Free-running Rhythms in Blood Pressure and Heart Rate in Normotensive and Transgenic Hypertensive Rats

Hypotensive effects of melatonin in rats: Focus on the model, measurement, application, and main mechanisms

The hypotensive effects of melatonin are based on a negative correlation between melatonin levels and blood pressure in humans. However, there is a positive correlation in nocturnal animals that are often used as experimental models in cardiovascular research, and the hypotensive effects and mechanism of melatonin action are often investigated in rats and mice. In rats, the hypotensive effects of melatonin have been studied in normotensive and spontaneously or experimentally induced hypertensive strains. In experimental animals, blood pressure is often measured indirectly during the light (passive) phase of the day by tail-cuff plethysmography, which has limitations regarding data quality and animal well-being compared to telemetry. Melatonin is administered to rats in drinking water, subcutaneously, intraperitoneally, or microinjected into specific brain areas at different times. Experimental data show that the hypotensive effects of melatonin depend on the experimental animal model, blood pressure measurement technique, and the route, time and duration of melatonin administration. The hypotensive effects of melatonin may be mediated through specific membrane G-coupled receptors located in the heart and arteries. Due to melatonin's lipophilic nature, its potential hypotensive effects can interfere with various regulatory mechanisms, such as nitric oxide and reactive oxygen species production and activation of the autonomic nervous and circadian systems. Based on the research conducted on rats, the cardiovascular effects of melatonin are modulatory, delayed, and indirect.

Continuous Light Exposure and Sympathectomy Suppress Circadian Rhythm of Blood Pressure in Rats

Background: Although the 24-hour rhythm in blood pressure is well known, it is not clear how environmental light controls circadian cardiovascular and behavioral rhythms.

Methods and Results: The prolonged exposure of Wistar rats to continuous light for 17 weeks, beginning at 5 weeks old, induced a complete suppression of their blood pressure, heart rate, spontaneous locomotor activity, and body temperature circadian rhythms. Daily subcutaneous melatonin injections at the theoretical onset of darkness for 21 days could not restore light-suppressed blood pressure circadian rhythm, whereas it partially synchronized heart rate and body temperature rhythms and it fully restored spontaneous locomotor activity rhythms, as measured by radiotelemetry. The transfer of these rats from constant light to a standard 12:12-hour light/dark photoperiod fully restored circadian rhythmicity within 2 to 5 days, although their 24-hour diastolic blood pressure remained elevated. Synchronized rats were then subjected to superior cervical ganglionectomy (SCGx) and 6-hydroxydopamine sympathectomy (SYMPx). SCGx plus SYMPx completely abolished the circadian rhythm in blood pressure and significantly reduced those in heart rate, spontaneous locomotor activity, and body temperature.

Conclusions: We conclude that in Wistar rats exposed to continuous light, the light-induced increase in sympathetic outflow can suppress blood pressure circadian rhythm, and sustained cardiac wall stress can alter diastolic function at rest. Preserved inotropy in these conditions must result from an adaptative hypertrophic response of myocytes.

Distinctive cardiac autonomic dysfunction following stress exposure in both sexes in an animal model of PTSD

It is unclear whether the poor autonomic flexibility or dysregulation observed in patients with posttraumatic stress disorder (PTSD) represents a pre-trauma vulnerability factor or results from exposure to trauma. We used an animal model of PTSD to assess the association between the behavioral response to predator scent stress (PSS) and the cardiac autonomic modulation in male and female rats. The rats were surgically implanted with radiotelemetry devices to measure their electrocardiograms and locomotor activity (LMA). Following baseline telemetric monitoring, the animals were exposed to PSS or sham-PSS. Continuous telemetric monitoring (24h/day sampling) was performed over the course of 7days. The electrocardiographic recordings were analyzed using the time- and frequency-domain indexes of heart rate variability (HRV). The behavioral response patterns were assessed using the elevated plus maze and acoustic startle response paradigms for the retrospective classification of individuals according to the PTSD-related cut-off behavioral criteria. During resting conditions, the male rats had significantly higher heart rates (HR) and lower HRV parameters than the female rats during both the active and inactive phases of the daily cycle. Immediately after PSS exposure, both the female and male rats demonstrated a robust increase in HR and a marked drop in HRV parameters, with a shift of sympathovagal balance towards sympathetic predominance. In both sexes, autonomic system habituation and recovery were selectively inhibited in the rats whose behavior was extremely disrupted after exposure to PSS. However, in the female rats, exposure to the PSS produced fewer EBR rats, with a more rapid recovery curve than that of the male rats. PSS did not induce changes to the circadian rhythm of the LMA. According to our results, PTSD can be conceptualized as a disorder that is related to failure-of-recovery mechanisms that impede the restitution of physiological homeostasis.

Cardiac autonomic activity has a circadian rhythm in summer but not in winter in non-lactating pregnant dairy cows

This investigation was conducted to examine circadian and seasonal rhythms of heart rate and heart rate variability (HRV) by means of hour-by-hour recordings over 24h in a large population of non-lactating Holstein-Friesian pregnant cows [N=56, summer (June-July); N=61, winter (November-December)]. Data were collected during a 5-day period from each animal. Besides parameters of cardiac autonomic function [the high-frequency (HF) component of HRV and the ratio between the low-frequency (LF) and the HF components (LF/HF ratio)], the RR triangular index and Lmax were calculated. A clear circadian profile was observed for every parameter in summer. Heart rate elevated gradually with the course of the day from 7:00 to 17:00 o'clock and then slightly decreased from 18:00 to 6:00. Sympathovagal balance shifted towards sympathetic dominance during the daytime (increased LF/HF ratio), whereas parasympathetic activity was predominant during the night (increased HF). Lmax reflected a chaotic behavior of heart rate fluctuations during the afternoon in summer. Decreased values of RR triangular index indicated a sensitive period for cows between 14:00 and 16:00 o'clock in summer. During winter, except for the RR triangular (RRtri) index reflecting a high overall variability in R-R intervals between 12:00 and 23:00 o'clock, heart rate and HRV showed no periodicity over the 24-h period. The results suggest an impaired cardiac autonomic function during daytime in summer. HF, Lmax and RRtri index showed seasonal differences for both daytime and nighttime. Heart rate was higher in summer than in winter during the daytime, whereas the LF/HF ratio was higher in winter during the nighttime. Circadian and seasonal rhythms of cardiovascular function are presumably related to the differing temperature, and animal activity associated with summer and winter. As all of the investigated parameters are commonly used in bovine HRV research, these findings have practical implications for behavioral, physiological and welfare studies on dairy cattle.

Repeated Phase Shifts in the Lighting Regimen Change the Blood Pressure Response to Norepinephrine Stimulation in Rats

Disturbed circadian activity of the sympathetic system may be involved in negative consequences of chronodisruption on the cardiovascular system. We studied daily changes in pressure response to adrenergic stimulation in rats exposed to repeated phase advance shifts (PAS) of light/dark (LD) regimen. Blood pressure (BP), heart rate (HR) and locomotor activity was measured by radiotelemetry in normotensive Wistar rats exposed to repeated PAS (three 8-h shifts per week) lasting for 12 weeks. Norepinephrine was administered subcutaneously in the middle of L and D during week 12 of PAS exposure. In the control LD cycle, cardiovascular parameters exhibited significant daily rhythms with expected higher values during D than L phase. Rats exposed to PAS showed disturbed rhythms without a BP and HR increase. Administration of norepinephrine to control rats revealed daily variability in the cardiovascular response with higher stimulation of BP during L than D. This daily pattern of BP response to norepinephrine was diminished in the PAS group. The damped daily variability in pressure response to norepinephrine and augmented response during the light phase of the day suggest that the increased and desynchronized activity of the sympathetic system may worsen responses of the cardiovascular system to load in individuals exposed to irregular LD conditions.

Stress and Its Effects on Glucose Metabolism and 11β-HSD Activities in Rats Fed on a Combination of High-Fat and High-Sucrose Diet with Glycyrrhizic Acid

Chronic stress has been shown to have a strong link towards metabolic syndrome (MetS). Glycyrrhizic acid (GA) meanwhile has been shown to improve MetS symptoms caused by an unhealthy diet by inhibiting 11 β -HSD 1. This experiment aimed to determine the effects of continuous, moderate-intensity stress on rats with and without GA intake on systolic blood pressure (SBP) across a 28-day period, as well as glucose metabolism, and 11 β -HSD 1 and 2 activities at the end of the 28-day period. Adaptation to the stressor (as shown by SBP) resulted in no significant defects in glucose metabolism by the end of the experimental duration. However, a weakly significant increase in renal 11 β -HSD 1 and a significant increase in subcutaneous adipose tissue 11 β -HSD 1 activities were observed. GA intake did not elicit any significant benefit in glucose metabolism, indicating that the stress response may block its effects. However, GA-induced improvements in 11 β -HSD activities in certain tissues were observed, although it is uncertain if these effects are manifested after adaptation due to the withdrawal of the stress response. Hence the ability of GA to improve stress-induced disturbances in the absence of adaptation needs to be investigated further.

Expressions of per1 clock gene and genes of signaling peptides vasopressin, vasoactive intestinal peptide, and oxytocin in the suprachiasmatic and paraventricular nuclei of hypertensive TGR[mREN2]27 rats

Hypertensive rats with multiple extra copies of the renin gene (TGR) exert an inverted circadian blood pressure (BP) profile. We investigated whether circadian oscillations in the hypothalamic suprachiasmatic nucleus (SCN), a main circadian oscillator, and the paraventricular nucleus (PVN), involved in BP control, are influenced in TGR rats. The expression of the clock gene per1, a marker of circadian timing, was measured in the SCN and PVN. Moreover, the expression of genes encoding vasopressin (AVP), vasoactive intestinal peptide (VIP) in the SCN, and AVP and oxytocin (OXT) in the PVN were studied by in situ hybridization. Expression of the per1 gene showed a distinct circadian rhythm in both the SCN and PVN with no differences observed between the TGR and control Sprague–Dawley (SD) rats. The expression of avp in the SCN was rhythmic in both strains and moderately higher in TGR than in SD rats while no significant changes were found in the PVN. The expression of vip in the SCN and oxt in the PVN did not differ between both strains. Our results may indicate that changes occurring downstream to the SCN are responsible for the development of the inverted BP rhythm in TGR hypertensive rats.

Circadian Rhythms in Heart Rate, Motility, and Body Temperature of Wild‐type C57 and eNOS Knock‐out Mice Under Light‐dark, Free‐run, and After Time Zone Transition

The nitric oxide (NO) system is involved in the regulation of the cardiovascular system in controlling central and peripheral vascular tone and cardiac functions. It was the aim of this study to investigate in wild-type C57BL/6 and endothelial nitric oxide synthase (eNOS) knock-out mice (eNOS-/-) the contribution of NO on the circadian rhythms in heart rate (HR), motility (motor activity [MA]), and body temperature (BT) under various environmental conditions. Experiments were performed in 12:12 h of a light:dark cycle (LD), under free-run in total darkness (DD), and after a phase delay shift of the LD cycle by -6 h (i.e., under simulation of a westward time zone transition). All parameters were monitored by radiotelemetry in freely moving mice. In LD, no significant differences in the rhythms of HR and MA were observed between the two strains of mice. BT, however, was significantly lower during the light phase in eNOS-/- mice, resulting in a significantly greater amplitude. The period of the free-running rhythm in DD was slightly shorter for all variables, though not significant. In general, rhythmicity was greater in eNOS-/- than in C57 mice both in LD and DD. After a delay shift of the LD cycle, HR and BT were resynchronized to the new LD schedule within 5-6 days, and resynchronization of MA occurred within 2-3 days. The results in telemetrically instrumented mice show that complete knock-out of the endothelial NO system--though expressed in the suprachiasmatic nuclei and in peripheral tissues--did not affect the circadian organization of heart rate and motility. The circadian regulation of the body temperature was slightly affected in eNOS-/- mice.

Importance of circadian rhythms for regulation of the cardiovascular system--studies in animal and man

In all mammals the cardiovascular system is highly organised in time. Pathophysiological cardiovascular events also do not occur at random (e.g. sudden cardiac death, stroke, ventricular arrhythmias, arterial embolism, symptoms of coronary heart disease, myocardial infarction). Radiotelemetry allows to get more insight into the circadian regulation of the cardiovascular system in unrestrained freely-moving animals. We monitored by telemetry blood pressure, heart rate (also be ECG-recordings), motility and body temperature in various strains of normotensive and hypertensive rats as well as in wildtype and knock-out mice. Our data gave evidence that the circadian rhythms in blood pressure and heart rate are controlled by the biological clock(s), since in rats and mice the rhythms persisted under free-running conditions in total darkness and were abolished in rats by lesioning of the "master clock" located in the suprachiasmatic nulcei.

Effect of upregulated renin–angiotensin system on per2 and bmal1 gene expression in brain structures involved in blood pressure control in TGR(mREN-2)27 rats

Circadian system regulates rhythms with 24 h period including those occurring in the cardiovascular system. Inverted blood pressure profile was demonstrated in hypertensive TGR(mREN-2)27 (TGR) rats with upregulated renin-angiotensin system. To depict structures involved in the generation of the inverted pattern of blood pressure in TGR rats, we analyzed daily expression of clock genes per2 and bmal1 in the brain areas involved in the regulation of the blood pressure. Heterozygous male TGR and control rats were synchronized to the light:dark cycle 12:12 and blood samples were taken in 4 h intervals within 24 h cycle. The levels of the plasma renin activity were increased in TGR rats in comparison with controls. Brain nuclei were isolated by dissection from frozen sections. The clock gene expression was determined in the hypothalamic paraventricular and dorsomedial nuclei, dorsal vagal motor nucleus, caudal ventrolateral medulla, nucleus ambiguus, area postrema, and anteroventral third ventricle. Daily pattern of per2 expression was rhythmic in most of the nuclei studied with its highest levels at the beginning of the nighttime in both groups of rats. Expression of bmal1 peaked at the beginning of the day. We found robust differences in the clock gene expression between the TGR and control rats in the area postrema. TGR rats exerted changes in the clock gene expression in the nucleus ambiguus which receives direct innervation from the area postrema. The area postrema seems to play a key role in the transmission of signals from the periphery to the CNS.

Altered cardiovascular responses to chronic angiotensin II infusion in aged rats

In this work we determined by telemetry the cardiovascular effects produced by Ang II infusion on blood pressure (BP) and heart rate (HR) in aged rats. Male Wistar aged (48-52 weeks) and young (12 weeks) rats were used. Ang II (6 microg/h, young, n=6; aged, n=6) or vehicle (0.9% NaCl 1 microl/h, young, n=4; aged, n=5) were infused subcutaneously for 7 days, using osmotic mini-pump. The basal diurnal and nocturnal BP values were higher in aged rats (day: 98+/-0.3 mm Hg, night: 104+/-0.4 mm Hg) than in the young rats (day: 92+/-0.2 mm Hg, night: 99+/-0.2 mm Hg). In contrast, the basal diurnal and nocturnal HR values were significantly smaller in the aged rats. Ang II infusion produced a greater increase in the diurnal BP in the aged rats (Delta MAP=37+/-1.8 mm Hg) compared to the young ones (Delta MAP=30+/-3.5 mm Hg). In contrast, the nocturnal MAP increase was similar in both groups (young rats; Delta MAP=22+/-3.0 mm Hg, aged rats; Delta MAP=24+/-2.6 mm Hg). During Ang II infusion HR decreased transiently in the young rats. An opposite trend was observed in the aged rats. Ang II infusion also inverted the BP circadian rhythm, in both groups. No changes in HR circadian rhythm were observed. These differences suggest that the aging process alters in a different way Ang II-sensitive neural pathways involved in the control of autonomic activity.

Indirect projections from the suprachiasmatic nucleus to the median preoptic nucleus in rat

We recently showed, using dual tract-tracing, that the suprachiasmatic nucleus (SCN), the site of the principal circadian clock in mammals, may have indirect projections to the sleep-promoting ventrolateral preoptic nucleus (VLPO) via relays in the medial preoptic area (MPA), dorsomedial hypothalamic nucleus (DMH), and, to a lesser extent, the subparaventricular zone (SPVZ). Here, we found that the injection of the rostral MPA, the periventricular nucleus/medial SPVZ, and the caudal DMH with a mixture of anterograde and retrograde tracers resulted in dense anterograde labeling in the median preoptic nucleus (MnPO), another key sleep-promoting nucleus in the preoptic region. The retrograde labeling in the SCN was evident as previously reported. The injections in either the MPA or the DMH produced similar densities of varicose fibers between the MnPO and the VLPO, while the injections in the SPVZ yielded a greater density of varicose fibers in the MnPO than in the VLPO. These results suggest that the MPA and DMH are potential relay nuclei to mediate SCN output to the MnPO, as well as to the VLPO, for the circadian control of sleep-wake states.

Transgenic TGR(mREN2)27 rats as a model for disturbed circadian organization at the level of the brain, the heart, and the kidneys

In transgenic hypertensive TGR(mREN2)27 rats (TGR) harboring the murine Ren-2 gene an inverse 24h blood pressure (BP) profile was described in relation to a normal pattern in heart rate (HR) and motility (MA), normotensive Sprague-Dawley rats (SDR) were used as controls. Transgenic rats as an animal model of human secondary hypertension (non-dipper) was studied in detail at different levels: (1) Radiotelemetry was applied to document gross circadian rhythms/rhythm disturbances in cardiovascular functions, MA and body temperature under normal LD conditions, under DD and after a light pulse. (2) Signal transduction of the overexpressed renin-angiotensin in TGR was studied by determation of AT1-receptors in kidney glomeruli together with kidney functions. (3) Expression of key processes involved in increased sympathetic regulation in TGR, mRNAs, the tyrosine-hydroxylase (TH) and norepinephrine (NE) reuptake1-carrier were determined. (4) In the SCN mRNA of c-fos and c-jun were determined under LD and after light pulse. (5) In primary cultures of pinealocytes the effects of adrenergic agonists and antagonists were evaluated on second messenger (cAMP, cGMP) accumulation and melatonin release. The results of these studies clearly demonstrate that the additional mouse renin genin in TGR greatly affected not only the renin-angiotensin-system and led--as expected--to an increased BP in this rat but also disturbed circadian rhythms from the BP pattern down to the level of hormones, processes of signal transduction, and expression of transcription factors and clock genes. In conclusion, the expression of a single additional gene is able to disturb the circadian system of an animal in a highly complex way. These findings are importance for chronobiologic as well as pharmacologic research.

Evaluation and applications of radiotelemetry in small laboratory animals

Radiotelemetry is the "state of the art" for monitoring physiological functions in awake and freely moving laboratory animals, while minimizing stress artifacts. For researchers, especially those in the fields of pharmacology and toxicology, the technique provides a valuable tool for defining the physiological and pathophysiological consequences derived from advances molecular, cellular, and tissue biology and in predicting the effectiveness and safety of new compounds in humans. There is ample evidence that radiotelemetry systems for measuring physiological functions has been sufficiently validated. Today, the technology is an important tool for collection of a growing number of physiological parameters, for contributing to animal welfare (reduction and refinement alternatives), and for reducing overall animal research costs.

A new inbred Wistar-Kyoto rat substrain exhibiting apparent salt sensitivity and borderline hypertension

The normotensive Wistar-Kyoto (WKY) rat strain is a traditional control for the spontaneously hypertensive rat (SHR). We found trait differences between two inbred normotensive WKY strains, derived originally from different vendors, and compared these two strains from La Jolla-Taconic Farms (WKY/lj-tf) and La Jolla-Charles River (WKY/lj-cr) with the inbred SHR/lj-cr for cardiovascular, diurnal, and activity traits under normal and high (8%) NaCl diets. Marked genetic diversity was found between the two vendor-derived WKY. By using an extended study design and radiotelemetry, we compared WKY/lj-cr, WKY/lj-tf, and SHR/lj-cr with the following results: systolic pressure (120 +/- 1, 133 +/- 1, 168 +/- 3 mmHg, respectively); diurnal variation in heart rate (DeltaHR: 46 +/- 3, 71 +/- 4, 57 +/- 2 beats/min, respectively); and salt sensitivity of arterial pressure (Deltasystolic: 10 +/- 1, 21 +/- 1, 20 +/- 1 mmHg, respectively). The WKY/lj-tf genotype apparently results in compromised control of arterial pressure and heart rate, especially during high NaCl intake, and greater susceptibility to high pressure (i.e., high NaCl-induced secondary changes). WKY/lj-tf thus constitutes a new inbred borderline hypertensive WKY substrain offering unique opportunities for genomic studies into the development of genetic hypertension.

Jet lag in athletes after eastward and westward time-zone transition

In healthy male top athletes several functions were measured after either a westbound flight over six time-zones (WEST: Frankfurt-Atlanta; n = 13) or an eastbound flight over eight time-zones (EAST: Munich-Osaka; n = 6). Under either condition the athletes performed two standardized exercise training units in the morning and in the afternoon within 24 h, investigations were done as controls in Germany and on day 1, 4, 6, and 11, after arrival. The primary aim of the study was to evaluate the effect of time-zone transitions on the 24h profiles of blood pressure (BP) and heart rate (HR) using an ambulatory BP device (SpaceLabs 90207), for up to 11 d after arrival at the destination. As additional parameters, we studied jet-lag symptoms, training performance, and training coordination by using visual analog scales. Finally, oral temperature and grip strength were measured, and saliva samples were analyzed for cortisol and melatonin. The study showed that all functions were disturbed on the first day after arrival at the destination, jet-lag symptoms remained until day 5-6 after WEST and day 7 after EAST, training performance was worst within the first 4 d after WEST. In accordance with earlier reports, cortisol, melatonin, body temperature, and grip strength were affected in their 24h profiles and additionally modified by the training units. Surprisingly, BP and HR were not only affected on the first day but also the time-zone transition led to an increase in BP after WEST and a decrease in BP after EAST. However, the training units seemed to influence the BP profile more than the time-zone transitions. HR rhythm was affected by both time-zone transitions and exercise. It is concluded that not only jet-lag symptoms but also alterations in physiological functions should be considered to occur in highly competitive athletes due to time-zone transition and, therefore, an appropriate time of reentrainment is recommended.

Effects of angiotensins on day-night fluctuations and stress-induced changes in blood pressure

In this study we evaluated by telemetry the effects of ANG II and ANG-(1-7) infusion on the circadian rhythms of blood pressure (BP) and heart rate (HR) and on the cardiovascular adjustment resulting from restraint stress in rats. ANG II or ANG-(1-7) or vehicle were infused subcutaneously for 7 days. Restraint stress was carried out before, during, and after infusion at 7-day intervals. Parallel with an increase in MAP, ANG II infusion produced an inversion of MAP circadian rhythm with a significant MAP acrophase inversion. It also produced bradycardia during the first 3 days of infusion. Thereafter, HR progressively increased, reaching values similar to or above those of the control period at the end of the infusion period. HR circadian variation was not changed by ANG II infusion. Strikingly, ANG II significantly attenuated the increase in MAP induced by restraint stress without altering the HR response. ANG-(1-7) infusion produced a slight but significant decrease in MAP restricted to the daytime period. No significant changes in the MAP acrophase were observed. In addition, ANG-(1-7) infusion produced a small but significant sustained bradycardia. ANG-(1-7) did not change cardiovascular responses to restraint stress. These data indicate that ANG II can influence the activity of brain areas involved in the determination of stress-induced or circadian-dependent variations of blood pressure without changing HR fluctuations. A significant modulatory influence of ANG-(1-7) on basal MAP and HR is also suggested.

Day-night variation in the in vitro contractility of aorta and mesenteric and renal arteries in transgenic hypertensive rats

TGR(mREN2)27 (TGR) rats develop severe hypertension and an inverted circadian blood pressure profile with peak blood pressure in the day-time rest phase. The present study investigated the in vitro responsiveness of different arteries of TGR rats during day and night. Twelve-week-old TGR rats and normotensive Sprague-Dawley (SPRD) controls, synchronized to 12h light, 12h dark (LD 12:12) (light 07:00-19:00), were killed at 09:00 (during rest) and 21:00 (during activity), and endothelium-dependent relaxation by acetylcholine and vascular contraction by angiotensin II were studied by measuring isometric force in ring segments of abdominal aorta and mesenteric and renal arteries. In SPRD rats, consistent day-night variation was found, with greater responses to angiotensin II during the daytime rest span. In TGR rats, biological time-dependent differences were found in the renal vasculature, but not in the aorta and mesenteric artery. Relaxation of SPRD rat aorta and mesenteric artery by acetylcholine was greater at 09:00, whereas in TGR rats, day-night variation was absent (mesenteric artery) or inverted (aorta). In conclusion, based on the study of two time points, day-night variation in vascular contractility of aorta and mesenteric artery is blunted in TGR rats, whereas renal artery segments showed an unchanged day-night pattern compared to SPRD controls.

Reduced baroreflex sensitivity and blunted endogenous nitric oxide synthesis precede the development of hypertension in TGR(mREN2)27 rats

Transgenic TGR(mREN2)27 (TGR) rats are an animal model of fulminant hypertension characterized by an inverse circadian blood pressure profile. The present study addressed the contribution of nitric oxide (NO) synthesis and baroreflex function to hypertension and the inverse blood pressure pattern. NO synthesis was measured at four different times of day indirectly by excretion of NO metabolites (NOx: NO2- and NO3-) in the urine of 5- and 11-week-old TGR and Sprague-Dawley (SPRD) controls. Blood pressure, heart rate, and motor activity were recorded in age-matched rats of both strains using an implantable telemetry system. Beat-to-beat recording of blood pressure and pulse interval was performed hourly in 6-week-old animals over 24 h. From these data, baroreflex sensitivity (BRS) was calculated by linear regression of spontaneous fluctuations of blood pressure and corresponding changes of pulse interval. Baroreflex sensitivity was lower in pre-hypertensive TGR rats than in SPRD rats, and the reduction was restricted to the daily resting period. In both strains, NOx excretion showed circadian rhythmicity, with peak values during the activity period at night. Interestingly, excretion of NOx was reduced during the resting period in 5-week-old TGR rats prior to the development of hypertension. Impairment of NO synthesis and baroreflex function precede the development of hypertension in TGR rats. The reduction of both parameters was restricted to the resting period and, therefore, could be involved in the development of the inverse circadian blood pressure profile of TGR rats.

Cardiovascular effects of endothelin-1 and endothelin antagonists in conscious, hypertensive ((mRen-2)27) rats

SB 209670 is a potent antagonist of the vasoconstrictor (ET(A)- and ET(B)-receptor-mediated) and vasodilator (ET(B)-receptor-mediated) effects of endothelin, whereas SB 234551 is relatively selective for the constrictor (ET(A)-receptor-mediated) effects. Since we had previously found SB 209670 exerted antihypertensive, vasodilator effects in conscious, heterozygous, transgenic ((mRen-2)27) (abbreviated to TG) rats, here we compared the two antagonists in that model, and assessed their chronic effects on responses to exogenous endothelin-1. We did this to test our global hypothesis, namely, that SB 209670, but not SB 234551, would cause inhibition of the depressor effects of exogenous endothelin-1 in vivo, and that this differential effect would be associated with a more marked antihypertensive action of SB 234551 in TG rats. SB 209670 and SB 234551 (infused for 50 h) exerted similar, sustained, antihypertensive effects in TG rats. The antihypertensive effects of the antagonists occurred at times when the pressor effects of exogenous endothelin-1 were not significantly inhibited. Furthermore, SB 234551 did not exert a greater antihypertensive effect than SB 209670 at a time (i.e., 2 - 4 h) when the depressor effects of endothelin-1 were abolished by the latter, but not by the former (although this differential action was lost after 24 h infusion). The results caused us to reject the hypothesis that selective antagonism of the vasoconstrictor effects of endothelin-1 would result in SB 234551 exerting a greater antihypertensive effect than SB 209670 in TG rats.

Loss of 24 h rhythm and light-induced c-fos mRNA expression in the suprachiasmatic nucleus of the transgenic hypertensive TGR(mRen2)27 rat and effects on cardiovascular rhythms

Immediate early genes, especially c-fos, are thought to play an essential role in photic entrainment of circadian rhythms. A special characteristic of the transgenic hypertensive TGR(mRen2)27 rat strain, expressing an additional mouse renin2 gene, is the inverse blood pressure rhythm in relation to those in heart rate and activity resulting in internal desynchronisation of these physiological rhythms. Assessment of c-fos mRNA expression by microdissection and RT-PCR in the suprachiasmatic nucleus showed, that in contrast to normotensive Sprague-Dawley rats the 24 h and circadian rhythm of c-fos mRNA expression in TGR(mRen2)27 rats is abolished. Moreover, light-induced c-fos expression within the nucleus could be found in the normotensive controls, but was absent in transgenic hypertensive rats. The light pulse applied during the subjective night, at CT 14, significantly phase delayed rhythms in blood pressure, heart rate and activity in the normotensive rats by about 2 h, whereas in the transgenic hypertensive animals rhythms in blood pressure and heart rate were unaffected, only activity showed a slight phase shift. In conclusion, these data suggest that the transgene in TGR leads not only to a disturbance of the cardiovascular system but also influences the light entrainment response, which is accompanied by a suppressed c-fos mRNA expression in the suprachiasmatic nucleus.

Urinary excretion of nitric oxide, cyclic GMP, and catecholamines during rest and activity period in transgenic hypertensive rats

Dysregulation of the system of nitric oxide (NO)-cyclic 3',5'-guanosine monophosphate (cGMP) might be involved in the development of hypertension in transgenic hypertensive TGR(mREN2)27 (TGR) rats. The present study was performed to determine possible differences in the day-night pattern and the urinary excretion rates of NO and cGMP in TGR rats in comparison to normotensive Sprague-Dawley (SPRD) controls. In addition, the urinary excretion of creatinine and catecholamines was measured in both rat strains. The day-night excretion patterns of NO, cGMP, catecholamines, and creatinine were preserved in TGR rats. Urinary excretion of NO was significantly decreased in TGR rats, whereas cGMP, the second messenger of NO, was elevated in the transgenic animals. Catecholamines and creatinine excretion rates did not differ between the strains. In conclusion, data suggest that a reduced NO synthesis could contribute to the increased blood pressure in the severely hypertensive rats. However, these data make it unlikely that the disturbances in the nitric oxide-cGMP system and the sympathetic nervous system are mainly responsible for the inverse circadian blood pressure rhythm in TGR rats.

Development of inverse circadian blood pressure pattern in transgenic hypertensive TGR(mREN2)27 rats

TGR(mREN2)27 (TGR) rats are transgenic animals with an additional mouse renin gene, which leads to overactivity of the renin-angiotensin system. Adult TGR rats are characterized by fulminant hypertension, hypertensive end-organ damage, and an inverse circadian blood pressure pattern. To study the ontogenetic development of cardiovascular circadian rhythms, telemetric blood pressure transmitters were implanted in male Sprague-Dawley (SPRD, n = 5) and heterozygous, transgenic TGR rats before 5 weeks of age. The TGR received either drinking water or enalapril 10 mg/L in drinking water (n = 5 per group). Drug intake was measured throughout the study by computerized monitoring of drinking volume. Circadian patterns in blood pressure and heart rate were analyzed from 5 to 11 weeks of age. In the first week after transmitter implantation, blood pressure did not differ among SPRD, untreated, and enalapril-treated TGR rats. In parallel with the rise in blood pressure of untreated TGR rats, a continuous delay of the circadian acrophase (time of fitted blood pressure maximum) was observed, leading to a complete reversal of the rhythm in blood pressure at an age of 8 weeks. Enalapril reduced blood pressure at night, but was less effective during the day, presumably due to the drinking pattern of the animals, which ingested about 90% of their daily water intake during the nocturnal activity period. After discontinuation of treatment, blood pressure returned almost immediately to values found in untreated TGR rats. In conclusion, the inverse circadian blood pressure profile in TGR rats develops in parallel with the increase in blood pressure. Direct effects of the brain renin-angiotensin system may be involved in the disturbed circadian rhythmicity in TGR(mREN2)27 rats.

Blood pressure and heart rate rhythmicity: differential effects of late pregnancy

Arterial blood pressure (BP) and heart rate (HR) of 31 hospitalized pregnant women at low risk of hypertension were automatically monitored for 48 h at 15-min intervals. Each of the recorded 56 data series for systolic arterial pressure (SAP), diastolic arterial pressure (DAP), and HR was chronobiologically assessed by linear-nonlinear rhythmometry. The rhythm-adjusted mean (MESOR), circadian amplitude, circadian acrophase, and best-fitting period were grouped by pregnancy trimester and further subjected to analysis of variance. BP MESOR remained unaltered, whereas HR MESOR increased significantly in middle and late pregnancy. Ultradian rhythms, with an amplitude higher than that of the circadian rhythm, were found in 25% of the SAP records in the second and third trimester. Such ultradian rhythms were not detected in the simultaneously recorded HR. Finally, the group BP and HR circadian acrophases coincided in the first trimester, but were significantly apart in mid and late pregnancy. These observations support the notion that the coordination of BP and HR rhythmicity involves different physiological mechanisms. Analysis of the individual variability in the chronobiological end points (based on the records of nine women monitored in each pregnancy trimester) revealed that only the BP MESOR was well reproducible in the course of pregnancy and may be useful in early diagnosis of gestational hypertension.

Effects of melatoninergic agonists on light-suppressed circadian rhythms in rats

This study addressed the question whether light-suppressed circadian rhythms in cardiovascular parameters in rats could be restored by melatonin and a synthetic analogue. Blood pressure, heart rate, and locomotor activity were monitored by radiotelemetry in six Sprague-Dawley rats. After synchronization to a 12:12 light/dark (LD) schedule (lights on at 0700 hours, 100 lux), rats were kept in constant light (LL) of low intensity (5-10 lux) for 11 weeks. After 3 weeks of LL, rats received daily intraperitoneal (i.p.) injections at 1900 hours of vehicle, the melatonin agonist S-21767 (5 mg/kg) and melatonin (1 mg/kg). Spectral power, 24-h amplitudes and the differences between day and night means were calculated as measures of circadian rhythmicity. During LL a lengthening of the endogenous period to 26 h was observed, which was accompanied by a continuous decrease in circadian amplitude in all parameters monitored until, in the third week of LL, circadian rhythmicity was almost abolished. Neither vehicle, S-21767 nor melatonin were able to restore circadian rhythms in blood pressure and locomotor activity. In contrast, both agonists induced circadian rhythmicity in heart rate in two out of six rats. The day/night difference in heart rate of all animals was significantly increased by S-21767 and, to a smaller extent, by melatonin, whereas the circadian amplitude was not affected. In conclusion, melatonin and the synthetic agonist were able to partially synchronize circadian rhythmicity in heart rate during constant light, but could not restore circadian rhythms in blood pressure.

Effects of SCN lesions on circadian blood pressure rhythm in normotensive and transgenic hypertensive rats

Transgenic hypertensive TGR(mREN2)27 (TGR) rats, carrying an additional mouse renin gene, have been found to show inverse circadian blood pressure profiles compared to normotensive Sprague-Dawley rats. In order to evaluate the contributions of the suprachiasmatic nucleus (SCN) and the neurohormone melatonin to cardiovascular circadian regulation in TGR(mREN2)27 rats and Sprague-Dawley (SPRD) controls, we investigated the effects of melatonin agonist and antagonist treatment in SCN-lesioned and nonlesioned rats, which were kept under conditions of alternating light and darkness (LD). After destruction of the SCN, circadian rhythmicity in blood pressure, heart rate (HR), and motor activity (MA) was almost abolished in rats of both strains. One week of treatment with a synthetic melatonin agonist S-21634 was not able to restore circadian variation in the parameters monitored. In nonlesioned TGR(mREN2)27 rats and Sprague-Dawley control rats, the melatonin antagonist S-22365 had no suppressive effect on LD-synchronized circadian rhythmicity, indicating that LD itself may have a stronger influence on the SCN than endogenous melatonin.

Analysis of telemetric time series data for periodic components using DQ-FIT

DQ-FIT and CV-SORT have been developed to facilitate the automatic analysis of data sampled by radiotelemetry, but they can also be used with other data sampled in chronobiological settings. After import of data, DQ-FIT performs conventional linear, as well as rhythm analysis according to user-defined specifications. Linear analysis includes calculation of mean values, load values (percentage of values above a defined limit), highest and lowest readings, and areas under the (parameter-time) curve (AUC). All of these parameters are calculated for the total sampling interval and for user-defined day and night periods. Rhythm analysis is performed by fitting of partial Fourier series with up to six harmonics. The contribution of each harmonic to the overall variation of data is tested statistically; only those components are included in the best-fit function that contribute significantly. Parameters calculated in DQ-FIT's rhythm analysis include mesor, amplitudes, and acrophases of all rhythmic components; significance and percentage rhythm of the combined best fit; maximum and minimum of the fitted curve and times of their occurrence. In addition, DQ-FIT uses the first derivative of the fitted curve (i.e., its slope) to determine the time and extent of maximal increases and decreases within the total sampling interval or user-defined intervals of interest, such as the times of lights on or off. CV-SORT can be used to create tables or graphs from groups of data sets analyzed by DQ-FIT. Graphs are created in CV-SORT by calculation of group mean profiles from individual best-fit curves rather than their curve parameters. This approach allows the user to combine data sets that differ in the number and/or period length of harmonics included. In conclusion, DQ-FIT and CV-SORT can be helpful in the analysis of time-dependent data sampled by telemetry or other monitoring systems. The software can be obtained on request by every interested researcher.

Circadian rhythms of cardiovascular functions are modulated by the baroreflex and the autonomic nervous system in the rat

BACKGROUND

We assessed the hypothesis that the baroreflex and the autonomic nervous system are important in the control of the circadian rhythms of cardiovascular functions.

METHODS AND RESULTS

We continuously measured blood pressure (BP), heart rate (HR), and locomotor activity in sinoaortic denervated (SAD), sympathectomized, and atropine-injected rats by use of a radiotelemetry system. The circadian rhythm of mean blood pressure (MBP) was selectively disrupted in SAD rats under 12-hour light-dark (LD12:12) cycles as a result of an increase in MBP during the light period and disappeared under constant darkness (DD). The locomotor activity and HR were not remarkably affected by SAD. The circadian rhythm of MBP was suppressed in sympathectomized rats by a decrease in the MBP during the dark period, and the abrupt changes in MBP when the lighting was altered were not seen under LD. Under DD, an MBP rhythm similar to that observed under LD was obtained. Sympathectomized rats also showed lower HR levels during the dark period than intact rats under LD cycles. In atropine-injected rats, the MBP and HR increased, especially during the light period, resulting in a reduction of light-dark differences in MBP and HR. The locomotor activity showed an apparent 24-hour variation in the sympathectomized and atropine-injected rats.

CONCLUSIONS

The disruption of the baroreflex selectively eliminates the circadian rhythm of BP, and the circadian rhythms of BP and HR are modulated by the autonomic nervous system in rats. The circadian rhythms of BP and HR are regulated by different mechanisms involving the autonomic nervous system.

24-hour recordings of blood pressure, heart rate and behavioural activity in rabbits by radio-telemetry: effects of feeding and hypertension

We used radio-telemetry to measure 24-hour rhythms of systolic, diastolic and mean blood pressure, heart rate and behavioural activity in conscious rabbits, which were maintained under normal day/night rhythms and restricted feeding. Over three consecutive days, all variables showed little change between day-period and night-period, except for a pronounced rise in the afternoon, coinciding with the presentation of pellet food. Mean blood pressure increased during this period from baseline values between 78-82 mm Hg to a peak of 89-91 mm Hg. At the same time heart rate rose from baseline values of 147-161 b/min to a peak of 206-234 b/min and behavioural activity scores rose from 11-31 counts/h to a peak of 52-81 counts/h. Changing the time at which pellet food was presented to the rabbits from the early afternoon to the early morning, caused a complete and immediate shift of the peak of blood pressure and heart rate to the morning period. Chronic intravenous infusion of angiotensin II caused a significant increase in blood pressure (24-hour average: 80 +/- 1 vs. 114 +/- 7 mm Hg) but did not alter basal heart rate or behavioural activity. The increase in heart rate and blood pressure seen with food presentation was attenuated with angiotensin II infusion. These data show that in rabbits diurnal changes in blood pressure, heart rate and activity were determined to a large extent by timed feeding. In addition, in rabbits with angiotensin-induced hypertension the food-induced changes in blood pressure and heart rate were blunted.