skeletal muscle of vitamin E-deficient monkeys.