Endocrine and cardiovascular rhythms differentially adapt to chronic phase-delay shifts in rats

Disturbances in regular circadian oscillations can have negative effects on cardiovascular function, but epidemiological data are inconclusive and new data from animal experiments elucidating critical biological mechanisms are needed. To evaluate the consequences of chronic phase shifts of the light/dark (LD) cycle on hormonal and cardiovascular rhythms, two experiments were performed. In Experiment 1, male rats were exposed to either a regular 12:12 LD cycle (CONT) or rotating 8-h phase-delay shifts of LD every second day (SHIFT) for 10 weeks. During this period, blood pressure (BP) was monitored weekly, and daily rhythms of melatonin, corticosterone, leptin and testosterone were evaluated at the end of the experiment. In Experiment 2, female rats were exposed to the identical shifted LD schedule for 12 weeks, and daily rhythms of BP, heart rate (HR) and locomotor activity were recorded using telemetry. Preserved melatonin rhythms were found in the pineal gland, plasma, heart and kidney of SHIFT rats with damped amplitude in the plasma and heart, suggesting that the central oscillator can adapt to chronic phase-delay shifts. In contrast, daily rhythms of corticosterone, testosterone and leptin were eliminated in SHIFT rats. Exposure to phase shifts did not lead to increased body weight and elevated BP. However, a shifted LD schedule substantially decreased the amplitude and suppressed the circadian power of the daily rhythms of BP and HR, implying weakened circadian control of physiological and behavioural processes. The results demonstrate that endocrine and cardiovascular rhythms can differentially adapt to chronic phase-delay shifts, promoting internal desynchronization between central and peripheral oscillators, which in combination with other negative environmental stimuli may result in negative health effects.     

Longitudinal studies on the effect of hypertension on circadian hemodynamic and autonomic rhythms in telemetered rats

This study dealt with the long-term effects of hypertension on circadian rhythms of hemodynamic and cardiovascular autonomic functions in radiotelemetered rats. Blood pressure (BP), heart rate (HR), spontaneous locomotor activity, and respiration.were monitored in spontaneously hypertensive rats (SHRs), a model of human hypertension, from 14 to 27 weeks of age and in Wistar-Kyoto rats (WKY) as controls. Cardiovascular autonomic changes were determined by time-domain analysis of the variability of BP (standard deviation of mean arterial pressure, SDMAP) and HR (standard deviation of R-R intervals, SDRR, and the root mean square of successive differences in R-R intervals, rMSSD). Compared with WKY rats, the 24-hr MAP and SDMAP were higher at week 14 in SHRs and showed stepwise increases over the study duration, suggesting progressive increases in vasomotor sympathetic activity in hypertensive rats. Also, higher SDRR, rMSSD, and activity and lower HR and respiration were demonstrated in SHRs. Normal circadian rhythms (higher dark-time values) of MAP, HR, SDMAP, and SDRR were evident in WKY rats at week 20 and continued thereafter. Compared with WKY rats, the circadian BP and HR patterns were abolished and inverted, respectively, in SHRs. Lower dark-time, compared with light-time, SDMAP values were observed in SHRs that were associated with temporal increases in HR variability indices. These findings demonstrate that hypertension elicits significant alterations in circadian autonomic and hemodynamic profiles. Further, the steady increases in BP, average level and oscillations, in SHRs may explain the reported progressive age-related vascular and cardiac hypertrophy in these rats.     

Altered circadian rhythm reentrainment to light phase shifts in rats with low levels of brain angiotensinogen

In this study, we aimed to investigate the adaptation of blood pressure (BP), heart rate (HR), and locomotor activity (LA) circadian rhythms to light cycle shift in transgenic rats with a deficit in brain angiotensin [TGR(ASrAOGEN)]. BP, HR, and LA were measured by telemetry. After baseline recordings (bLD), the light cycle was inverted by prolonging the light by 12 h and thereafter the dark period by 12 h, resulting in inverted dark-light (DL) or light-dark (LD) cycles. Toward that end, a 24-h dark was maintained for 14 days (free-running conditions). When light cycle was changed from bLD to DL, the acrophases (peak time of curve fitting) of BP, HR, and LA shifted to the new dark period in both SD and TGR(ASrAOGEN) rats. However, the readjustment of the BP and HR acrophases in TGR(ASrAOGEN) rats occurred significantly slower than SD rats. The LA acrophases changed similarly in both strains. When light cycle was changed from DL to LD by prolonging the dark period by 12 h, the reentrainment of BP and LA occurred faster than the previous shift in both strains. The readjustment of the BP and HR acrophases in TGR(ASrAOGEN) rats occurred significantly slower than SD rats. In free-running conditions, the circadian rhythms of the investigated parameters adapted in TGR(ASrAOGEN) and SD rats in a similar manner. These results demonstrate that the brain RAS plays an important role in mediating the effects of light cycle shifts on the circadian variation of BP and HR. The adaptive behavior of cardiovascular circadian rhythms depends on the initial direction of light-dark changes.     

Characteristics of autonomic nervous function in Zucker-fatty rats: investigation by power spectral analysis of heart rate variability.

We investigated the characteristics of autonomic nervous function in Zucker-fatty and Zucker-lean rats. For this purpose, a long-term electrocardiogram (ECG) was recorded from conscious and unrestrained rats using a telemetry system, and the autonomic nervous function was investigated by power spectral analysis of heart rate variability (HRV). Although heart rate (HR) in Zucker-fatty rats was lower than that in Zucker-lean rats throughout 24 h, apparent diurnal variation in HR was observed in both strains and HR during the dark period was significantly higher than that in light period. Diurnal variation in locomotor activity (LA) in Zucker-fatty rats was also observed, but LA was lower than that in Zucker lean rats, especially during the dark period. There were no significant differences, however, in high-frequency (HF) power, low-frequency (LF) power, and the LF/HF ratio between Zucker-fatty and Zucker-lean rats. The circadian rhythm of these parameters was mostly preserved in both strains of rats. Moreover, the effect of autonomic blockades on HRV was nearly the same in Zucker-fatty and Zucker-lean rats. These results suggest that the autonomic nervous function of insulin-resistant Zucker-fatty rats remain normal, from the aspect of power spectral analysis of HRV.     

Gender dependency of circadian blood pressure and heart rate profiles in spontaneously hypertensive rats: effects of beta-blockers

This study investigated (i) blood pressure (BP), heart rate (HR), and their relation to urinary NOx and eNOS protein expression in male and female spontaneously hypertensive rats (SHR), as well as (ii) gender-dependent cardiovascular effects of nebivolol (NEB) in comparison to metoprolol (MET) in SHR. BP and HR were measured telemetrically after a single intraperitoneal application of NEB or MET at 07.00 and 19.00 h in male rats and at 19.00 h in proestrus female rats. The two β-blockers varied in time of decreasing BP and HR and also in duration. In males, MET decreased BP and HR for few hours exclusively when applied at the onset of the activity phase (i.e., at 19.00 h), while after its application at 07.00 h, BP and HR were unchanged. In females, MET also caused a short-lasting BP and HR reduction, with the effect being more pronounced than in males. In males, NEB at either dosing time decreased HR and BP to a greater extent than did MET. This effect was evident both during the activity and rest periods and persisted for at least five days. In females, NEB provoked a similar, but more pronounced, effect on BP and HR in comparison to males. These findings demonstrate that significant gender-dependent differences in the circadian profile of BP and HR exist. BP and urinary NOx as well as eNOS expression are inversely correlated, and the cardiovascular effects of NEB and MET vary, depending on the time of application as well as gender.     

Gender Dependency of Circadian Blood Pressure and Heart Rate Profiles in Spontaneously Hypertensive Rats: Effects of Beta‐Blockers

This study investigated (i) blood pressure (BP), heart rate (HR), and their relation to urinary NOx and eNOS protein expression in male and female spontaneously hypertensive rats (SHR), as well as (ii) gender‐dependent cardiovascular effects of nebivolol (NEB) in comparison to metoprolol (MET) in SHR. BP and HR were measured telemetrically after a single intraperitoneal application of NEB or MET at 07.00 and 19.00 h in male rats and at 19.00 h in proestrus female rats. The two β‐blockers varied in time of decreasing BP and HR and also in duration. In males, MET decreased BP and HR for few hours exclusively when applied at the onset of the activity phase (i.e., at 19.00 h), while after its application at 07.00 h, BP and HR were unchanged. In females, MET also caused a short‐lasting BP and HR reduction, with the effect being more pronounced than in males. In males, NEB at either dosing time decreased HR and BP to a greater extent than did MET. This effect was evident both during the activity and rest periods and persisted for at least five days. In females, NEB provoked a similar, but more pronounced, effect on BP and HR in comparison to males. These findings demonstrate that significant gender‐dependent differences in the circadian profile of BP and HR exist. BP and urinary NOx as well as eNOS expression are inversely correlated, and the cardiovascular effects of NEB and MET vary, depending on the time of application as well as gender.     

Heart rate circadian rhythm as a biological marker of desynchronization in major depression: a methodological and preliminary report

Heart rate (HR) was continuously monitored during successive 24-hr periods in 19 healthy subjects and 26 major depressed patients (DSM III-R). Recordings were performed after a 2-week wash-out period and the morningness or eveningness typology of each subject was determined. The chronobiological parameters and rhythm percentage (RP) were calculated by the single cosinor method from the smoothed HR curves of each subject. In normal subjects, HR follows a circadian rhythm (RP greater than 65%) with the lowest values at night. Morning type subjects have an earlier peak time (13:30) than evening type subjects (17:30). Major depressive patients were split into two groups; in the first one HR circadian rhythm was still present (RP greater than 63%) with a decrease in amplitude (24%) while in the second group, no circadian rhythm of HR could be detected (RP less than 25%, decrease in amplitude greater than 70%). In the group of patients with a persisting HR circadian rhythm, no veritable phase advance was observed. Our results suggest that circadian HR rhythm, which can be easily studied with non-invasive methods, might represent a chronobiological marker of some depressions. Given the lag that exists between the rhythms of morning type and evening type subjects, our study also stresses the importance of taking into account this behavioural trait in chronobiological studies.     

Heart Rate Orcadian Rhythm as a Biological Marker of Desynchronization in Major Depression: A Methodological and Preliminary Report

Heart rate (HR) was continuously monitored during successive 24-hr periods in 19 healthy subjects and 26 major depressed patients (DSM III-R). Recordings were performed after a 2-week wash-out period and the morningness or eveningness typology of each subject was determined. The chronobiological parameters and rhythm percentage (RP) were calculated by the single cosinor method from the smoothed HR curves of each subject. In normal subjects, HR follows a circadian rhythm (RP > 65%) with the lowest values at night. Morning type subjects have an earlier peak time (13:30) than evening type subjects (17:30). Major depressive patients were split into two groups; in the first one HR circadian rhythm was still present (RP > 63%) with a decrease in amplitude (24%) while in the second group, no circadian rhythm of HR could be detected (RP < 25%, decrease in amplitude > 70%). In the group of patients with a persisting HK circadian rhythm, no veritable phase advance was observed. Our results suggest that circadian HR rhythm, which can be easily studied with non-invasive methods, might represent a chronobiological marker of some depressions. Given the lag that exists between the rhythms of morning type and evening type subjects, our study also stresses the importance of taking into account this behavioural trait in chronobiological studies.     

Blood pressure regulation in neurally intact human vs. acutely injured paraplegic and tetraplegic patients during passive tilt

We investigated autonomic control of cardiovascular function in able-bodied (AB), paraplegic (PARA), and tetraplegic (TETRA) subjects in response to head-up tilt following spinal cord injury. We evaluated spectral power of blood pressure (BP), baroreflex sensitivity (BRS), baroreflex effectiveness index (BEI), occurrence of systolic blood pressure (SBP) ramps, baroreflex sequences, and cross-correlation of SBP with heart rate (HR) in low (0.04-0.15 Hz)- and high (0.15-0.4 Hz)-frequency regions. During tilt, AB and PARA effectively regulated BP and HR, but TETRA did not. The numbers of SBP ramps and percentages of heartbeats involved in SBP ramps and baroreflex sequences increased in AB, were unchanged in PARA, and declined in TETRA. BRS was lowest in PARA and declined with tilt in all groups. BEI was greatest in AB and declined with tilt in all groups. Low-frequency power of BP and the peak of the SBP/HR cross-correlation magnitude were greatest in AB, increased during tilt in AB, remained unchanged in PARA, and declined in TETRA. The peak cross-correlation magnitude in HF decreased with tilt in all groups. Our data indicate that spinal cord injury results in decreased stimulation of arterial baroreceptors and less engagement of feedback control as demonstrated by lower 1) spectral power of BP, 2) number (and percentages) of SBP ramps and barosequences, 3) cross-correlation magnitude of SBP/HR, 4) BEI, and 5) changes in delay between SBP/HR. Diminished vasomotion and impaired baroreflex regulation may be major contributors to decreased orthostatic tolerance following injury.     

Effects of three-hour restricted food access during the light period on circadian rhythms of temperature, locomotor activity, and heart rate in rats

The effects of food on biological rhythms may influence the findings of chronopharmacological studies. The present study evaluated the influence of a restricted food access during the rest (light) span of nocturnally active Wistar rats on the 24 h time organization of biological functions in terms of the circadian rhythms of temperature (T), heart rate (HR), and locomotor activity (LA) in preparation for subsequent studies aimed at evaluating the influence of timed food access on the pharmacokinetics and pharmacodynamics of medications. Ten-wk-old male Wistar rats were housed under controlled 12:12 h light:dark (LD) environmental conditions. Food and water were available ad libitum, excepted during a 3 wk period of restriction. Radiotelemetry transmitters were implanted to record daily rhythms in T, HR, and LA. The study lasted 7 wk and began after a 21-d recovery span following surgery. Control baseline data were collected during the first wk (W1). The second span of 3 wk duration (W2 to W4) consisted of the restricted feeding regimen (only 3 h access to food between 11:00 and 14:00 h daily) during the L (rest span) under 12:12 h LD conditions. The third period of 3 wk duration (W5 to W7) consisted of the recovery span with ad libitium normal feeding. Weight loss in the amount of 5% of baseline was observed during W1 with stabilization of body weight thereafter during the remaining 2 wk of food restriction. The 3 h restricted food access during the L rest span induced a partial loss of circadian rhythmicity and the emergence of 12 h rhythms in T, HR, and LA. Return to ad libitum feeding conditions restored circadian rhythmicity in the manner evidenced during the baseline control span. Moreover, the MESORS and amplitudes of the T, HR, and LA 24 h patterns were significantly attenuated during food restriction (p < 0.001) and then returned to initial values during recovery. These changes may be interpreted as a masking effect, since T, HR, and LA are known to directly react to food intake. The consequences of such findings on the methods used to conduct chronokinetic studies, such as the fasting of animals the day before testing, are important since they may alter the temporal structure of the organism receiving the drug and thereby compromise findings.     

Blunting of circadian rhythms and increased acrophase variability in sleep-time hypertensive subjects

24 h and ultradian rhythms of blood pressure (BP) have been previously shown to be disorganized in nocturnal hypertensive subjects. The present study was undertaken to further analyze the ultradian and circadian BP rhythm structure in sleep-time hypertensive subjects with normal or elevated awake-time BP levels. Fourier analysis was used to fit 24, 12, 8, and 6 h curves to mean BP as well as heart rate (HR) time series data derived from 24 h ambulatory blood pressure monitoring. Awake and sleep periods were defined according to individual sleep diaries. Awake-time hypertension was defined as diurnal systolic (SBP) and/or diastolic BP (DBP) means ≥135/85 mmHg. Sleep-time hypertension was defined as nocturnal SBP and/or DBP means ≥120/70 mmHg. The sample included 240 awake-time normotensive subjects (180 sleep-time normotensives and 60 sleep-time hypertensives) and 138 untreated awake-time hypertensive subjects (31 sleep-time normotensives and 107 sleep-time hypertensives). The amplitude and integrity (i.e., percent rhythm) of the 24 and 12 h BP rhythms were lower in the sleep-time hypertensive subjects and higher in the awake-time hypertensive subjects. However, no differences were detected when the integrity and amplitude of the 6 and 8 h mean BP rhythms were analyzed. The sleep-time hypertensive group showed significantly higher 24 h BP rhythm acrophase variability. No differences could be found in any of the HR rhythm parameters. Altogether, the findings suggest a disorganization of the BP circadian rhythm in sleep-time hypertensives that results in reduced 24 h rhythm amplitude and integrity that could be related to cardiovascular risk.     

Effects of photoperiod reduction on rat circadian rhythms of BP, heart rate, and locomotor activity

The effects of a photoperiod reduction in the entrainment of circadian rhythms of systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), and spontaneous locomotor activity (SLA) were determined in conscious Wistar rats by using radiotelemetry. Two groups of seven rats were maintained in a 12:12-h light-dark (12L/12D) photoperiod for 11 wk and then placed in a reduced photoperiod of 8:16-h light-dark (8L/16D) by advancing a 4-h darkness or by advancing and delaying a 2-h darkness for 6 wk. Finally, they were resynchronized to 12L/12D. Advancing a 4-h dark phase induced a 1-h advance of acrophase for SBP, DBP, and HR, but not for SLA. The percent rhythm, amplitude, and the 12-h mean values of all parameters were significantly decreased by the photoperiod reduction. When symmetrically advancing and delaying a 2-h dark phase, a 1 h 20 min delay of acrophases and a decrease in percent rhythms and amplitudes of SBP, DBP, HR, and SLA were observed. Only the 12-h mean values of HR and SLA were decreased. Our findings show that the cardiovascular parameters differ from SLA in phase-shift response to photoperiod reduction and that the adjustment of circadian rhythms to change from 12L/12D to 8L/16D photoperiod depends on the direction of the extension of the dark period.     

Hypertension and disrupted blood pressure circadian rhythm in type 2 diabetic db/db mice

Human Type 2 diabetes is associated with increased incidence of hypertension and disrupted blood pressure (BP) circadian rhythm. Db/db mice have been used extensively as a model of Type 2 diabetes, but their BP is not well characterized. In this study, we used radiotelemetry to define BP and the circadian rhythm in db/db mice. We found that the systolic, diastolic, and mean arterial pressures were each significantly increased by 11, 8, and 9 mmHg in db/db mice compared with controls. In contrast, no difference was observed in pulse pressure or heart rate. Interestingly, both the length of time db/db mice were active (locomotor) and the intensity of locomotor activity were significantly decreased in db/db mice. In contrast to controls, the 12-h light period average BP in db/db mice did not dip significantly from the 12-h dark period. A partial Fourier analysis of the continuous 72-h BP data revealed that the power and the amplitude of the 24-h period length rhythm were significantly decreased in db/db mice compared with the controls. The acrophase was centered at 0141 in control mice, but became scattered from 1805 to 0236 in db/db mice. In addition to BP, the circadian rhythms of heart rate and locomotor activity were also disrupted in db/db mice. The mean arterial pressure during the light period correlates with plasma glucose, insulin, and body weight. Moreover, the oscillations of the clock genes DBP and Bmal1 but not Per1 were significantly dampened in db/db mouse aorta compared with controls. In summary, our data show that db/db mice are hypertensive with a disrupted BP, heart rate, and locomotor circadian rhythm. Such changes are associated with dampened oscillations of clock genes DBP and Bmal1 in vasculature.     

Circadian rhythm of activity and sleep‐wakefulness in elderly institutionalized persons

Abstract

The circadian rhythms of blood pressure (BP) and heart rate (HR) were documented in 30 patients for a 24‐hour period before and during the 24 hours that included unilateral surgery for senile cataract or retinal detachment. The patients were premedicated with diazepam. Anaesthesia was induced at a fixed time (09.00) in all patients with thiopentone, and muscle relaxation was with pancuronium. Maintenance was with enflurane in 15 patients and with fentanyl and droperidol in the rest. Though the intraoperative changes in haemodynamic parameters were dissimilar with the two types of maintenance agents, but both types had a similar effect on the circadian rhythms of blood pressure and heart rate. Whereas preoperatively the BP and HR circadian rhythms were nearly in phase, with their peaks in the late morning to early afternoon, the postoperative rhythms underwent a dissociation to a phase shift in the BP 24‐h pattern. The phase effect may be hypothetically attributed to direct pharmacological actions or to masking effects.     

Effects of Three‐Hour Restricted Food Access during the Light Period on Circadian Rhythms of Temperature,Locomotor Activity,and Heart Rate in Rats

The effects of food on biological rhythms may influence the findings of chronopharmacological studies. The present study evaluated the influence of a restricted food access during the rest (light) span of nocturnally active Wistar rats on the 24 h time organization of biological functions in terms of the circadian rhythms of temperature (T), heart rate (HR), and locomotor activity (LA) in preparation for subsequent studies aimed at evaluating the influence of timed food access on the pharmacokinetics and pharmacodynamics of medications. Ten‐wk‐old male Wistar rats were housed under controlled 12:12 h light:dark (LD) environmental conditions. Food and water were available ad libitum, excepted during a 3 wk period of restriction. Radiotelemetry transmitters were implanted to record daily rhythms in T, HR, and LA. The study lasted 7 wk and began after a 21‐d recovery span following surgery. Control baseline data were collected during the first wk (W1). The second span of 3 wk duration (W2 to W4) consisted of the restricted feeding regimen (only 3 h access to food between 11:00 and 14:00 h daily) during the L (rest span) under 12:12 h LD conditions. The third period of 3 wk duration (W5 to W7) consisted of the recovery span with ad libitium normal feeding. Weight loss in the amount of 5% of baseline was observed during W1 with stabilization of body weight thereafter during the remaining 2 wk of food restriction. The 3 h restricted food access during the L rest span induced a partial loss of circadian rhythmicity and the emergence of 12 h rhythms in T, HR, and LA. Return to ad libitum feeding conditions restored circadian rhythmicity in the manner evidenced during the baseline control span. Moreover, the MESORS and amplitudes of the T, HR, and LA 24 h patterns were significantly attenuated during food restriction (p<0.001) and then returned to initial values during recovery. These changes may be interpreted as a masking effect, since T, HR, and LA are known to directly react to food intake. The consequences of such findings on the methods used to conduct chronokinetic studies, such as the fasting of animals the day before testing, are important since they may alter the temporal structure of the organism receiving the drug and thereby compromise findings.     

Effects of wire-bottom caging on heart rate, activity and body temperature in telemetry-implanted rats

Some experimental procedures are associated with placement of animals in wire-bottom cages. The goal of this study was to evaluate stress-related physiological parameters (heart rate [HR], body temperature [BT], locomotor activity [LA], body weight [BW] and food consumption) in rats under two housing conditions, namely in wire-bottom cages and in bedding-bottom cages. Telemetry devices were surgically implanted in male Sprague-Dawley rats. HR, BT and LA were recorded at 5 min intervals. Analysis under each housing condition was performed from 16:00 to 08:00 h of the following day (4 h light, 12 h dark). During almost all of the light phase, the HR of rats housed in wire-bottom cages remained high (371 ± 35 bpm; mean ± SD; n = 6) and was significantly different from that of rats housed in bedding-bottom cages (340 ± 29 bpm; n = 6; P < 0.001; Student's t-test). In general, BT was similar under the two housing conditions. However, when rats were in wire-bottom cages, BT tended to fluctuate more widely during the dark phase. LA decreased when animals were housed in wire-bottom cages, in particular during the dark phase. Moreover, there was a significant difference with respect to the gain in BW: BW of rats housed in bedding-bottom cages increased 12 ± 2 g, whereas that of rats in wire-bottom cages decreased by 2 ± 3 g (P < 0.001). Our results demonstrate that housing rats in wire-bottom cages overnight leads to immediate alterations of HR, BW and LA, which might be related to a stress response.     

Influence of Time of Day on Body Temperature,Heart Rate,Arterial Pressure,and Other Biological Variables in Horses during Incremental Exercise

We examined the response to exercise of selected physiological variables in horses performing the identical routine for eight days, in the morning (a.m.) or in the afternoon (p.m.). Heart rate (HR), systolic and diastolic blood pressure (BP), and body temperature (BT) were all consistently greater in the p.m. For BP and BT, the absolute increase above the a.m. values was the same at rest and during exercise. For HR, the absolute increase was greater during exercise, but the percent increase was the same as during rest. During exercise, blood glucose decreased, while blood lactate and skin temperature increased; these changes were the same during the a.m. and p.m. sessions. We conclude that there is no indication in horses of a difference in the responses of HR, BP, and BT to exercise between the a.m. and p.m. The circadian oscillations, however, alter the absolute values of these variables both at rest and during exercise, raising the possibility that the safety margins against hyperthermia and hypertension may decrease during p.m. exercise.     

Scaling the amplitudes of the circadian pattern of resting oxygen consumption, body temperature and heart rate in mammals

We questioned whether the amplitudes of the circadian pattern of body temperature (T(b)), oxygen consumption (V (O(2))) and heart rate (HR) changed systematically among species of different body weight (W). Because bodies of large mass have a greater heat capacitance than those of smaller mass, if the relative amplitude (i.e., amplitude/mean value) of metabolic rate was constant, one would expect the T(b) oscillation to decrease with the increase in the species W. We compiled data of T(b), V (O(2)) and HR from a literature survey of over 200 studies that investigated the circadian pattern of these parameters. Monotremata, Marsupials and Chiroptera, were excluded because of their characteristically low metabolic rate and T(b). The peak-trough ratios of V (O(2)) (42 species) and HR (35 species) averaged, respectively, 1.57+/-0.08, and 1.35+/-0.07, and were independent of W. The daily high values of T(b) did not change, while the daily low T(b) values slightly increased, with the species W; hence, the high-low T(b) difference (57 species) decreased with W (3.3 degrees C.W(-0.13)). However, the decrease in T(b) amplitude with W was much less than expected from physical principles, and the high-low T(b) ratio remained significantly above unity even in the largest mammals. Thus, it appears that in mammals, despite the huge differences in physical characteristics, the amplitude of the circadian pattern is a fixed (for V (O(2)) and HR), or almost fixed (for T(b)), fraction of the 24-h mean value. Presumably, the amplitudes of the oscillations are controlled parameters of physiological significance.     

Ranges of diurnal variation and the pattern of body temperature, blood pressure and heart rate in laboratory beagle dogs.

Ranges in diurnal variation and the patterns of body temperature (T), blood pressure (BP), heart rate (HR) and locomotor activity (LA) in 61 laboratory beagle dogs were analyzed using a telemetry system. Body temperature, BP, HR and LA increased remarkably at feeding time. Locomotor activity increased sporadically during the other periods. Body temperature was maintained at the higher value after feeding but had decreased by 0.2 C by early the next morning. Blood pressure fell to a lower value after feeding but had increased by 2.8% by early the next morning. Heart rate decreased progressively after feeding and was 14.5% lower the next morning. This study determined that in laboratory beagles the ranges of diurnal variation and patterns of T, BP and HR are significantly different from those reported in humans and rodents, and that over 24 hr these physiological changes were associated with their sporadic wake-sleep cycles of the dogs.     

Baroreflex modulation of ultradian oscillations of blood pressure and heart rate in unanesthetized dogs

Telemetered, free-running dogs were studied to determine the role of cardiovascular control systems in modulation of ultradian oscillations of arterial pressure (MAP) and heart rate (HR). Data, aquired (2 Hz) by a stable telemetry system, was stored on a digital computer and analyzed for its harmonic content by a Fast Fourier Transform (FFT) algorithm. Both AP and HR consistently demonstrated rhythms having a period of from 0.6 to 1.0 h. Modulation of these rhythms by arterial pressure control systems was assessed in dogs studied before and carotid sinus baroreceptor denervation, before and after denervation of the aortic arch baroreceptors and before and after a combination of both these procedures. The data indicate the power spectral density (PSD) of MAP, but not HR, is increased (p less than 0.05) after denervation of the carotid sinuses alone, while the primary frequency of the oscillations was unchanged. On the other hand, denervation of the aortic arch baroreceptors alone was without effect on either the frequency or PSD of these oscillations. A combination of both carotid sinus and aortic arch denervation resulted in an increased (p less than 0.05) PSD of MAP oscillations but not in their frequency. These data indicate that the carotid sinuses modulate rhythmic behavior of MAP by buffering the magnitude, but not frequency, of the oscillations. Moreover, since oscillations were present in dogs after denervation of both the carotid sinus and aortic arch baroreceptors, these ultradian oscillations are not a result of a non-linear negative feedback mechanisms arising from these pressure sensitive regions.