Effect of Endothelin-1 on Baroreflexes and the Cardiovascular Action of Clonidine in Conscious Rabbits

We studied the influence of pretreatment with endothelin–1 on cardiac baroreflexes and on the effect of clonidine on blood pressure and heart rate. In order to avoid the complication of the direct vasoconstrictor effects of endothelin-1, initial dose-response studies in animals treated with a ganglion blocker were performed. Intravenous administration of 50, 200, and 1200 ng/kg of endothelin-1 produced biphasic changes in blood pressure, consisting of an immediate depressor response, followed by a long lasting and dose-dependent pressor effect (peak response 3 ± 1, 9 ± 3, and 33 ± 5 mmHg, respectively). Thus, the 50 ng/kg dose of endothelin-1 was used in subsequent studies. Conscious rabbits were pretreated on separate days with endothelin-1, either intravenously (50 ng/kg) or intracisternally (10 and 50 ng/kg), or with vehicle. The animals then received an intravenous dose (20 μg/kg) or an intracisternal dose (1 μg/kg) of clonidine and the effects on blood pressure and heart rate were measured. In vehicle-treated rabbits, the intravenous administration of clonidine induced a significant decrease in blood pressure and heart rate (15 min after injection: −15.7 ± 4.7 mmHg and −33 ± 4 b/min, respectively). Similarly, the intracisternal injection of clonidine lowered blood pressure (−16.0 ± 2.5 mmHg), but produced a less pronounced bradycardia (−18 ± 4 b/min). Endothelin pretreatment, either 50 ng/kg centrally or peripherally, had no significant effect on the hypotension or bradycardia produced either by central or peripheral injection of clonidine. At this dose, endothelin by itself did not produce significant changes in blood pressure or heart rate. There was a reduction of the gain of the baroreceptor-heart rate reflex with intracisternal endothelin-1. These results suggest that central ₂–adrenoceptor mechanisms involved in clonidine-induced hypotension and bradycardia do not appear to be influenced by activation of endothelin receptors.

Associations of blood pressure variability and retinal arteriolar diameter in participants with type 2 diabetes

Blood pressure variability is associated with macrovascular complications and stroke, but its association with the microcirculation in type II diabetes has not been assessed. This study aimed to determine the relationship between blood pressure variability indices and retinal arteriolar diameter in non-diabetic and type II diabetes participants. Digitized retinal images were analysed to quantify arteriolar diameters in 35 non-diabetic (aged 52 ± 11 years; 49% male) and 28 type II diabetes (aged 61 ± 9 years; 50% male) participants. Blood pressure variability was derived from 24-h ambulatory blood pressure. Arteriolar diameter was positively associated with daytime rate of systolic blood pressure variation (p = 0.04) among type II diabetes participants and negatively among non-diabetics (p = 0.008; interaction p = 0.001). This finding was maintained after adjusting for age, sex, body mass index and mean daytime systolic blood pressure. These findings suggest that the blood pressure variability-related mechanisms underlying retinal vascular disease may differ between people with and without type II diabetes.

Contribution of Orexin to the Neurogenic Hypertension in BPH/2J Mice

BPH/2J mice are a genetic model of hypertension associated with an overactive sympathetic nervous system. Orexin is a neuropeptide which influences sympathetic activity and blood pressure. Orexin precursor mRNA expression is greater in hypothalamic tissue of BPH/2J compared with normotensive BPN/3J mice. To determine whether enhanced orexinergic signaling contributes to the hypertension, BPH/2J and BPN/3J mice were preimplanted with radiotelemetry probes to compare blood pressure 1 hour before and 5 hours after administration of almorexant, an orexin receptor antagonist. Mid frequency mean arterial pressure power and the depressor response to ganglion blockade were also used as indicators of sympathetic nervous system activity. Administration of almorexant at 100 (IP) and 300 mg/kg (oral) in BPH/2J mice during the dark-active period (2 hours after lights off) markedly reduced blood pressure (-16.1 ± 1.6 and -11.0 ± 1.1 mm Hg, respectively;P<0.001 compared with vehicle). However, when almorexant (100 mg/kg, IP) was administered during the light-inactive period (5 hours before lights off) no reduction from baseline was observed (P=0.64). The same dose of almorexant in BPN/3J mice had no effect on blood pressure during the dark (P=0.79) or light periods (P=0.24). Almorexant attenuated the depressor response to ganglion blockade (P=0.018) and reduced the mid frequency mean arterial pressure power in BPH/2J mice (P<0.001), but not BPN/3J mice (P=0.70). Immunohistochemical labeling revealed that BPH/2J mice have 29% more orexin neurons than BPN/3J mice which are preferentially located in the lateral hypothalamus. The results suggest that enhanced orexinergic signaling contributes to sympathetic overactivity and hypertension during the dark period in BPH/2J mice.

Comparison of sympathetic nerve activity normalization procedures in conscious rabbits

One of the main constraints associated with recording sympathetic nerve activity (SNA) in both humans and experimental animals is that microvolt values reflect characteristics of the recording conditions and limit comparisons between different experimental groups. The nasopharyngeal response has been validated for normalizing renal SNA (RSNA) in conscious rabbits, and in humans muscle SNA is normalized to the maximum burst in the resting period. We compared these two methods of normalization to determine whether either could detect elevated RSNA in hypertensive rabbits compared with normotensive controls. We also tested whether either method eliminated differences based only on different recording conditions by separating RSNA of control (sham) rabbits into two groups with low or high microvolts. Hypertension was induced by 5 wk of renal clipping (2K1C), 3 wk of high-fat diet (HFD), or 3 mo infusion of a low dose of angiotensin (ANG II). Normalization to the nasopharyngeal response revealed RSNA that was 88, 51, and 34% greater in 2K1C, HFD, and ANG II rabbits, respectively, than shams (P < 0.05), but normalization to the maximum burst showed no differences. The RSNA baroreflex followed a similar pattern whether RSNA was expressed in microvolts or normalized. Both methods abolished the difference between low and high microvolt RSNA. These results suggest that maximum burst amplitude is a useful technique for minimizing differences between recording conditions but is unable to detect real differences between groups. We conclude that the nasopharyngeal reflex is the superior method for normalizing sympathetic recordings in conscious rabbits.

How to measure home blood pressure: Recommendations for healthcare professionals and patients

BACKGROUND

Home blood pressure (BP) monitoring is the self-measurement of BP in the home environment. It is complementary to 24-hour ambulatory BP, for better diagnosis and management of patients with high BP. Home BP monitoring is in widespread use, but variation in monitoring protocols could lead to inaccurate assessment of BP.

OBJECTIVE

The aim of this article is to provide a practical guide (with resources) for patients and doctors on how to measure home BP according to a standardised, evidence-based protocol.

DISCUSSION

Home BP should be measured using a validated, automatic BP device (preferably with memory storage), using an appropriately sized upper arm cuff. Measurements should be taken after five minutes of seated rest and before medication, food or vigorous exercise. BP should be recorded for seven days (five days minimum) in the morning and evening (two readings each). Overall, home BP is the average systolic and diastolic BP over seven days (excluding the first day); an average of ≥135/85 mmHg is indicative of hypertension.

Characteristics of renal sympathetic nerve single units in rabbits with angiotensin-induced hypertension

We examined the effect of chronic angiotensin (Ang II)-induced hypertension on activity of postganglionic renal sympathetic units to determine whether altered whole renal nerve activity is due to recruitment or changes in firing frequency. Rabbits were treated with a low (20 ng kg(-1) min(-1), 8 weeks) or high dose (50 ng kg(-1) min(-1), 4 weeks) of Ang II before the experiment under chloralose-urethane anaesthesia. Spontaneously active units were detected from multiunit recordings using an algorithm that separated units by action potential shape using templates that matched spikes within a prescribed standard deviation. Multiunit sympathetic nerve activity was 40% higher in rabbits treated with low-dose Ang II than in sham (P = 0.012) but not different in high-dose Ang II. Resting firing frequency was similar in sham rabbits (1.00 ± 0.09 spikes s(-1), n = 144) and in those treated with high-dose Ang II (1.10 ± 0.08 spikes s(-1), n = 112) but was lower with low-dose Ang II (0.65 ± 0.08 spikes s(-1), n = 149, P < 0.05). Unit firing rhythmicity was linked to the cardiac cycle and was similar in sham and low-dose Ang II groups but 29-32% lower in rabbits treated with high-dose Ang II (P < 0.001). Cardiac linkage followed a similar pattern during hypoxia. All units showed baroreceptor dependency. Baroreflex gain and range were reduced and curves shifted to the right in Ang II groups. Firing frequency during hypoxia increased by +39% in low-dose Ang II and +82% in shams, but the greatest increase was in the high-dose Ang II group (+103%, P(dose) = 0.001). Responses to hypercapnia were similar in all groups. Increases in sympathetic outflow in hypertension caused by low-dose chronic Ang II administration are due to recruitment of neurons, but high-dose Ang II increases firing frequency in response to chemoreceptor stimuli independently of the arterial baroreceptors.

Endothelial cationic amino acid transporter-1 overexpression blunts the effects of oxidative stress on pressor responses to behavioural stress in mice

Observational studies indicate that psychological stress may contribute to the pathogenesis of hypertension and this may be further accentuated by factors such as endothelial dysfunction. On this basis, we aimed to determine whether oxidative stress enhances pressor responses to stressful stimuli and whether augmenting endothelial function by increasing the transport of L-arginine can counter the effects of oxidative stress. Telemetry probes were used to measure mean arterial pressure (MAP) in wild-type (WT; n = 6) and endothelial cationic amino acid transporter-1 (CAT-1)-overexpressing (CAT+) mice (n = 6) before and during an aversive (restraint) and non-aversive (almond feeding) stressor. The superoxide dismutase inhibitor diethyldithiocarbamic acid (DETCA; 30 mg/kg per day; 14 days) was then administered via a minipump to induce oxidative stress. Stress responses to feeding and restraint were repeated during Days 11-12 of DETCA infusion. In WT mice, pressor responses to restraint and feeding were augmented during infusion of DETCA (35 ± 1 and 28 ± 1 mmHg, respectively) compared with respective pretreatment responses (28 ± 2 and 24 ± 1 mmHg, respectively; P ≤ 0.01). In CAT+ mice, pressor responses to feeding were blunted during DETCA (20 ± 1 mmHg) compared with the control response (23 ± 1 mmHg; P = 0.03). In these mice, pressor responses to restraint were similar before (28 ± 1 mmHg) and during (26 ± 1 mmHg) DETCA infusion (P = 0.26). We conclude that endothelial CAT-1 overexpression can counter the ability of oxidative stress to augment pressor responses to behavioural stress.

Augmented Endothelial-Specific L-Arginine Transport Blunts the Contribution of the Sympathetic Nervous System to Obesity Induced Hypertension in Mice

Augmenting endothelial specific transport of the nitric oxide precursor L-arginine via cationic amino acid transporter-1 (CAT1) can prevent obesity related hypertension. We tested the hypotheses that CAT1 overexpression prevents obesity-induced hypertension by buffering the influence of the sympathetic nervous system (SNS) on the maintenance of arterial pressure and by buffering pressor responses to stress. Wild type (WT; n=13) and CAT1 overexpressing mice (CAT+; n=13) were fed a normal or a high fat diet for 20 weeks. Mice fed a high fat diet were returned to the control diet before experiments commenced. Baseline mean arterial pressure (MAP) and effects of restraint-, shaker- and almond feeding-stress and ganglionic blockade (pentolinium; 5 mg/kg; i.p.) on MAP were determined in conscious mice. Fat feeding increased body weight to a similar extent in WT and CAT+ but MAP was greater only in WT compared to appropriate controls (by 29%). The depressor response to pentolinium was 65% greater in obese WT than lean WT (P < 0.001), but was similar in obese and lean CAT+ (P = 0.65). In lean WT and CAT+, pressor responses to shaker and feeding stress, but not restraint stress, were less in the latter genotype compared to the former (P ≤ 0.001). Pressor responses to shaker and feeding stress were less in obese WT than lean WT (P ≤ 0.001), but similar in obese and lean CAT+. The increase in MAP in response to restraint stress was less in obese WT (22 ± 2%), but greater in obese CAT+ (37 ± 2%), when compared to respective lean WT (31 ± 3%) and lean CAT+ controls (27 ± 2%; P ≤ 0.02). We conclude that CAT1 overexpression prevents obesity-induced hypertension by reducing the influence of the SNS on the maintenance of arterial pressure but not by buffering pressor responses to stress.

Specific role of dietary fat in modifying cardiovascular and locomotor activity 24-h rhythms

Meal-fed conscious rabbits normally exhibit postprandial elevation in blood pressure, heart rate (HR) and locomotor activity, which is abolished by consumption of a high-fat diet (HFD). Here, we assessed whether the cardiovascular changes are attributable to the increased caloric intake due to greater fat content or to hyperphagia. Rabbits were meal-fed during the baseline period then maintained on either an ad libitum normal fat diet (NFD) or ad libitum HFD for 2 weeks. Blood pressure, HR and locomotor activity were measured daily by radio-telemetry alongside food intake and body weight. Caloric intake in rabbits given a NFD ad libitum rose 50% from baseline but there were no changes in cardiovascular parameters. By contrast, HR increased by 10% on the first day of the ad libitum HFD (p < 0.001) prior to any change in body weight while blood pressure increased 7% after 4 d (p < 0.01) and remained elevated. Baseline 24-h patterns of blood pressure and HR were closely associated with mealtime, characterised by afternoon peaks and morning troughs. When the NFD was changed from meal-fed to ad libitum, blood pressure and HR did not change but afternoon activity levels decreased (p < 0.05). By contrast, after 13 d ad libitum HFD, morning HR, blood pressure and activity increased by 20%, 8% and 71%, respectively. Increased caloric intake specifically from fat, but not as a result of hyperphagia, appears to directly modulate cardiovascular homeostasis and circadian patterns, independent of white adipose tissue accumulation.

Central nervous system dysfunction in obesity-induced hypertension

The activation of the sympathetic nervous system is a major mechanism underlying both human and experimental models of obesity-related hypertension. While insulin and the adipokine leptin have long been thought to contribute to obesity-related neurogenic mechanisms, the evidence is now very strong that they play a major role, shown particularly in animal studies using selective receptor antagonists. There is not just maintenance of leptin's sympatho-excitatory actions as previously suggested but considerable amplification particularly in renal sympathetic nervous activity. Importantly, these changes are not dependent on short-term elevation or reduction in plasma leptin or insulin, but require some weeks to develop indicating a slow "neural adaptivity" within hypothalamic signalling. These effects can be carried across generations even when offspring are raised on a normal diet. A better understanding of the underlying mechanism should be a high research priority given the prevalence of obesity not just in the current population but also for future generations.

Short term fat feeding rapidly increases plasma insulin but does not result in dyslipidaemia

Although the association between obesity and hypertension is well-known, the underlying mechanism remains elusive. Previously, we have shown that 3 week fat feeding in rabbits produces greater visceral adiposity, hypertension, tachycardia and elevated renal sympathetic nerve activity (RSNA) compared to rabbits on a normal diet. Because hyperinsulinaemia, hyperleptinemia, and dyslipidaemia are independent cardiovascular risk factors associated with hypertension we compared plasma insulin, leptin, and lipid profiles in male New Zealand White rabbits fed a normal fat diet (NFD 4.3% fat, n = 11) or high fat diet (HFD 13.4% fat, n = 13) at days 1, 2, 3 and weeks 1, 2, 3 of the diet. Plasma concentrations of diacylglyceride (DG), triacylglyceride (TG), ceramide and cholesteryl esters (CE) were obtained after analysis by liquid chromatography mass spectrometry. Plasma insulin and glucose increased within the first 3 days of the diet in HFD rabbits (P < 0.05) and remained elevated at week 1 (P < 0.05). Blood pressure and heart rate (HR) followed a similar pattern. By contrast, in both groups, plasma leptin levels remained unchanged during the first few days (P > 0.05), increasing by week 3 in fat fed animals alone (P < 0.05). Concentrations of total DG, TG, CE, and Ceramide at week 3 did not differ between groups (P > 0.05). Our data show plasma insulin increases rapidly following consumption of a HFD and suggests that it may play a role in the rapid rise of blood pressure. Dyslipidaemia does not appear to contribute to the hypertension in this animal model.

Sympathetic activity and markers of cardiovascular risk in nondiabetic severely obese patients: the effect of the initial 10% weight loss

BACKGROUND

Obesity is associated with elevated cardiovascular mortality, which may be attributed, in part, to sympathetic nervous system (SNS) activation and an associated poor metabolic profile. We examined the effects of laparoscopic adjustable gastric band (LAGB) on SNS activity and cardiovascular profile when the initial weight loss of 10%, corresponding to the recommendation of clinical guidelines, was reached.

METHODS

Direct muscle sympathetic nerve activity (MSNA, microneurography), baroreflex function, and cardiovascular profile were examined before and after a predetermined weight loss of 10% in 23 severely obese nondiabetic individuals.

RESULTS

The 10% weight loss was achieved at an average of 7.3 ± 1.4 months (range = 1.3-23.3 months). This was associated with significant improvement in office systolic and diastolic blood pressure (BP) (-12 mm Hg and -5 mm Hg, respectively), a decrease in MSNA (33 ± 3 to 22 ± 3 bursts per minute), improvement in cardiac (16 ± 3 to 31 ± 4 ms/mm Hg) and sympathetic (-2.23 ± 0.39 to -4.30 ± 0.96 bursts/100 heartbeats/mm Hg) baroreflex function, total cholesterol (5.33 ± 0.13 to 4.97 ± 0.16 mmol/L), fasting insulin (29.3 ± 2.4 to 19.6 ± 1.1 mmol/L), and creatinine clearance (172 ± 11 to 142 ± 8 ml/min). None of the cardiovascular risk improvement related to the rate of weight loss. The change in systolic and diastolic BP correlated with change in waist circumference (r = 0.46, P = 0.04; r = 0.50, P = 0.02, respectively).

CONCLUSIONS

The initial 10% weight loss induced by LAGB was associated with substantial hemodynamic, metabolic, SNS, and renal function improvements. Changes in waist circumference appear to be an important factor contributing to BP adaptation after LAGB surgery.

Pressor responsiveness to angiotensin II in female mice is enhanced with age: role of the angiotensin type 2 receptor

Background

The pressor response to angiotensin II (AngII) is attenuated in adult females as compared to males via an angiotensin type 2 receptor (AT₂R)-dependent pathway. We hypothesized that adult female mice are protected against AngII-induced hypertension via an enhanced AT₂R-mediated pathway and that in reproductively senescent females this pathway is no longer operative.

Methods

Mean arterial pressure was measured via telemetry in 4-month-old (adult) and 16-month-old (aged) and aged ovariectomized (aged-OVX) wild-type and AT₂R knockout (AT₂R-KO) female mice during baseline and 14-day infusion of vehicle (saline) or AngII (600 ng/kg/min s.c.). Real-time reverse transcription polymerase chain reaction (RT-PCR) was used to determine renal gene expression of angiotensin receptors and angiotensin-converting enzyme 2 in response to 14-day treatment with vehicle or AngII.

Results

Basal mean arterial pressure was similar between the groups. The pressor response to AngII was augmented in adult AT₂R-KO compared to adult wild-type mice (29 ± 3 mmHg versus 10 ± 4 mmHg, respectively, on day 14 as compared to basal mean arterial pressure, P = 0.002). In wild-type mice, pressor responsiveness to AngII was augmented with age, such that the pressor response to AngII was similar between aged AT₂R-KO and wild-type female mice (31 ± 4 mmHg versus 34 ± 3 mmHg, respectively, on day 14, P = 0.9). There were no significant differences in pressor responsiveness to AngII between aged and aged-OVX mice. Vehicle-treated aged wild-type mice had a lower renal AT₂R/AT₁R balance as compared to adult counterparts. In response to AngII, the renal AT₂R/AT₁R balance in aged wild-type females was greater than that observed in vehicle-treated aged wild-type females and adult wild-type females, yet the protective effects of AT₂R activation were not restored.

Conclusions

The protective role of the AT₂R depressor pathway is lost with age in female mice. Therefore, targeting deficits in AT₂R expression and/or signaling may represent a novel anti-hypertensive approach in aged females.

Origin of the Y chromosome influences intrarenal vascular responsiveness to angiotensin I and angiotensin (1-7) in stroke-prone spontaneously hypertensive rats

The lineage of the Y chromosome accounts for up to 15 to 20 mm Hg in arterial pressure. Genes located on the Y chromosome from the spontaneously hypertensive rat (SHR) are associated with the renin-angiotensin system. Given the important role of the renin-angiotensin system in the renal regulation of fluid homeostasis and arterial pressure, we hypothesized that the origin of the Y chromosome influences arterial pressure via interaction between the intrarenal vasculature and the renin-angiotensin system. Sixteen-week-old normotensive rats (Wistar Kyoto [WKY]), spontaneously hypertensive stroke-prone rat (SHRSP), and 2 reciprocal Y consomic rat strains, 1 comprising the WKY autosomes and X chromosome with the Y chromosome from the hypertensive rat strain (WKY.SPGlaY) and vice versa (SP.WKYGlaY), were examined. SP.WKYGlaY had lower systolic blood pressure than SHRSP (195±5 versus 227±8 mm Hg; P<0.03), whereas WKY.SPGlaY had higher systolic blood pressure compared with WKY (157±3 versus 148±3 mm Hg; P<0.05), measured by radiotelemetry. Compared with WKY rats, SHRSP had higher plasma angiotensin(1-7) (Ang (1-7)):Ang II ratio (WKY: 0.13±0.01 versus SHRSP: 1.33±0.4; P<0.005), greater angiotensin II receptor type 2 and Mas receptor mRNA expression, and a blunted renal constrictor response to intrarenal Ang I and Ang(1-7) infusions. Introgression of the normotensive Y chromosome into the SHRSP background (SP.WKYGlaY) restored responses in the SHRSP to WKY levels, evidenced by a reduction in plasma Ang(1-7):Ang II ratio (SP.WKYGlaY: 0.24±0.02; P<0.01), angiotensin II receptor type 2, and Mas receptor mRNA expression and an increased vasoconstrictor response to intrarenal Ang I and Ang(1-7) infusion. This study demonstrates that the origin of the Y chromosome significantly impacts the renal vascular responsiveness and therefore may influence the long-term renal regulation of blood pressure.

Abstract 038: MicroRNA-181a Mimic Inhibits the Renin-Angiotensin System and Attenuates Hypertension in a Neurogenic Model of Hypertension

Background: Schlager BPH/2J mice are a genetic model of hypertension driven by greater activity of the sympathetic nervous system (SNS) and renin-angiotensin system (RAS). During the dark period of the 24-hour light cycle when hypertension is at its greatest, BPH/2J mice display enhanced renal renin mRNA possibly related to lower levels of microRNA-181a, which is a negative regulator of renin mRNA.

Aim: To determine whether lower renal miR-181a abundance contributes to elevated RAS activity and hypertension in BPH/2J mice.

Methods: BPH/2J mice (n=6) were administered a mirVana miR-181a mimic (1, 5, 10 and 25nmol i.v) using an in vivo kidney specific transfection reagent and compared with established values in untreated normotensive BPN/3J and BPH/2J mice (n=6-8). Blood pressure (BP) was measured before and for two days after mimic treatment via pre-implanted radiotelemetry probes. The BP response to ACE inhibition (enalaprilat) and ganglion blockade (pentolinium) was determined during the dark period ~26-hrs after the 25nmol dose and kidney tissue was collected at ~50 hours.

Results: The 25nmol dose of the miR-181a mimic caused a 4.0±1.4mmHg reduction from baseline in diastolic BP during the dark period ( P <0.01) whilst the 1, 5 and 10nmol doses had no detectable effect ( P >0.27). Renal renin mRNA abundance in mice treated with the miR-181a mimic was 0.87±0.1 which is lower than untreated BPH/2J mice (1.47±0.2, P =0.02) and comparable with untreated normotensive BPN/3J control mice (0.95±0.1, P =0.93), suggesting that the mimic effectively inhibited renin mRNA in vivo . Furthermore the depressor response to enalaprilat in untreated BPH/2J mice was abolished in BPH/2J mice treated with the mimic (-11.2±2mmHg vs 1.5±3mmHg respectively, P <0.001), suggesting the mimic reduced the RAS contribution to BP maintenance. The peak depressor response to pentolinium following enalaprilat pre-treatment, was comparable between untreated and mimic treated BPH/2J mice (-58±3 vs -50±3mmHg, P =0.08), suggesting the mimic does not overtly affect the SNS contribution to BP in BPH/2J mice. Importantly these findings provide the first in vivo evidence that low miR-181a levels contribute to greater activity of the RAS and hypertension in the BPH/2J model of hypertension.

Morning surge in blood pressure is associated with reactivity of the sympathetic nervous system

BACKGROUND

An exaggerated morning surge in blood pressure (BP) closely relates to target organ damage and cardiovascular risk, but whether the causative mechanism involves greater reactivity of the sympathetic nervous system (SNS) is unknown. We determined whether the response of the SNS to a cold pressor test predicted the BP morning surge.

METHODS

Ambulatory BP recordings were obtained from 14 men and 19 women (age = 41±4 years), and the amplitude (day-night difference), rate of rise (RoR), rate by amplitude product (BPPower), and morning BP surge (MBPS; post-awake minus pre-awake) of morning mean arterial pressure (MAP) were determined. The reactivity of the SNS to CPT was assessed by recording of muscle sympathetic nerve activity (MSNA).

RESULTS

CPT induced a marked increase in MAP and all parameters of MSNA, including burst amplitude. Log-normalized BPPower positively correlated with the overall average CPT-induced increases in total MSNA (r = 0.38; P = 0.04) and burst amplitude (r = 0.43; P = 0.02) but was not related to the increase in MSNA frequency. Furthermore, a strong positive linear trend in the CPT-induced changes in burst amplitude across tertiles of BPPower and RoR was observed. BPPower and RoR were not related to CPT-induced hemodynamic changes. The MBPS did not correlate with any of the CPT-induced changes in vascular or MSNA variables.

CONCLUSIONS

These results suggest that the central nervous system mechanisms influencing the increase in MSNA burst amplitude during arousal may also be fundamental in determining the rate and power of BP rise during the morning period.

Predictors of mean arterial pressure morning rate of rise and power function in subjects undergoing ambulatory blood pressure recording

BACKGROUND

We determined clinical predictors of the rate of rise (RoR) in blood pressure in the morning as well as a novel measure of the power of the BP surge (BP(power)) derived from ambulatory blood pressure recordings.

METHODS

BP(power) and RoR were calculated from 409 ambulatory blood pressure (ABP) recordings from subjects attending a cardiovascular risk clinic. Anthropometric data, blood biochemistry, and history were recorded. The 409 subjects were 20-82 years old (average 57, SD = 13), 46% male, 9% with hypertension but not on medication and 34% on antihypertensive medication.

RESULTS

Average RoR was 11.1 mmHg/hour (SD = 8) and BP(power) was 273 mmHg(2)/hour (SD = 235). Only cholesterol, low density lipoprotein and body mass index (BMI) were associated with higher BP(power) and RoR (P<0.05) from 25 variables assessed. BP(power) was lower in those taking beta-blockers or diuretics. Multivariate analysis identified that only BMI was associated with RoR (4.2% increase/unit BMI, P = 0.020) while cholesterol was the only remaining associated variable with BP(power) (17.5% increase/mmol/L cholesterol, P = 0.047). A follow up of 213 subjects with repeated ABP after an average 1.8 years identified that baseline cholesterol was the only predictor for an increasing RoR and BP(power) (P<0.05). 37 patients who commenced statin subsequently had lower BP(power) whereas 90 age and weight matched controls had similar BP(power) on follow-up.

CONCLUSIONS

Cholesterol is an independent predictor of a greater and more rapid rise in morning BP as well as of further increases over several years. Reduction of cholesterol with statin therapy is very effective in reducing the morning blood pressure surge.