Circadian rhythm of the angiotensin converting enzyme (ACE) activity in serum of healthy adult subjects

Angiotensin converting enzyme (ACE) activity was measured in the peripheral serum of 10 healthy, diurnally-active and nocturnally-resting adult subjects (5 women and 5 men). Blood was drawn serially at 4-h intervals throughout a 24-h cycle with the subjects hospitalized and synchronized. They were not kept in recumbency. Data were evaluated by conventional statistical analysis and by single- and population mean- cosinor procedures. A low-amplitude circadian rhythm was statistically validated for the group. Acrophase was located in the afternoon, at about 1630. The recognition that ACE activity oscillates physiologically in the peripheral blood according to a circadian rhythm may serve to amplify the clinical use of ACE measurements.     

Effect of Sex, Menstrual Cycle Phase, and Oral Contraceptive Use on Circadian Temperature Rhythms

The circadian rhythm of rectal temperature was continuously recorded over several consecutive days in young men and women on regular nocturnal sleep schedules. There were 50 men, 21 women with natural menstrual cycles [i.e., not taking oral contraceptives (OCs) (10 in the follicular phase and 11 in the luteal phase)], and 14 women using OCs (6 in the pseudofollicular phase and 8 in the pseudoluteal phase). Circadian phase and amplitude were estimated using a curve-fitting procedure, and temperature levels were determined from the raw data. A two-way analysis of variance (ANOVA) on the data from the four groups of women, with factors menstrual cycle phase (follicular, luteal) and OC use (yes, no), showed that temperature during sleep was lower during the follicular phase than during the luteal phase. Since waking temperatures were similar in the two phases, the circadian amplitude was also larger during the follicular phase. The lower follicular phase sleep temperature also resulted in a lower 24-h temperature during the follicular phase. The two-way ANOVA showed that temperature during sleep and 24-h temperature were lower in naturally cycling women than in women taking OCs. A one-way ANOVA on the temperature rhythm parameters from the five groups of subjects showed that the temperature rhythms of the men and of the naturally cycling women in the follicular phase were not significantly different. Both of these groups had lower temperatures during sleep, lower 24-h temperatures, and larger circadian amplitudes than the other groups. There were no significant differences in circadian phase among the five groups studied. In conclusion, menstrual cycle phase, OC use, and sex affect the amplitude and level, but not the phase, of the overt circadian temperature rhythm.     

Effect of Sex,Menstrual Cycle Phase,and Oral Contraceptive Use on Circadian Temperature Rhythms

The circadian rhythm of rectal temperature was continuously recorded over several consecutive days in young men and women on regular nocturnal sleep schedules. There were 50 men, 21 women with natural menstrual cycles [i.e., not taking oral contraceptives (OCs) (10 in the follicular phase and 11 in the luteal phase)], and 14 women using OCs (6 in the pseudofollicular phase and 8 in the pseudoluteal phase). Circadian phase and amplitude were estimated using a curve-fitting procedure, and temperature levels were determined from the raw data. A two-way analysis of variance (ANOVA) on the data from the four groups of women, with factors menstrual cycle phase (follicular, luteal) and OC use (yes, no), showed that temperature during sleep was lower during the follicular phase than during the luteal phase. Since waking temperatures were similar in the two phases, the circadian amplitude was also larger during the follicular phase. The lower follicular phase sleep temperature also resulted in a lower 24-h temperature during the follicular phase. The two-way ANOVA showed that temperature during sleep and 24-h temperature were lower in naturally cycling women than in women taking OCs. A one-way ANOVA on the temperature rhythm parameters from the five groups of subjects showed that the temperature rhythms of the men and of the naturally cycling women in the follicular phase were not significantly different. Both of these groups had lower temperatures during sleep, lower 24-h temperatures, and larger circadian amplitudes than the other groups. There were no significant differences in circadian phase among the five groups studied. In conclusion, menstrual cycle phase, OC use, and sex affect the amplitude and level, but not the phase, of the overt circadian temperature rhythm.     

Repeated assessment of the endogenous 24-hour profile of blood pressure under constant routine

The impact of environmental and behavioral factors on the 24-h profile of blood pressure (BP) has been well established. Various attempts have been made to control these exogenous factors, in order to investigate a possible endogenous circadian variation of BP. Recently, we reported the results of the first environmentally and behaviorally controlled laboratory study with 24-h recordings of BP and heart rate (HR) during maintained wakefulness. In this constant-routine study, a pronounced endogenous circadian rhythm of HR was found, but circadian variation of BP was absent. This result suggested that the circadian rhythm of BP observed in earlier controlled studies, with sleep allowed, was evoked by the sleep-wake cycle as opposed to the endogenous circadian pacemaker. In order to verify our previous finding during maintained wakefulness, we repeated the experiment five times with six normotensive, healthy young subjects. Statistical analyses of the hourly measurements of BP and HR confirmed the replicable presence of an endogenous circadian rhythm of HR, as well as the consistent absence of an endogenous circadian variation of BP. Thus, this study provided additional evidence that the 24-h profile of BP—as observed under normal circumstances—is the sole result of environmental and behavioral factors such as the occurrence of sleep, and has no endogenous circadian component. (Chronobiology International, 18(1), 85-98, 2001)     

Altered circadian periodicities in oral temperature and mood in men on an 18-hour work/rest cycle during a nuclear submarine patrol

A group of nuclear submariners was studied to examine whether an 18-h routine (6-h on, 12-h off watch) during a 10-week submerged patrol affected the 24-h circadian rhythm in oral temperature, Thayer's activation, Mood 'Activity' (MA) and Mood 'Happiness' (MH). They were observed during three phases of the patrol: Phase 1, the beginning 8-day period; Phase 2, the middle of the voyage; and Phase 3, the last 7-8 day period. The group-synchronized 24-h rhythm in oral temperature disappeared during Phase 3. The group-synchronized 24-h rhythms in Thayer's activation and in MA and MH disappeared during Phases 2 and 3. A group-synchronized 18-h rhythm was not produced in any of the variables in any phase, except MH during Phase 2. Periodicity analysis of the individuals' data showed that a loss of 24-h rhythmicity in oral temperature was due not only to reduced circadian amplitude but also to a dispersion of Time of Peak (TOPs). Loss of 24-h rhythm in 'Activation', 'Happiness', and 'Activity' was predominantly due to a wider dispersion of TOPs. The 18-h routine did appear to exert a small modulating effect on rhythmic activity in the variables examined in this study. Since the sleep/wakefulness cycle was well entrained by the 18-h routine, the submariners experienced a spontaneous internal desynchronization between the activity cycle and the cycles or oral temperature and psychological states. The performance and health consequences of this chronic dyschronism have yet to be explored. We suggest further research to determine the usefulness of an index of synchronization among the physiological and psychological variables, and the relationship of the desynchronizing effects to performance.     

Features of the circadian rhythm of skin temperature in eight- to nine-year-old children and young adults

The age-related features of the circadian rhythm of skin temperature were studied using the Termochron iButton method in two age groups of subjects: eight-to nine-year-old boys and girls and young men and women aged 20–22 years. For this purpose, the temperature was monitored for 48 h at a recording interval of 10 min. The following authentic chronobiological differences were revealed: the temperature mesor was higher in the girls than in the boys and in the young men it was higher than in the young women. The circadian rhythm’s amplitude was higher in the boys and the young women. Studies of the chronological values in different 24-h periods showed that the average level of temperature at night was lower in all subjects. The amplitude in the adults did not differ in different 24-h periods, whereas it was higher in the children at night.     

REPEATED ASSESSMENT OF THE ENDOGENOUS 24-HOUR PROFILE OF BLOOD PRESSURE UNDER CONSTANT ROUTINE*

The impact of environmental and behavioral factors on the 24-h profile of blood pressure (BP) has been well established. Various attempts have been made to control these exogenous factors, in order to investigate a possible endogenous circadian variation of BP. Recently, we reported the results of the first environmentally and behaviorally controlled laboratory study with 24-h recordings of BP and heart rate (HR) during maintained wakefulness. In this constant-routine study, a pronounced endogenous circadian rhythm of HR was found, but circadian variation of BP was absent. This result suggested that the circadian rhythm of BP observed in earlier controlled studies, with sleep allowed, was evoked by the sleep–wake cycle as opposed to the endogenous circadian pacemaker. In order to verify our previous finding during maintained wakefulness, we repeated the experiment five times with six normotensive, healthy young subjects. Statistical analyses of the hourly measurements of BP and HR confirmed the replicable presence of an endogenous circadian rhythm of HR, as well as the consistent absence of an endogenous circadian variation of BP. Thus, this study provided additional evidence that the 24-h profile of BP—as observed under normal circumstances—is the sole result of environmental and behavioral factors such as the occurrence of sleep, and has no endogenous circadian component. (Chronobiology International, 18(1), 85–98, 2001)     

Linear demasking techniques are unreliable for estimating the circadian phase of ambulatory temperature data.

Clinical investigators often use ambulatory temperature monitoring to assess the endogenous phase and amplitude of an individual's circadian pacemaker for diagnostic and research purposes. However, an individual's daily schedule includes changes in levels of activity, in posture, and in sleep-wake state, all of which are known to have masking or evoked effects on core body temperature (CBT) data. To compensate for or to correct these masking effects, many investigators have developed "demasking" techniques to extract the underlying circadian phase and amplitude data. However, the validity of these methods is uncertain. Therefore, the authors tested a variety of analytic methods on two different ambulatory data sets from two different studies in which the endogenous circadian pacemaker was not synchronized to the sleep-wake schedule. In both studies, circadian phase estimates calculated from CBT collected when each subject was ambulatory (i.e., free to perform usual daily activities) were compared to those calculated during the same study when the same subject's activities were controlled. In the first study, 24 sighted young and older subjects living on a 28-h scheduled "day" protocol were studied for approximately 21 to 25 cycles of 28-h each. In the second study, a blind man whose endogenous circadian rhythms were not synchronized to the 24-h day despite his maintenance of a regular 24-h sleep-wake schedule was studied for more than 80 consecutive 24-h days. During both studies, the relative phase of the endogenous (circadian) and evoked (scheduled activity-rest) components of the ambulatory temperature data changed progressively and relatively slowly, enabling analysis of the CBT rhythm at nearly all phase relationships between the two components. The analyses of the ambulatory temperature data demonstrate that the masking of the CBT rhythm evoked by changes in activity levels, posture, or sleep-wake state associated with the evoked schedule of activity and rest can significantly obscure the endogenous circadian component of the signal, the object of study. In addition, the masking effect of these evoked responses on temperature depends on the circadian phase at which they occur. These nonlinear interactions between circadian phase and sleep-wake schedule render ambulatory temperature data unreliable for the assessment of endogenous circadian phase. Even when proposed algebraic demasking techniques are used in an attempt to reveal the endogenous temperature rhythm, the phase estimates remain severely compromised.     

Activity in the ferret: oestradiol effects and circadian rhythms

The present study was conducted to determine whether oestradiol increases activity in the European ferret (Mustela furo), whether this effect is sexually dimorphic, and whether a 24-h rhythm is present in the ferret's daily activity. The activity of male and female adult, postpubertally gonadectomized ferrets was monitored while they were maintained singly on a 13:11 light-dark cycle, before and after implantation with oestradiol-17β. Gonadectomized male and female ferrets exhibited equal levels of activity, and neither sex exhibited a significant change in activity following oestradiol implantation. None of the ferrets exhibited a strong circadian rhythm, although weak 24-h rhythms and shorter harmonic rhythms were present. Golden hamsters (Mesocricetus auratus), monitored in an identical manner, exhibited strong circadian rhythms. It was concluded that oestradiol administration may not cause an increase in activity in the ferret, and that this species lacks a strong circadian activity rhythm.     

Circadian rhythm parameters of endocrine functions in elderly subjects during the seventh to the ninth decade of life

The circadian rhythm of 17 endocrine parameters (ACTH, aldosterone, cortisol, C-peptide, DHEA-S, FSH, growth hormone, insulin, LH, 17-OH progesterone, prolactin, testosterone, total T3, total T4 and TSH and estradiol and progesterone in women only) were studied in 63 clinically apparently healthy men (124 profiles) and 86 women (154 profiles) during the 7th to 9th decade of life. The subjects lived under very uniform conditions in a home for the aged with their daily schedule standardized by institutional routine with rest at night on the average from 21:30 to 06:30 local time and 3 daily meals at 08:30, 13:00 and 18:30. Blood was drawn over a 24-h span at 4-h intervals. Circadian periodicity was ascertained and the rhythm parameters quantified by cosinor analysis. In clinically healthy elderly subjects, circadian periodicity persisted in most parameters studied well into the 9th decade of life. The timing of the circadian rhythm was comparable between subjects in their 7th decade and 9th decade of life with the exception of cortisol and DHEA-S, which showed a phase advance with advancing age. A decrease in circadian amplitude is limited during this part of the human life span to only a few of the functions investigated and with the exception of prolactin in the women, a decrease in amplitude did accompany a decrease in MESOR.     

Salivary melatonin rhythm as a marker of the circadian system in healthy children and those with attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is the most common neurobehavioral disorder of childhood. Problems with sleep structure, efficiency, and timing have been reported in some, but not all, studies on ADHD children. As the sleep-wake cycle belongs to circadian rhythms, the timekeeping circadian system might be involved in ADHD. To assess whether the circadian system of ADHD children differs from that of controls, the rhythm of the pineal hormone melatonin was used as a reliable marker of the system. Saliva from 34 ADHD and 43 control 6- to 12-yr-old children was sampled at 2-h intervals throughout the entire 24-h cycle, and the melatonin profiles of the ADHD and control children were compared. The nocturnal melatonin peaks of the ADHD and control group did not differ significantly. The high nocturnal interindividual variability of the peaks seen in adulthood was present already in the studied children. The 24-h melatonin profiles of all the ADHD subjects did not differ significantly from those of the control subjects. Categorization of subjects according to age, into groups of 6- to 7-yr-old (9 ADHD, 5 control), 8- to 9-yr-old (16 ADHD, 26 control), and 10- to 12-yr-old (9 ADHD, 12 control) children, revealed significant differences between the ADHD and control group in the melatonin rhythm waveform, but not in nocturnal melatonin peaks; the peaks were about the same in both groups and did not change significantly with increasing age. In the oldest, but not in the younger, children, the melatonin signal duration in the ADHD group was shorter than in the control group. The difference might be due to the fact that whereas in the control group both the evening melatonin onset and the morning offset phase delayed in the oldest children relative to those in the youngest children, in the ADHD group only the onset, but not the offset, phase delayed with increasing age. The data may indicate subtle differences between the circadian system of ADHD and control children during development.     

The effects of low-dose 0.5-mg melatonin on the free-running circadian rhythms of blind subjects

Exogenous melatonin (0.5-10 mg) has been shown to entrain the free-running circadian rhythms of some blind subjects. The aim of this study was to assess further the entraining effects of a daily dose of 0.5 mg melatonin on the cortisol rhythm and its acute effects on subjective sleep in blind subjects with free-running 6-sulphatoxymelatonin (aMT6s) rhythms (circadian period [tau] 24.23-24.95 h). Ten subjects (9 males) were studied, aged 32 to 65 years, with no conscious light perception (NPL). In a placebo-controlled, single-blind design, subjects received 0.5 mg melatonin or placebo p.o. daily at 2100 h (treatment duration 26-81 days depending on individuals' circadian period). Subjective sleep was assessed from daily sleep and nap diaries. Urinary cortisol and aMT6s were assessed for 24 to 48 h weekly and measured by radioimmunoassay. Seven subjects exhibited an entrained or shortened cortisol period during melatonin treatment. Of these, 4 subjects entrained with a period indistinguishable from 24 h, 2 subjects continued to free run for up to 25 days during melatonin treatment before their cortisol rhythm became entrained, and 1 subject appeared to exhibit a shortened cortisol period throughout melatonin treatment. The subjects who entrained within 7 days did so when melatonin treatment commenced in the phase advance portion of the melatonin PRC (CT6-18). When melatonin treatment ceased, cortisol and aMT6s rhythms free ran at a similar period to before treatment. Three subjects failed to entrain with initial melatonin treatment commencing in the phase delay portion of the PRC. During melatonin treatment, there was a significant increase in nighttime sleep duration and a reduction in the number and duration of daytime naps. The positive effect of melatonin on sleep may be partly due to its acute soporific properties. The findings demonstrate that a daily dose of 0.5 mg melatonin is effective at entraining the free-running circadian systems in most of the blind subjects studied, and that circadian time (CT) of administration of melatonin may be important in determining whether a subject entrains to melatonin treatment. Optimal treatment with melatonin for this non-24-h sleep disorder should correct the underlying circadian disorder (to entrain the sleep-wake cycle) in addition to improving sleep acutely.     

Hypoxic alterations of cortisol circadian rhythm in man after simulation of a long duration flight

Fatigue is often reported after long duration flights. Mild hypobaric hypoxia caused by pressurisation may be involved in this effect through disruption of circadian rhythms, independently of the number of time zones crossed. In this controlled crossover study, we assessed the effects of two levels of hypoxia equivalent to 8000 and 12,000 ft on the circadian rhythm of plasma cortisol, a marker of the circadian time structure. Sixteen healthy young male volunteers (23-39 years) were exposed in a hypobaric chamber for 8 h (08:00-16:00 h) to 8000 ft, followed 4 weeks later to 12,000 ft. Plasma cortisol was assayed during two 24-h cycles (control and hypoxic exposure) every 2h in all subjects. We found a significant change in the pattern of cortisol secretion during both hypoxic exposures, with an initial fall in cortisol followed by a transient rebound, whereas the phase and the 24-h mean level remained unchanged. The change in cortisol pattern followed the alterations in autonomic balance assessed by heart rate variability (HRV) spectral analysis. The normalised high frequencies and the low-to-high frequencies ratio showed a significant shift toward sympathetic dominance with some differences in time course for both altitudes studied. HRV analysis improved the interpretation of cortisol 24-h profiles. Our data, which strongly suggest that prolonged mild hypoxia alters the expression of cortisol circadian rhythm, should be taken into account to interpret secretory rhythm changes after transmeridian flights.     

Circadian Rhythms of Serum Concentrations of 12 Enzymes of Clinical Interest

A total of 25 apparently healthy adults (13 men and 12 women), 29.5 years (SD = 3.6 years) of age, served as subjects in a 24-h study conducted in Barcelona, Spain, in the spring of 1990. The group had a homogeneous pattern of meals, activity, and behavior. Six blood samples were collected at 4-h intervals over a single 24-h period beginning at 10:00 h. The oral temperature was measured at 2-h intervals to facilitate an independent biological time reference for the local population being studied. The serum concentration of 12 enzymes of clinical interest were measured in each sample: creatine kinase, creatine kinase 2, alanine aminotransferase, aspartate aminotransferase, γ-glutamyltransferase, alkaline phosphatase, cholinesterase, lactate dehydrogenase, lactate dehydrogenase 1, 5′-nucleotidase, pancreatic α-amylase, and triacylglycerol lipase. We supposed that all experimental data obtained for a quantity came from a single “hypothetical subject” that represented the central tendency of the population and then these data were analyzed for circadian rhythm by single cosinor. A statistically significant circadian rhythm was detected in all quantities studied (p ≤ 0.05) except for serum concentrations of pancreatic α-amylase and triacylglycerol lipase. The maximum daily rhythmic variation was ～ 10% (interval, 6–14%) for all quantities studied except pancreatic α-amylase (2.6%). This rhythmic variation is greater than the analytical variation except for 5′-nucleotidase and pancreatic α-amylase. The acrophases for the quantities studied (except that of triacylglycerol lipase) coincide with times near those of the oral temperature acrophase (18:01 local time). The results of this study will doubtless contribute to further documentation of the structure of the human circadian timing system and to establishment of time-qualified reference intervals for a defined group of subjects.     

Circadian variation of fibrinolytic activity in blood.

Approximately 35 years ago, it was discovered that spontaneous fibrinolytic activity in blood showed a sinusoidal variation with a period of 24 h; it increased severalfold during the day, reaching a peak at 6:00 p.m. and then dropped to trough levels at 3:00-4:00 a.m. The range of the fluctuation and the 24-h mean levels were highly reproducible within an individual; moreover, the timing of the oscillation was remarkably consistent among individuals, with a fixed phase relationship to external clock time. The biorhythm could not be accounted for simply by variations in physical activity, body posture, or sleep/wake schedule. Gender, ethnic origin, meals, or resting levels of blood fibrinolytic activity also did not influence the basic features of the rhythm. Older subjects, compared to younger ones, showed a blunted diurnal increase in fibrinolytic activity in blood. Recent studies have established that, of the known components of the fibrinolytic system, only tissue-type plasminogen activator (tPA) and its fast-acting inhibitor, plasminogen activator inhibitor-1 (PAI-1), show a marked circadian variation in plasma. In contrast, levels of plasminogen, alpha 2-antiplasmin, urinary-type plasminogen activator, and a reversible tPA inhibitor vary little or none during the 24 h. Quenching antibodies to tPA have shown that the circadian rhythm of fibrinolytic activity in blood is due exclusively to changes in tPA activity. However, the 24-h fluctuation of plasma tPA activity is phase shifted in relation to the rhythm of immunoreactive tPA, but shows a precise phase inversion with respect to the 24-h variation of PAI-1 activity and antigen. Therefore, plasma tPA activity, as currently measured in vitro, is tightly and inversely related to the levels of PAI-1 throughout the 24-h cycle. The factors controlling the rhythmicity of plasma PAI-1 are not fully elucidated but probably involve a humoral mechanism; changes in endothelial function, circulating platelet release products, corticosteroids, catecholamines, insulin, activated protein C, or hepatic clearance do not appear to be responsible. Shift workers on weekly shift rotations show a disrupted 24-h rhythm of plasma tPA and PAI-1. In acute and chronic diseases, the circadian rhythmicity of fibrinolytic activity may show a variety of alterations, affecting the 24-h mean, the amplitude, or the timing of the fluctuation. It is advisable, therefore to define the 24-h pattern of plasma tPA and PAI-1 in patient groups, before levels based on a single blood sampling time are compared to those of a control population.(ABSTRACT TRUNCATED AT 400 WORDS)     

Plasticity of the intrinsic period of the human circadian timing system

Human expeditions to Mars will require adaptation to the 24.65-h Martian solar day-night cycle (sol), which is outside the range of entrainment of the human circadian pacemaker under lighting intensities to which astronauts are typically exposed. Failure to entrain the circadian time-keeping system to the desired rest-activity cycle disturbs sleep and impairs cognitive function. Furthermore, differences between the intrinsic circadian period and Earth's 24-h light-dark cycle underlie human circadian rhythm sleep disorders, such as advanced sleep phase disorder and non-24-hour sleep-wake disorders. Therefore, first, we tested whether exposure to a model-based lighting regimen would entrain the human circadian pacemaker at a normal phase angle to the 24.65-h Martian sol and to the 23.5-h day length often required of astronauts during short duration space exploration. Second, we tested here whether such prior entrainment to non-24-h light-dark cycles would lead to subsequent modification of the intrinsic period of the human circadian timing system. Here we show that exposure to moderately bright light ( approximately 450 lux; approximately 1.2 W/m(2)) for the second or first half of the scheduled wake episode is effective for entraining individuals to the 24.65-h Martian sol and a 23.5-h day length, respectively. Estimations of the circadian periods of plasma melatonin, plasma cortisol, and core body temperature rhythms collected under forced desynchrony protocols revealed that the intrinsic circadian period of the human circadian pacemaker was significantly longer following entrainment to the Martian sol as compared to following entrainment to the 23.5-h day. The latter finding of after-effects of entrainment reveals for the first time plasticity of the period of the human circadian timing system. Both findings have important implications for the treatment of circadian rhythm sleep disorders and human space exploration.     

Circadian Variation of Fibrinolytic Activity in Blood

Approximately 35 years ago, it was discovered that spontaneous fibrinolytic activity in blood showed a sinusoidal variation with a period of 24 h; it increased severalfold during the day, reaching a peak at 6:OO p.m. and then dropped to trough levels at 3:00–4:00 a.m. The range of the fluctuation and the 24-h mean levels were highly reproducible within an individual; moreover, the timing of the oscillation was remarkably consistent among individuals, with a fixed phase relationship to external clock time. The biorhythm could not be accounted for simply by variations in physical activity, body posture, or sleepfwake schedule. Gender, ethnic origin, meals, or resting levels of blood fibrinolytic activity also did not influence the basic features of the rhythm. Older subjects, compared to younger ones, showed a blunted diurnal increase in fibrinolytic activity in blood. Recent studies have established that, of the known components of the fibrinolytic system, only tissue-type plasminogen activator (tPA) and its fast-acting inhibitor, plasminogen activator inhibitor- 1 (PAL l), show a marked circadian variation in plasma. In contrast, levels of plasminogen, α₂-antiplasmin, urinarytype plasminogen activator, and a reversible tPA inhibitor vary little or none during the 24 h. Quenching antibodies to tPA have shown that the circadian rhythm of fibrinolytic activity in blood is due exclusively to changes in tPA activity. However, the 24-h fluctuation of plasma tPA activity is phase shifted in relation to the rhythm of immunoreactive tPA, but shows a precise phase inversion with respect to the 24-h variation of PAL 1 activity and antigen. Therefore, plasma tPA activity, as currently measured in vitro, is tightly and inversely related to the levels of PAL 1 throughout the 24-h cycle. The factors controlling the rhythmicity of plasma PAI-1 are not fully elucidated but probably involve a humoral mechanism; changes in endothelial function, circulating platelet release. products, corticosteroids, catecholamines, insulin, activated protein C, or hepatic clearance do not appear to be responsible. Shift workers on weekly shift rotations show a disrupted 24-h rhythm of plasma tPA and PAL 1. In acute and chronic diseases, the circadian rhythmicity of fibrinolytic activity may show a variety of alterations, affecting the 24-h mean, the amplitude, or the timing of the fluctuation. It is advisable, therefore, to define the 24-h pattern of plasma tPA and PAI- 1 in patient groups, before levels based on a single blood sampling time are compared to those of a control population. In normal conditions, the 24-h variation of plasma tPA and PAI- 1 is not associated with parallel circadian changes in effective fibrinolysis, assessed as plasma D-dimer concentrations, presumably because fibrin generation in the circulation is low. In diseases in which fibrin formation is increased, however, the physiological drop of fibrinolytic activity in the morning hours may favour thrombus development at this time of day, in agreement with the reported higher morning frequency of acute thrombotic events.     

Circadian and Ultradian Variations in the Plasma Level of Maturational Gonadotropin (GTH II) in the Indian Catfish,Clarias batrachus

Analysis of changes in the plasma levels of GTH II over a 24-h day/night cycle revealed statistically significant circadian or ultradian variation or both in its circulating level during the gonadally active months of the reproductive cycle. The 24-h average and amplitude of circadian or ultradian rhythm in GTH II increased with the advancement of ovarian development and maturation. In April, a single peak in GTH II level was noticed in the night at 20.6 h. In contrast, in May, June and July a biphasic pattern (ultradian rhythm) was noticed with two characteristic peaks, one in the photophase and the other in the scotophase. In addition, in May a statistically significant circadian rhythm in plasma GTH II was validated with a peak located at 20.1 h. These rhythms seem to have physiological significance. The ultradian rhythm with two peaks during the reproductively active phase appears to provide a suitable physiological milieu for the temporally different yet synchronous population of oocytes for the secretion of steroids. Thus, the observed temporal organization in GTH II may have physiological consequences leading to accomplishment of reproductive process at appropriate time of the year.     

Circadian and Ultradian Variations in the Plasma Level of Maturational Gonadotropin (GTH II) in the Indian Catfish, Clarias batrachus

Analysis of changes in the plasma levels of GTH II over a 24-h day/night cycle revealed statistically significant circadian or ultradian variation or both in its circulating level during the gonadally active months of the reproductive cycle. The 24-h average and amplitude of circadian or ultradian rhythm in GTH II increased with the advancement of ovarian development and maturation. In April, a single peak in GTH II level was noticed in the night at 20.6 h. In contrast, in May, June and July a biphasic pattern (ultradian rhythm) was noticed with two characteristic peaks, one in the photophase and the other in the scotophase. In addition, in May a statistically significant circadian rhythm in plasma GTH II was validated with a peak located at 20.1 h. These rhythms seem to have physiological significance. The ultradian rhythm with two peaks during the reproductively active phase appears to provide a suitable physiological milieu for the temporally different yet synchronous population of oocytes for the secretion of steroids. Thus, the observed temporal organization in GTH II may have physiological consequences leading to accomplishment of reproductive process at appropriate time of the year.     

Oscillations in the Activities of Enzymes of Nitrate Reduction and Ammonia Assimilation in Glycine max and Zea mays

The activities of glutamate dehydrogenase (GDH), glutamine synthetase (GS), and nitrate reductase (NR) and the levels of soluble protein and NO^-₃ were assayed in soybean (Glycine max [L.] Merr.) leaves over a 48-h period with the initial 24 h under a light-dark cycle (LD 16:8) followed by 24 h of continuous light (LL). Plants had been entrained for 30 days under the LD regime. Maize (Zea mays) leaves (10 days old) under a LD 15:9 cycle were assayed only for NR and nitrite reductase (NiR). Data were subjected to frequency analysis by the least squares method to determine probabilities for cosine function periods (τ's) between 10 and 30 h. NR activities for both soybean and Zea leaves had 24 h τ's with P values < 0.05 indicating circadian periodicity. GDH in soybeans had a 24-h rhythm under LD conditions which lengthened under LL conditions. The 24-h rhythm of GDH displayed maximal activity toward the end of the dark period of the LD cycle whereas the highest activity of NR was early in the light period. Total soluble protein displayed a rhythm with a best fitting τ of greater than 24 h under both LD and LL. GDH, GS, NR, NO₃, and soluble protein in soybeans and NiR in Zea, all displayed that were ultradian (10–18 h), indicating that a τ of about one half a circadian periodicity may be a common characteristic of the enzymes of primary nitrogen metabolism in higher plants. These data also demonstrate that although both NR and GDH are circadian in their activity, the 24-h rhythm may be greatly influenced by ultradian oscillations in activity.