Baroreflex control of plasma norepinephrine and heart period in healthy subjects and diabetic patients

A High intensity Interval training (running and swimming) and resistance training intervention on heart rate variability and the selected biochemical factors in boys with type 1 diabetes

OBJECTIVE

The purpose of this research is to investigate the effect of High Intensity Interval Training and Resistance training (HIITR) on heart rate variability (HRV), blood glucose, and plasma biomarkers levels in adolescents with type 1 diabetes (T1D).

RESEARCH DESIGN AND METHODS

24 boys with T1D (FBS: 274.66 ± 52.99, age: 15.2 ± 1.78 years, and BMI = 19.61 ± 1.11) and 12 healthy boys (FBS: 92.75 ± 5.22, age: 15.08 ± 1.67 years, and BMI = 20.26 ± 2.66) were divided into three groups: Diabetes Training (DT, n = 12), Diabetes Control (DC, n = 12), and Healthy Control (HC, n = 12). HRV (24 h) was computed in time and frequency domains, anthropometric, biochemical parameters at rest, and aerobic capacity (VO2peak) obtained during a graded exercise testing (GXT). All variables were evaluated at the baseline and following 12 weeks of exercise training, done 3 days weekly. The statistical method used for data analysis was analysis of covariance (ANCOVA) test.

RESULTS

HRV, Hemoglobin A1c (HbA1c) and Fasting blood sugar (FBS), VO2peak, norepinephrine (NEP), and HDL-C indicated significant differences between both T1D groups compared to HC at baseline (p < 0.001). BMI, LDL-C, TC, and TG parameters were similar in all groups. HRV parameters, VO2peak and HDL-C, and NEP were significantly improved by exercise training, and HbA1c and FBS levels were significantly reduced (p < 0.001). There is a negative and significant correlation between LF/HF Ratio Difference (post-test minus pre-test) and VO2Peak Difference variables (post-test minus pre-test) (p < 0.001).

CONCLUSIONS

The present study suggests the importance of early screening for CVD risk factors in adolescent males with T1D. Also, it was revealed HIITR training compared to other training patterns, and cardiovascular health improves via enhancement of autonomic modulation, VO2peak, plasma lipids, and catecholamine levels.

Roles of catechol neurochemistry in autonomic function testing

Catechols are a class of compounds that contain adjacent hydroxyl groups on a benzene ring. Endogenous catechols in human plasma include the catecholamines norepinephrine, epinephrine (adrenaline), and dopamine; the catecholamine precursor DOPA, 3,4-dihydroxyphenylglycol (DHPG), which is the main neuronal metabolite of norepinephrine; and 3,4-dihydroxyphenylacetic acid (DOPAC), which is the main neuronal metabolite of dopamine. In the diagnostic evaluation of patients with known or suspected dysautonomias, measurement of plasma catechols is rarely diagnostic but often is informative. This review summarizes the roles of clinical catechol neurochemistry in autonomic function testing.

Impact of Restless Legs Syndrome on Cardiovascular Autonomic Control

STUDY OBJECTIVES

To examine whether patients with restless legs syndrome demonstrate specific alterations in cardiovascular autonomic control.

METHODS

Patients with moderate-severe restless legs syndrome (n = 20, 80% female) and controls (n = 20) matched for age, sex, body mass index, and free of hypertension and cardiovascular disease were enrolled. We assessed cardiovagal baroreflex gain via the modified Oxford technique, sympathetically mediated vascular responses to isometric exercise to fatigue, bradycardiac response to Valsalva maneuver, and respiratory sinus arrhythmia during paced breathing. Standard electrocardiography, beat-by-beat arterial pressure, respiration, and popliteal blood flow velocity were recorded continuously.

RESULTS

Resting blood pressure and heart rate were similar between groups. However, baroreflex gain averaged 14.3 ± 1.4 msec/mm Hg in restless legs syndrome and was lower than in controls (22.6 ± 3.5 msec/mm Hg, P = 0.04). Hemodynamic responses to isometric exercise were similar between groups, though participants with restless legs syndrome had lower leg blood flow (P < 0.001), with greater leg vascular resistance (P < 0.0001), before and during isometric exercise. Respiratory sinus arrhythmia and Valsalva ratios were similar between groups. Neither baroreflex gain nor vascular resistance was correlated with sleep duration, sleep quality, or symptom duration.

CONCLUSION

Patients with restless legs syndrome demonstrate compromised cardiovagal control, specific to the arterial baroreflex, with greater peripheral vascular resistance, potentially due to heightened sympathetic outflow. These autonomic alterations may directly relate to the higher prevalence of cardiovascular disease in restless legs syndrome.

Morphological and functional aspects of acute kidney injury after fetal programing in the offspring of diabetic rats

OBJECTIVE

To evaluate the effects of folic acid (FA)-induced renal failure in young offspring of diabetic mothers.

METHODS

The offspring of streptozotocin-induced diabetic dams were divided into four groups: CC (controls receiving vehicle); DC (diabetics receiving vehicle); CA (controls receiving FA solution, 250 mg/kg) and DA (diabetics receiving FA solution, 250 mg/kg). Renal function tests and morphometry results were analyzed.

RESULTS

An increase in creatinine and urea levels was observed in CA and DA groups at two and five months. FA administration caused a significant reduction in the number of glomeruli in the offspring of diabetic dams. The diabetes group treated with FA had fewer glomeruli compared to controls at two and five months. FA caused an increase in the area of the urinary space both in controls and offspring of diabetic dams at two and five months. The number of glomeruli and area of the urinary space at two months were negatively correlated.

CONCLUSIONS

Fetal programing promotes remarkable changes in kidney morphology and function in offspring. We suggest that the morphological changes in the kidneys are more pronounced when fetal programing is associated with newly acquired diseases, e.g. renal failure induced by FA.

Rosiglitazone increases cerebral klotho expression to reverse baroreflex in type 1-like diabetic rats

Reduced baroreflex sensitivity (BRS) is widely observed in diabetic human and animals. Rosiglitazone is one of the clinically used thiazolidinediones (TZD) known as PPARγ agonist. Additionally, the klotho protein produced from choroid plexus in the central nervous system is regulated by PPARγ. In an attempt to develop the new therapeutic strategy, we treated streptozotocin-induced diabetic rats (STZ) with rosiglitazone (STZ + TZD) orally at 10 mg/kg for 7 days. Also, STZ rats were subjected to intracerebroventricular (ICV) infusion of recombinant klotho at a dose of 3 μg/2.5 μL via syringe pump (8 μg/hr) daily for 7 days. The BRS and heart rate variability were then estimated under challenge with a depressor dose of sodium nitroprusside (50 μg/kg) or a pressor dose of phenylephrine (8 μg/kg) through an intravenous injection. Lower expression of klotho in medulla oblongata of diabetic rats was identified. Cerebral infusion of recombinant klotho or oral administration of rosiglitazone reversed BRS in diabetic rats. In conclusion, recovery of the decreased klotho in brain induced by rosiglitazone may restore the impaired BRS in diabetic rats. Thus, rosiglitazone is useful to reverse the reduced BRS through increasing cerebral klotho in diabetic disorders.

Renal nerve ultrastructural alterations in short term and long term experimental diabetes

BACKGROUND

Despite the evidence that renal hemodynamics is impaired in experimental diabetes, associated with glomeruli structural alterations, renal nerves were not yet investigated in experimental models of diabetes and the contribution of nerve alterations to the diabetic nephropathy remains to be investigated. We aimed to determine if ultrastructural morphometric parameters of the renal nerves are affected by short term and/or long term experimental diabetes and if insulin treatment reverses these alterations. Left renal nerves were evaluated 15 days or 12 weeks (N = 10 in each group) after induction of diabetes, with a single injection of streptozotocin (STZ). Control rats (N = 10 in each group) were injected with vehicle (citrate buffer). Treated animals (N = 10 in each group) received a single subcutaneous injection of insulin on a daily basis. Arterial pressure, together with the renal nerves activity, was recorded 15 days (short-term) or 12 weeks (long-term) after STZ injection. After the recordings, the renal nerves were dissected, prepared for light and transmission electron microscopy, and fascicle and fibers morphometry were carried out with computer software.

RESULTS

The major diabetic alteration on the renal nerves was a small myelinated fibers loss since their number was smaller on chronic diabetic animals, the average morphometric parameters of the myelinated fibers were larger on chronic diabetic animals and distribution histograms of fiber diameter was significantly shifted to the right on chronic diabetic animals. These alterations began early, after 15 days of diabetes induction, associated with a severe mitochondrial damage, and were not prevented by conventional insulin treatment.

CONCLUSIONS

The experimental diabetes, induced by a single intravenous injection of STZ, in adult male Wistar rats, caused small fiber loss in the renal nerves, probably due to the early mitochondrial damage. Conventional treatment with insulin was able to correct the weight gain and metabolic changes in diabetic animals, without, however, correcting and / or preventing damage to the thin fibers caused by STZ-induced diabetes. The kidney innervation is impaired in this diabetic model suggesting that alterations of the renal nerves may play a role in the development of the diabetic nephropathy.

Choice of anti-hypertensive agents in diabetic subjects

Hypertension is an extremely common co-morbid condition in diabetes leading to acceleration in micro-vascular and macro-vascular complications. The use of anti-hypertensives in diabetic patients should be considered in the context of preventing the development of complications. Various factors contribute to the pathophysiology of diabetes in hypertension. With the advancements in technology, the understanding of the pathophysiological mechanisms has increased, and this can contribute in providing evidence for beneficial role of certain anti-hypertensives. Many clinical trials have been carried out for use of diuretics, beta blockers, calcium channel blockers, angiotensin-converting enzyme inhibitors and angiotensin receptor blockers. The present review gives an overview of pathophysiological mechanisms of hypertension and diabetes in addition to the details of clinical trials of anti-hypertensives in diabetic patients. This is an attempt to provide some evidences for the clinicians, which may serve as a guide for use of anti-hypertensives in clinical practice.

Sex differences in blood pressure control during 6° head-down tilt bed rest

Spaceflight-induced orthostatic intolerance has been studied for decades. Although ∼22% of the astronaut corps are women, most mechanistic studies use mostly male subjects, despite known sex differences in autonomic control and postflight orthostatic intolerance. We studied adrenergic, baroreflex, and autonomic indexes during continuous infusions of vasoactive drugs in men and women during a 60-day head-down bed rest. Volunteers were tested before bed rest (20 men and 10 women) and around day 30 (20 men and 10 women) and day 60 (16 men and 8 women) of bed rest. Three increasing doses of phenylephrine (PE) and sodium nitroprusside were infused for 10 min after an infusion of normal saline. A 20-min rest period separated the phenylephrine and sodium nitroprusside infusions. Autonomic activity was approximated by spectral indexes of heart rate and blood pressure variability, and baroreflex sensitivity was measured by the spontaneous baroreflex slope. Parasympathetic modulation and baroreflex sensitivity decreased with bed rest, with women experiencing a larger decrease in baroreflex sensitivity by day 30 than men. The sympathetic activation of men and parasympathetic responsiveness of women in blood pressure control during physiological stress were preserved throughout bed rest. During PE infusions, women experienced saturation of the R-R interval at high frequency, whereas men did not, revealing a sex difference in the parabolic relationship between high-frequency R-R interval, a measurement of respiratory sinus arrhythmia, and R-R interval. These sex differences in blood pressure control during simulated microgravity reveal the need to study sex differences in long-duration spaceflight to ensure the health and safety of the entire astronaut corps.

Heart rate variability and plasma norepinephrine concentration in diabetic dogs at rest

Cardiac autonomic neuropathy in dogs with diabetic mellitus (DM) was evaluated using measurement of heart rate variability (HRV) and plasma norepinephrine (NE) concentration. Dogs were divided into 2 groups; the control non-DM group (n = 13) and the diabetic group (n = 22) which was further divided into the well-controlled DM (n = 11) and the poorly-controlled DM subgroups (n = 11) according to their fasting plasma fructosamine concentrations. The electrocardiogram (ECG) was recorded continuously for at least 30 min to yield HRV. The results showed that in the poorly-controlled DM subgroup, the average of normal R-R interval (mean N-N), SD of the mean of all 5-min segments of normal RR intervals (SDANN) were lower than the control group while heart rate was higher (P < 0.05). The NNA, SDNN, SDNN index and pNN50% were significantly lower when compared with the well-controlled DM subgroup (P < 0.05). The high frequency (HF) and total power were significantly lower while the ratio of low to high frequency (LF/HF) was higher (P < 0.05) when compared with the well-controlled DM subgroup. Moreover, in the poorly-controlled DM subgroup, plasma NE concentration was lower than the control group (210 ± 37 vs. 479 ± 74 pg/ml, P < 0.05). There was a significantly negative correlation between plasma NE and plasma fructosamine concentrations. It is concluded that cardiac autonomic neuropathy occurred in poorly-controlled DM dogs. The sympathetic activity was suppressed as shown by decrease in both plasma NE concentration and LF component.

The Relation between Fructose-Induced Metabolic Syndrome and Altered Renal Haemodynamic and Excretory Function in the Rat

This paper explores the possible relationships between dietary fructose and altered neurohumoral regulation of renal haemodynamic and excretory function in this model of metabolic syndrome. Fructose consumption induces hyperinsulinemia, hypertriglyceridaemia, insulin resistance, and hypertension. The pathogenesis of fructose-induced hypertension is dubious and involves numerous pathways acting both singly and together. In addition, hyperinsulinemia and hypertension contribute significantly to progressive renal disease in fructose-fed rats. Moreover, increased activity of the renin-angiotensin and sympathetic nervous systems leading to downregulation of receptors may be responsible for the blunted vascular sensitivity to angiotensin II and catecholamines, respectively. Various approaches have been suggested to prevent the development of fructose-induced hypertension and/or metabolic alteration. In this paper, we address the role played by the renin-angiotensin and sympathetic nervous systems in the haemodynamic alterations that occur due to prolonged consumption of fructose.

Circadian Variations in Blood Pressure, Heart Rate, and HR-BP Cross-Correlation Coefficient during Progression of Diabetes Mellitus in Rat

Circadian changes in cardiovascular function during the progression of diabetes mellitus in the diabetes prone rat (BBDP) (n = 8) were studied. Age-matched diabetes-resistant rats (BBDR) served as controls. BP was recorded via telemetry in contiguous 4 hr time periods over 24 hours starting with 12 midnight to 4 am as period zero (P0). Prior to onset of diabetes BP was high at P0, peaked at P2, and then fell again at P3; BP and heart rate (HR) then increased gradually at P4 and leveled off at P5, thereby exhibiting a bipodal rhythm. These patterns changed during long-term diabetes. The cross-correlation coefficient of BP and HR was not significantly different across groups at onset, but it fell significantly at 9 months of duration of diabetes (BBDP: 0.39 ± 0.06; BBDR: 0.65 ± 0.03; P < .05). These results show that changes in circadian cardiovascular rhythms in diabetes mellitus became significant at the late stage of the disease.

Dynamic assessment of baroreflex control of heart rate during induction of propofol anesthesia using a point process method

In this article, we present a point process method to assess dynamic baroreflex sensitivity (BRS) by estimating the baroreflex gain as focal component of a simplified closed-loop model of the cardiovascular system. Specifically, an inverse Gaussian probability distribution is used to model the heartbeat interval, whereas the instantaneous mean is identified by linear and bilinear bivariate regressions on both the previous R-R intervals (RR) and blood pressure (BP) beat-to-beat measures. The instantaneous baroreflex gain is estimated as the feedback branch of the loop with a point-process filter, while the RR-->BP feedforward transfer function representing heart contractility and vasculature effects is simultaneously estimated by a recursive least-squares filter. These two closed-loop gains provide a direct assessment of baroreflex control of heart rate (HR). In addition, the dynamic coherence, cross bispectrum, and their power ratio can also be estimated. All statistical indices provide a valuable quantitative assessment of the interaction between heartbeat dynamics and hemodynamics. To illustrate the application, we have applied the proposed point process model to experimental recordings from 11 healthy subjects in order to monitor cardiovascular regulation under propofol anesthesia. We present quantitative results during transient periods, as well as statistical analyses on steady-state epochs before and after propofol administration. Our findings validate the ability of the algorithm to provide a reliable and fast-tracking assessment of BRS, and show a clear overall reduction in baroreflex gain from the baseline period to the start of propofol anesthesia, confirming that instantaneous evaluation of arterial baroreflex control of HR may yield important implications in clinical practice, particularly during anesthesia and in postoperative care.

Aerobic training restores arterial baroreflex sensitivity in older adults with type 2 diabetes, hypertension, and hypercholesterolemia

OBJECTIVE

Lowered baroreflex sensitivity (BRS) predicts mortality and occurs with increasing age and diabetes. We examined whether aerobic exercise could restore arterial BRS in adults at high cardiovascular risk (diabetes, geriatric age group, hypercholesterolemia, and hypertension).

DESIGN

Randomized, controlled, single-blind study.

SETTING

VITALiTY (Vancouver Initiative to Add Life to Years) Research Laboratory.

PARTICIPANTS

Thirty-nine older adults (mean age, 71.5 +/- 0.7 years) with diet-controlled or oral hypoglycemic-controlled type 2 diabetes, hypertension, and hypercholesterolemia.

INTERVENTIONS

Subjects were recruited to each of 2 groups: an aerobic group (3 months of vigorous aerobic exercise as defined by 80% to 85% of maximal heart rate), and a nonaerobic (no aerobic exercise) group. Exercise sessions were supervised by a certified exercise trainer 3 times per week.

MAIN OUTCOME MEASURES

: Baroreflex function was assessed using the spontaneous baroreflex method. Main outcome measures included BRS, BRS(up), BRS(down), and [latin capital V with dot above]o(2)max.

RESULTS

The aerobic group demonstrated an increase in BRS that was not demonstrated in the nonaerobic group (+60.9 +/- 23.5 vs +2.2 +/- 7.9%; P = 0.010).

CONCLUSIONS

Our findings indicate that a relatively short aerobic exercise intervention can reverse functional impairments of the arterial baroreflex function in older adults at high cardiovascular risk.

Chronic central leptin infusion restores cardiac sympathetic-vagal balance and baroreflex sensitivity in diabetic rats

This study tested whether leptin restores sympathetic-vagal balance, heart rate (HR) variability, and cardiac baroreflex sensitivity (BRS) in streptozotocin (STZ)-induced diabetes. Sprague-Dawley rats were instrumented with arterial and venous catheters, and a cannula was placed in the lateral ventricle for intracerebroventricular (ICV) leptin infusion. Blood pressure (BP) and HR were monitored by telemetry. BRS and HR variability were estimated by linear regression between HR and BP responses to phenylephrine or sodium nitroprusside and autoregressive spectral analysis. Measurements were made during control period, 7 days after induction of diabetes, and 7 days after ICV leptin infusion. STZ diabetes was associated with hyperglycemia (422 +/- 17 mg/dl) and bradycardia (-79 +/- 4 beats/min). Leptin decreased glucose levels (165 +/- 16 mg/dl) and raised HR to control values (303 +/- 10 to 389 +/- 10 beats/min). Intrinsic HR (IHR) and chronotropic responses to a full-blocking dose of propranolol and atropine were reduced during diabetes (260 +/- 7 vs. 316 +/- 6, -19 +/- 2 vs. -43 +/- 6, and 39 +/- 3 vs. 68 +/- 8 beats/min), and leptin treatment restored these variables to normal (300 +/- 7, -68 +/- 10, and 71 +/- 8 beats/min). Leptin normalized BRS (bradycardia, -2.6 +/- 0.3, -1.7 +/- 0.2, and -3.0 +/- 0.5; and tachycardia, -3.2 +/- 0.4, -1.9 +/- 0.3, and -3.4 +/- 0.3 beats.min(-1).mmHg(-1) for control, diabetes, and leptin) and HR variability (23 +/- 4 to 11 +/- 1.5 ms2). Chronic glucose infusion to maintain hyperglycemia during leptin infusion did not alter the effect of leptin on IHR but abolished the improved BRS. These results show rapid impairment of autonomic nervous system control of HR after the induction of diabetes and that central nervous system actions of leptin can abolish the hyperglycemia as well as the altered IHR and BRS in STZ-induced diabetes.

Blunted excitability of aortic baroreceptor neurons in diabetic rats: involvement of hyperpolarization-activated channel

AIMS

Although dysfunction of arterial baroreflex occurs in human and animal models of type-1 diabetes (T1D), the mechanisms involved in the impairment of the baroreflex still remain unclear. The nodose ganglion (NG) contains the cell bodies of the aortic baroreceptor (AB) neurons. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are expressed in AB neurons and play an important role in regulating the cell excitability. We investigated whether the excitability of AB neurons is depressed in streptozotocin (STZ)-induced T1D rats and whether HCN channels are involved in this depression.

METHODS AND RESULTS

Using the whole-cell patch clamp technique, we found that AB neuron excitability (action potential frequency at 50 pA current stimulation) in the T1D rats was lower than that in the sham rats (0.4 +/- 0.5 vs. 4.8 +/- 0.6 spikes/s, P < 0.05; AB neurons were identified by DiI staining). In addition, HCN current density in AB neurons from the T1D rats was bigger than that from the sham rats (60.2 +/- 6.1 vs. 30.7 +/- 4.9 pA/pF at test pulse -140 from holding potential -40 mV, P < 0.05). Furthermore, HCN channel blockers (5 mM cesium chloride and 100 microM ZD7288) significantly reduced HCN currents and increased action potential frequency of the AB neurons in sham and T1D rats. Immunofluorescent and western blot analyses demonstrated that the expression of HCN1 and HCN2 channel protein in the NG from the T1D rats was higher than that from the sham rats.

CONCLUSION

These results indicate that the HCN channels influence the excitability of AB neurons, and more importantly, contribute to the decreased excitability of AB neurons in T1D rats.

Diabetes-induced changes in 5-hydroxytryptamine modulation of vagally-induced bradycardia in rat heart

1. In the present study, we investigated how alloxan-induced diabetes affects the ability of 5-hydroxytryptamine (5-HT) to modulate bradycardia induced in vivo by electrical stimulation of the vagus nerve in pithed rats. We also analysed the type and/or subtype of 5-HT receptors involved. 2. Diabetes was induced in male Wistar rats with a single injection of alloxan (150 mg/kg, s.c.). Four weeks later, rats were anaesthetized, pretreated with atenolol and pithed. Electrical stimulation (3, 6 and 9 Hz) of the vagus nerve resulted in frequency dependent decreases in heart rate (HR). 3. In diabetic rats, intravenous bolus administration of high doses of 5-HT (100 and 200 microg/kg) increased the bradycardia induced by vagal electrical stimulation. Similarly, low doses (10 microg/kg) of the 5-HT(1/7) receptor agonist 5-carboxamidotryptamine (5-CT), increased vagally induced bradycardia. However, at high doses (50, 100 and 150 microg/kg), 5-CT reduced the bradycardia. Attenuation of the vagally induced bradycardia evoked by the higher doses of 5-CT was reproduced by L-694,247 (50 microg/kg), a selective agonist for the non-rodent 5-HT(1B) and 5-HT(1D) receptors. Enhancement of the vagally induced bradycardia elicited by low doses of 5-CT was reproduced by the selective 5-HT(1A) receptor agonist 8-hydroxydipropylaminotretalin hydrobromide (8-OH-DPAT; 50 microg/kg). These stimulatory and inhibitory actions on vagal stimulation-induced bradycardia in diabetic rats were also observed after administration of exogenous acetylcholine. 4. Vagally induced bradycardia in diabetic rats was not affected by administration of the selective 5-HT(2) receptor agonist alpha-methyl-5-HT (150 microg/kg), the selective 5-HT(3) receptor agonist 1-phenylbiguanide (150 microg/kg) or the selective 5-HT(1B) receptor agonist CGS-12066B (50 microg/kg). 5. Enhancement of the electrical stimulation-induced bradycardia in diabetic rats caused by 5-CT (10 microg/kg) or 8-OH-DPAT (50 microg/kg) was abolished by the selective 5-HT(2/7) receptor antagonist mesulergine (1 mg/kg) and the selective 5-HT(1A) receptor antagonist WAY-100,635 (100 microg/kg), respectively. Similarly, pretreatment with the non-selective 5-HT(1) receptor antagonist methiothepin (0.1 mg/kg) blocked the inhibitory effect of 5-CT (50 microg/kg) on the bradycardia induced by vagal electrical stimulation in diabetic rats. BRL-15572 (2 microg/kg), a selective 5-HT(1D) receptor antagonist, inhibited the action of L-694,247 (50 microg/kg), a selective agonist for the non-rodent 5-HT(1B) and 5-HT(1D) receptors, on the vagally induced bradycardia. 6. In conclusion, in the present study, experimental diabetes evoked changes in both the nature and 5-HT receptor types/subtypes involved in vagally induced bradycardia.

Impaired regulation function in cardiovascular neurons of nucleus tractus solitarii in streptozotocin-induced diabetic rats

This study characterizes neural firing activity of the nucleus tractus solitarii (NTS) and baroreflex sensitivity (BRS) in streptozotocin (STZ)-induced diabetic rats relative to control rats by implantation of multi-wire electrode into rat NTS for direct monitoring of barosensitive NTS neurons before and after baroreflex system challenge by phenylephrine (PE) injection. NTS firing data is correlated with arterial pressure for both control and diabetic rats. In control rats, NTS firing rate and systolic arterial pressure correlate significantly with both pre-PE (baseline) and post-PE (p<0.01). In STZ-induced diabetic rats, positive correlation is observed only after PE injection (p<0.05). Although NTS firing rate was not significantly different between control and diabetic rats (p=0.085) in the baseline condition, it was significantly reduced in STZ-induced diabetic rats (p=0.042) with adjustment for BRS. After PE injection, NTS firing rate is significantly lower in diabetic rats relative to control rats (p<0.01). With adjustment for BRS, multivariate analysis shows that diabetes is independently associated with NTS firing rate after PE injection (p=0.034). Prior physiological and immunofluorescent studies found differing NTS data for control and diabetic rat only after PE challenge, but our data show diabetes-induced barosensitive NTS impairment in the baseline condition for STZ-induced diabetic rats. This latter finding suggests greater sensitivity of multi-wire electrode study of NTS relative to earlier methods.

Effect of propranolol on sympathetically mediated leg vasoconstriction in humans

Sympatho-excitatory manoeuvres are used to study vascular responsiveness in humans, but it is unclear if circulating adrenaline attenuates peripheral vasoconstriction during these manoeuvres. We hypothesized that vasoconstrictor responses to three manoeuvres (neck pressure, unilateral thigh-cuff release and isometric handgrip) would be greater after the administration of the beta-adrenergic blocker propranolol. Seven men and six women underwent these manoeuvres while beat-by-beat arterial pressure (finger photoplethysmography), femoral mean blood velocity (Doppler ultrasound) and femoral artery diameter (edge-detection software) were measured. Femoral vascular conductance was calculated as flow/pressure. Propranolol had no effect on baseline femoral vascular conductance (P > 0.05). As a result of neck pressure, femoral vascular conductance was reduced 23.9 +/- 3.5% before vs. 33.2 +/- 3.2% after infusion of propranolol (P = 0.033). After thigh-cuff release, femoral vascular conductance declined 50.2 +/- 5.8% before vs. 57.4 +/- 9.6% after propranolol infusion (P = 0.496). During handgrip, femoral vascular conductance was reduced 47.2 +/- 9.6% before vs. 55.2 +/- 9.2% after propranolol administration (P = 0.447). After handgrip, women had a greater rise in conductance than men (women: 153 +/- 16.2%; men: 36.4 +/- 10.6%; P < 0.001), which was blunted by 54.8% by propranolol (P < 0.001 vs. control), but unaffected by propranolol in men (P = 0.355 vs. control). The finding that beta-adrenergic receptor-mediated vasodilatation minimally affects vascular responses to these sympatho-excitatory manoeuvres reinforces their utility in the investigation of sympathetic vascular regulation in humans. Interestingly, post-handgrip hyperaemia is greater in women than men and is, in part, beta-adrenergic receptor mediated.

Effect of age on adrenergic and vagal baroreflex sensitivity in normal subjects

The baroreflex maintains a stable blood pressure (BP) by dynamically adjusting heart rate (vagal component) and total peripheral resistance (adrenergic component). Vagal baroreflex sensitivity (BRS-v) is widely used but no methodology existed to quantitate adrenergic baroreflex sensitivity (BRS-a) until we developed the indices of BP recovery time (PRT) and BRS-a. The aims of this study were to generate a normative database and to evaluate whether there is an age effect on the cardiovagal and adrenergic sensitivities. We evaluated recordings of heart rate (HR) and BP in 255 normal subjects during the Valsalva maneuver (VM) and determined both BRS-v and BRS-a sensitivities. PRT increased with age whereas all other parameters declined with age. The adrenergic parameters correlated well with each other but not significantly with BRS-v. The results indicate that both BRS-a and BRS-v become blunted with increasing age and that these indices behave independently of each other.

Hospitalizations for new heart failure among subjects with diabetes mellitus in the RENAAL and LIFE studies

We sought to study the risk factors for heart failure (HF) and the relation between antihypertensive treatment with losartan and the first hospitalization for HF in patients with diabetes mellitus in the Losartan Intervention For Endpoint reduction in hypertension (LIFE) and Reduction of Endpoints in NIDDM with the Angiotensin II Antagonist Losartan (RENAAL) studies. We evaluated 1,195 patients with hypertension, left ventricular hypertrophy, and diabetes from the LIFE study and 1,513 patients with type 2 diabetes and nephropathy from the RENAAL study. The comparative treatments were atenolol in the LIFE study and placebo in the RENAAL study. Patients with a history of HF were excluded from this analysis. Losartan significantly reduced the incidence of first hospitalizations for HF versus placebo in the RENAAL study (hazard ratio 0.74, p=0.037) and versus atenolol in the LIFE study (hazard ratio 0.57, p=0.019). Patients enrolled in the RENAAL study were at a higher risk of developing HF (hazard ratio for RENAAL vs LIFE diabetics 3.0, p<0.0001). The significant, independent baseline risk factors for the development of HF in the RENAAL study were urinary albumin/creatinine ratio, age, peripheral vascular disease, the Cornell product, body mass index, and previous angina; in the LIFE study they were the Cornell product, previous myocardial infarction, peripheral vascular disease, baseline atrial fibrillation, alcohol use (inverse relation), and urinary albumin/creatinine ratio. The beneficial effect of losartan on the reduction of risk for hospitalization for new HF was demonstrated in patients who were at high renal and/or high cardiovascular risk.

Vasomotor responses to hypoxia in type 2 diabetes

Type 2 diabetes is associated with vascular dysfunction, accelerated atherosclerotic morbidity, and mortality. Abnormal vasomotor responses to chemoreflex activation may contribute to the acceleration of atherosclerotic diabetes complications, but these responses have not previously been investigated. We measured forearm mean blood flow (MBF) and mean vascular conductance (MVC) responses to isocapnic hypoxia in seven healthy and eight type 2 diabetic subjects during local intra-arterial saline infusion and alpha-adrenergic blockade (phentolamine). The effects of hypoxia on saline and phentolamine responses significantly differed between groups; relative to normoxia, the %DeltaMVC with hypoxia during saline was -3.3 +/- 11.2% in control and 24.8 +/- 13.3% in diabetic subjects, whereas phentolamine increased hypoxic %DeltaMVC to similar levels (39.4 +/- 9.7% in control subjects and 48.0 +/- 11.8% in diabetic subjects, P < 0.05, two-way ANOVA). Absolute normoxic MBF responses during saline infusion were 91.9 +/- 21.1 and 77.9 +/- 15.3 in control and diabetic subjects, respectively, and phentolamine increased normoxic MBF to similar levels (165.2 +/- 40.1 ml/min in control subjects and 175.9 +/- 32.0 ml/min in diabetic subjects; both P < 0.05). These data indicate that diabetic and control subjects exhibit similar responses to hypoxia in the presence of alpha-adrenergic blockade despite evidence of exaggerated alpha-mediated vasoconstriction at rest.

Hypertension in the metabolic syndrome and diabetes: pathogenesis, clinical studies, and treatment

The increasing prevalence of obesity, hypertension, and abnormal glucose metabolism is a major challenge to health care in industrialized societies. This brief review summarizes the mechanistic evidence and clinical studies related to the problem of hypertension in the metabolic syndrome and diabetes to allow clinicians to provide more effective and enlightened management of these common, interrelated problems.

Effect of blockade of endogenous angiotensin II on baroreflex function in conscious diabetic rats

Little is known about baroreflex control of renal nerve sympathetic activity (RSNA) or the effect of angiotensin II (ANG II) on the baroreflex in diabetes. We examined baroreflex control of RSNA and heart rate (HR) in conscious, chronically instrumented rats 2 wk after citrate vehicle (normal) or 55 mg/kg iv streptozotocin (diabetic) before and after losartan (5 mg/kg iv) or enalapril (2.5 mg/kg iv). Resting HR and RSNA were lower in diabetic versus normal rats. The range of baroreflex control of HR and the gain of baroreflex-mediated bradycardia were impaired in diabetic rats. Maximum gain was unchanged. The baroreflex control of RSNA was reset to lower pressures in the diabetic rats but remained otherwise unchanged. Losartan decreased mean arterial pressure (MAP) and increased HR and RSNA in both groups but had no influence on the baroreflex. Enalapril decreased MAP only in normal rats, yet the increase in HR and RSNA was similar in both groups. Thus in diabetic rats enalapril produced a pressure-independent increase in HR and RSNA. Enalapril exerted no effect on the baroreflex control of HR or RSNA in either group. These data indicate that in conscious rats resting RSNA is lower but baroreflex control of RSNA is preserved after 2 wk of diabetes. At this time, the baroreflex control of HR is already impaired and blockade of endogenous ANG II does not improve this dysfunction.

Neuronal expression of fos protein in the forebrain of diabetic rats

We sought to identify the areas that have altered neuronal activity within the hypothalamus of diabetic rats by mapping neuronal expression of c-fos protein (Fos) and Fos-related antigens. After a standard PAP immunocytochemical protocol, Fos-like immunoreactivity was observed in the paraventricular nucleus (PVN), supraoptic nucleus (SON), median preoptic area (MnPO), anterior hypothalamus (AH) and posterior hypothalamus (PH) of control (vehicle; n=6) and diabetic rats (Sprague-Dawley rats injected with STZ 65 mg/kg/ip 4 weeks prior to the experiment; n=6). Blood glucose levels were significantly elevated in the diabetic group (370+/-8 mg/dl) compared to control group (104+/-3 mg/dl). Diabetic rats had a significantly higher number of Fos-positive cells in PVN (2.5x), SON (7x) and MnPO (2x) compared to the control rats. However, diabetic rats had significantly fewer Fos-positive cells in the AH (0.3x) and no difference was observed in the PH between the diabetic and control rats. Despite the elevated number of Fos-positive cells in the diabetic rats, dehydration (water withdrawal for 24 h) or hypertonic challenge (1.5 ml of 0.1 M NaCl i.p. injection) produced a further increase in the number of Fos-positive cells in the PVN, SON and MnPO. Dehydration did not alter the number of Fos-positive cells in the AH or PH, but hypertonic challenge produced a significant increase in the Fos-positive cells in both the AH and PH of diabetic rats. This study demonstrates that: (1) there is increased basal neuronal activity in the PVN, SON and MnPO, a decrease in neuronal activity in the AH and no change in neuronal activity in the PH as indicated by Fos staining in diabetic rats; and (2) dehydration or hypertonic challenge produces a further increase in the number of Fos-positive cells in the PVN, SON, and MnPO which is comparable to control rats. These data support the conclusion that vasopressin producing neurons in the PVN and SON and autonomic areas within the lamina terminalis and hypothalamus are activated during diabetes and may contribute to the elevated levels of vasopressin and autonomic dysfunction during diabetes.

Cardiovascular control in experimental diabetes

Several studies have reported impairment in cardiovascular function and control in diabetes. The studies cited in this review were carried out from a few days up to 3 months after streptozotocin administration and were concerned with the control of the circulation. We observed that early changes (5 days) in blood pressure control by different peripheral receptors were maintained for several months. Moreover, the impairment of reflex responses observed after baroreceptor and chemoreceptor stimulation was probably related to changes in the efferent limb of the reflex arc (sympathetic and parasympathetic), but changes also in the central nervous system could not be excluded. Changes in renal sympathetic nerve activity during volume expansion were blunted in streptozotocin-treated rats, indicating an adaptive natriuretic and diuretic response in the diabetic state. The improvement of diabetic cardiovascular dysfunction induced by exercise training seems to be related to changes in the autonomic nervous system. Complementary studies about the complex interaction between circulation control systems are clearly needed to adequately address the management of pathophysiological changes associated with diabetes.

Reflex control of arterial pressure and heart rate in short-term streptozotocin diabetic rats

Impaired baroreflex sensitivity in diabetes is well described and has been attributed to autonomic diabetic neuropathy. In the present study conducted on acute (10-20 days) streptozotocin (STZ)-induced diabetic rats we examined: 1) cardiac baroreflex sensitivity, assessed by the slope of the linear regression between phenylephrine- or sodium nitroprusside-induced changes in arterial pressure and reflex changes in heart rate (HR) in conscious rats; 2) aortic baroreceptor function by means of the relationship between systolic arterial pressure and aortic depressor nerve (ADN) activity, in anesthetized rats, and 3) bradycardia produced by electrical stimulation of the vagus nerve or by the iv injection of methacholine in anesthetized animals. Reflex bradycardia (-1.4 +/- 0.1 vs -1.7 +/- 0.1 bpm/mmHg) and tachycardia (-2.1 +/- 0.3 vs -3.0 +/- 0.2 bpm/mmHg) were reduced in the diabetic group. The gain of the ADN activity relationship was similar in control (1.7 +/- 0.1% max/mmHg) and diabetic (1.5 +/- 0.1% max/mmHg) animals. The HR response to vagal nerve stimulation with 16, 32 and 64 Hz was 13, 16 and 14% higher, respectively, than the response of STZ-treated rats. The HR response to increasing doses of methacholine was also higher in the diabetic group compared to control animals. Our results confirm the baroreflex dysfunction detected in previous studies on short-term diabetic rats. Moreover, the normal baroreceptor function and the altered HR responses to vagal stimulation or methacholine injection suggest that the efferent limb of the baroreflex is mainly responsible for baroreflex dysfunction in this model of diabetes.

Characteristic patterns of circadian variation in plasma catecholamine levels, blood pressure and heart rate variability in Type 2 diabetic patients

AIMS

To investigate whether Type 2 diabetic patients exhibit characteristic patterns of circadian variation in plasma levels of catecholamines, blood pressure (BP) and heart rate variability (HRV).

METHODS

Ten Type 2 diabetic and eight control in-patients were studied. Blood for catecholamine measurement was collected every 4 h, and non-invasive ambulatory BP and heart rate were monitored throughout the day. HRV was determined using frequency domain methods.

RESULTS

Diabetic patients showed a different pattern of circadian variation in BP and HRV from that of controls, the diurnal-nocturnal differences (D-N) being significantly smaller. The mean 24-h HRV levels were reduced in diabetic subjects. The mean 24-h plasma noradrenaline level of 1.36 +/- 0.12 nmol/l in diabetic patients was significantly lower than the 2.03 +/- 0.20 nmol/l in controls (P < 0.01). In contrast, no significant difference in adrenaline levels was observed. The mean 24-h plasma noradrenaline level demonstrated a significant positive correlation with D-N in systolic BP (r = 0.49, P = 0.0153).

CONCLUSIONS

The present study demonstrated distinctive patterns of circadian variation in plasma noradrenaline level, BP and HRV in Type 2 diabetic patients, associated with an abnormal circadian pattern of sympathovagal modulation.

Time- and frequency-domain estimation of early diabetic cardiovascular autonomic neuropathy

The risk related to cardiovascular autonomic neuropathy dysautonomia should lead to a specific assessment of this complication of diabetes. The aim of this study was to estimate the accuracy of a battery of blood pressure (BP) and heart rate (HR) variability indexes obtained in different subgroups of diabetic subjects classified according to the conventional laboratory autonomic function tests (Ewing scores). Blood pressure was measured continuously at the finger level with a Finapres monitor while subjects were in the supine position and again while they were standing. Pulse intervals were derived from BP recordings and were taken as surrogates for R-R intervals. Subjects with borderline or definite cardiovascular autonomic neuropathy showed a similar degree of alterations of both HR and BP variability (spectral measures) and in the relationship between BP and HR (cross-spectral and sequence analysis). Subjects with no evidence of cardiovascular autonomic neuropathy on the basis of the conventional tests showed an altered relationship between BP and HR. This baroreceptor-HR reflex dysfunction could represent an early stage of cardiovascular autonomic neuropathy undetected by the conventional tests. The areas under the receiver operating characteristic plots indicated that the high-frequency peak of pulse interval was highly discriminant in the supine and standing positions. The cross-spectral analysis showed the best discrimination for the gain in the high-frequency range. For the sequence analysis, the slope was the best discriminant factor for any degree of cardiovascular autonomic neuropathy. In conclusion, these estimates of baroreceptor-HR function may provide a powerful tool for assessing cardiovascular autonomic neuropathy at any stage, including the early stage, which is not detected by the conventional tests.

Blunted sympathetic response in diabetic patients with decompensated congestive heart failure

BACKGROUND

The risk for congestive heart failure is strongly increased in diabetes, and the prognosis of diabetic patients with established heart failure is worse compared to nondiabetic patients. Heart failure entails complex alterations in autonomic and neurohormonal responses, which exert a direct deleterious effect on the heart and contribute to progressive circulatory failure. Altered neurohumoral physiology may underlie the poor prognosis of diabetic patients with heart failure.

METHODS

We studied 88 patients (mean age 61+/-13 years) admitted for decompensated heart failure. Neurohormonal and cytokine profiles, including plasma renin activity, aldosterone, norepinephrine, endothelin-1, tumor necrosis factor-alpha, and interleukin-6, were obtained in all patients. In addition, a 24-h Holter recording was performed, and time and frequency domain heart rate variability indices were calculated.

RESULTS

Of 88 patients, 48 were classified as having diabetes based on history, diet therapy, or use of oral hypoglycemic agents or insulin. The only difference in the neurohormonal and cytokine profile between the diabetic and nondiabetic groups was a significantly lower norepinephrine level in diabetic patients (668+/-64 vs. 489+/-50 pg/ml, P=0.009). Heart rate variability analysis revealed that the low-frequency power in normalized units (an index of sympathetic modulation) was significantly lower in diabetic patients (4.7+/-1.4 vs. 5.9+/-0.9, P=0.04). No significant differences occurred in any of the time (the percentage of RR intervals with >50 ms variation and the square root of mean squared differences of successive RR intervals) or frequency domain (high frequency power) indices of parasympathetic modulation between the two groups.

CONCLUSIONS

Patients with diabetes mellitus exhibit a blunted sympathetic response during heart failure decompensation. Blunted sympathetic activation in the setting of symptomatic heart failure may impair the ability of the myocardium to compensate and contribute to the high incidence of symptomatic heart failure among diabetic patients.

Heightened norepinephrine-mediated vasoconstriction in type 2 diabetes

Adrenergic responsiveness (AR) appears to be increased in subjects with diabetes, but measurement of arterial AR in normotensive people with type 2 diabetes mellitus has not been previously reported. We sought to determine whether, compared with control subjects, there is increased arterial AR in type 2 diabetes mellitus and its relationship to the level of systemic sympathetic nervous system activity (SNSa). We studied 15 type 2 diabetic subjects aged 57 +/- 3 years without hypertension or clinical signs of autonomic neuropathy and 13 age-matched control subjects aged 55 +/- 2 years. We assessed vascular alpha-AR by measuring forearm blood flow (FABF) by venous occlusion plethysmography during intrabrachial artery norepinephrine (NE) and phentolamine infusions, as well as arterial plasma NE levels and the extravascular NE release rate (NE2) derived from 3H-NE kinetics, as estimates of systemic SNSa. The vasoconstricting effect of NE during intrabrachial artery NE infusion was greater in type 2 diabetes compared with control subjects (P = .02). The vasodilating effect of phentolamine was greater in type 2 diabetics compared with control subjects (P = .05), suggesting increased endogenous arterial alpha-adrenergic tone. Arterial plasma NE levels (control v type 2, 1.8 +/- 0.10 v 1.84 +/- 0.14 nmol/L, P = .86) and NE2 (control vtype 2, 11.8 +/- 1.54 v 13.3 +/- 0.89 nmol/min/m2, P = .39) were similar in the two groups. In summary, in type 2 diabetes compared with control subjects, (1) the vasoconstriction response to intraarterial NE is greater, (2) plasma NE and NE2 are similar, suggesting similar levels of systemic SNSa, and (3) arterial alpha-adrenergic tone is greater. We conclude that subjects with type 2 diabetes demonstrate inappropriately increased alpha-AR for their level of systemic SNSa.

Cardiopulmonary reflex impairment in experimental diabetes in rats

The aim of the present study was to evaluate the sensitivity of the cardiopulmonary receptors in experimental diabetes induced by streptozotocin by the use of 2 different methods: (1) administration of increasing doses of serotonin to analyze peak changes of arterial pressure and heart rate for each given dose in conscious intact normal and diabetic rats; (2) expanding blood volume with the use of dextran (6%) to produce similar increases in left ventricular end-diastolic pressure to quantify the arterial pressure, heart rate, and renal sympathetic nerve activity in sinoaortic, denervated, anesthetized normal and diabetic rats. Blood samples were collected to measure blood glucose. Diabetic rats showed hyperglycemia (22+/-0. 7 versus 7+/-0.2 mmol/L), reduced body weight (226+/-12 versus 260+/-4 g) and heart rate (294+/-14 versus 350+/-10 bpm), and similar arterial pressure (104+/-4 versus 113+/-4 mm Hg) when compared with control rats. Serotonin induced significant bradycardia and hypotension, which were similar and proportional to the dose injected in both groups. Mean arterial pressure and heart rate decreases in response to volume overload were significantly lower in diabetic than in control rats. The reflex reduction of the renal sympathetic nerve activity as expressed by percentage changes in nerve activity in response to increasing left end-diastolic pressure was abolished in diabetic animals (1.9+/-0.8% versus -14+/-4%/mm Hg in controls). These results showed an impairment of cardiopulmonary reflex control of circulation in diabetes during acute volume expansion. The normal responses to serotonin administration indicated that the cardiopulmonary reflex is still preserved in diabetic rats.

Is undetected autonomic dysfunction responsible for sudden death in Type 1 diabetes mellitus? The 'dead in bed' syndrome revisited

AIMS

Sudden nocturnal death in young persons with Type 1 diabetes mellitus has been recently described, and is known as the 'dead in bed' syndrome. Its aetiology is unknown, and we have therefore explored the details of all papers recording the syndrome, to formulate a hypothesis of causation.

METHODS

Literature review of 'dead in bed' reports as well as of nocturnal hypoglycaemia, and autonomic dysfunction in relation to baroreceptor-cardiac reflex sensitivity.

RESULTS

Clinical reports of 'dead in bed' cases strongly suggest that nocturnal hypoglycaemia is a likely precipitant, but that the death is sudden and probably arrhythmic. Ventricular dysrhythmias may occur in the context of early autonomic neuropathy, with relative sympathetic overactivity, in young Type 1 diabetic persons.

CONCLUSION

We conclude that the 'dead in bed' syndrome probably occurs in Type 1 diabetic persons with early autonomic neuropathy, resulting in relative sympathetic overactivity. In such persons, risks of ventricular dysrhythmias will be compounded by nocturnal hypoglycaemia, which may be associated with an increase in the electrocardiographic Q-T interval, and Q-T dispersion. This could lead to the observed sudden death in undisturbed beds. Further research in this area is urgently needed, in particular into the possible protective use of drugs that modulate the autonomic nervous system.

Human sympathetic and vagal baroreflex responses to sequential nitroprusside and phenylephrine

We evaluated a method of baroreflex testing involving sequential intravenous bolus injections of nitroprusside followed by phenylephrine and phenylephrine followed by nitroprusside in 18 healthy men and women, and we drew inferences regarding human sympathetic and vagal baroreflex mechanisms. We recorded the electrocardiogram, photoplethysmographic finger arterial pressure, and peroneal nerve muscle sympathetic activity. We then contrasted least squares linear regression slopes derived from the depressor (nitroprusside) and pressor (phenylephrine) phases with 1) slopes derived from spontaneous fluctuations of systolic arterial pressures and R-R intervals, and 2) baroreflex gain derived from cross-spectral analyses of systolic pressures and R-R intervals. We calculated sympathetic baroreflex gain from integrated muscle sympathetic nerve activity and diastolic pressures. We found that vagal baroreflex slopes are less when arterial pressures are falling than when they are rising and that this hysteresis exists over pressure ranges both below and above baseline levels. Although pharmacological and spontaneous vagal baroreflex responses correlate closely, pharmacological baroreflex slopes tend to be lower than those derived from spontaneous fluctuations. Sympathetic baroreflex slopes are similar when arterial pressure is falling and rising; however, small pressure elevations above baseline silence sympathetic motoneurons. Vagal, but not sympathetic baroreflex gains vary inversely with subjects' ages and their baseline arterial pressures. There is no correlation between sympathetic and vagal baroreflex gains. We recommend repeated sequential nitroprusside followed by phenylephrine doses as a simple, efficientmeans to provoke and characterize human vagal and sympathetic baroreflex responses.

Effects of streptozotocin-induced diabetes on heart rate, blood pressure and cardiac autonomic nervous control

Diabetes-associated alterations in resting heart rate and blood pressure have been demonstrated in clinical studies and in animal models of insulin-dependent diabetes mellitus (IDDM). These alterations may result from changes in the heart, vasculature or autonomic nervous system control. Using the streptozotocin- (STZ-) treated rat model of IDDM, the current study was designed to: (1) monitor changes in heart rate and blood pressure continually during a 10-week period in conscious unrestrained animals; and (2) determine if observed alterations in heart rate were mediated by changes in sympathetic and/or parasympathetic nervous control. Biotelemetry techniques were used. Heart rate and blood pressure were recorded 24 h a day at 10 min intervals before and after induction of diabetes. Diabetes was induced by i.v. administration of 50 mg/kg STZ. Resting autonomic nervous system tone was estimated by chronotropic responses to full-blocking doses of nadolol (5 mg/kg i.p.) and atropine (10 mg/kg i.p.). STZ-induced diabetes was associated with time-dependent reductions in heart rate and its circadian variation. Diastolic blood pressure and mean arterial pressure did not differ significantly when compared between control and STZ-treated animals; however, pulse pressure was diminished in diabetic rats. Chronotropic responses to both nadolol and atropine were blunted significantly in diabetic animals suggesting that resting levels of both vagal and sympathetic nervous tone to the heart were diminished. Heart rate in the presence of both nadolol and atropine was also decreased in diabetic rats. All effects observed following administration of STZ were reversed, at least in part, by insulin treatment. These results suggest that IDDM is associated with time-dependent reductions in resting heart rate and autonomic nervous control of cardiac function.

Vagal and sympathetic mechanisms in patients with orthostatic vasovagal syncope

BACKGROUND

Autonomic and particularly sympathetic mechanisms play a central role in the pathophysiology of vasovagal syncope. We report direct measurements of muscle sympathetic nerve activity in patients with orthostatic vasovagal syncope.

METHODS AND RESULTS

We studied 53 otherwise healthy patients with orthostatic syncope. We measured RR intervals and finger arterial pressures and in 15 patients, peroneal nerve muscle sympathetic activity before and during passive 60 degree head-up tilt, with low-dose intravenous isoproterenol if presyncope did not develop by 15 minutes. We measured baroreflex gain before tilt with regression of RR intervals or sympathetic bursts on systolic or diastolic pressures after sequential injections of nitroprusside and phenylephrine. Orthostatic vasovagal reactions occurred in 21 patients, including 7 microneurography patients. Presyncopal and nonsyncopal patients had similar baseline RR intervals, arterial pressure, and muscle sympathetic nerve activity. Vagal baroreflex responses were significantly impaired at arterial pressures below (but not above) baseline levels in presyncopal patients. Initial responses to tilt were comparable; however, during the final 200 seconds of tilt, presyncopal patients had lower RR intervals and diastolic pressures than nonsyncopal patients and gradual reduction of arterial pressure and sympathetic activity. Frank presyncope began abruptly with precipitous reduction of arterial pressure, disappearance of muscle sympathetic nerve activity, and RR interval lengthening.

CONCLUSIONS

Patients with orthostatic vasovagal reactions have impaired vagal baroreflex responses to arterial pressure changes below resting levels but normal initial responses to upright tilt. Subtle vasovagal physiology begins before overt presyncope. The final trigger of human orthostatic vasovagal reactions appears to be the abrupt disappearance of muscle sympathetic nerve activity.

Sympathetic and baroreceptor reflex function in neurally mediated syncope evoked by tilt

The pathophysiology of neurally mediated syncope is poorly understood. It has been widely assumed that excessive sympathetic activation in a setting of left ventricular hypovolemia stimulates ventricular afferents that trigger hypotension and bradycardia. We tested this hypothesis by determining if excessive sympathetic activation precedes development of neurally mediated syncope, and if this correlates with alterations in baroreflex function. We studied the changes in intraarterial blood pressure (BP), heart rate (HR), central venous pressure (CVP), muscle sympathetic nerve activity (MSNA), and plasma catecholamines evoked by upright tilt in recurrent neurally mediated syncope patients (SYN, 5+/-1 episodes/mo, n = 14), age- and sex-matched controls (CON, n = 23), and in healthy subjects who consistently experienced syncope during tilt (FS+, n = 20). Baroreflex responses were evaluated from changes in HR, BP, and MSNA that were obtained after infusions of phenylephrine and sodium nitroprusside. Compared to CON, patients with SYN had blunted increases in MSNA at low tilt levels, followed by a progressive decrease and ultimately complete disappearance of MSNA with syncope. SYN patients also had attenuation of norepinephrine increases and lower baroreflex slope sensitivity, both during tilt and after pharmacologic testing. FS+ subjects had the largest decrease in CVP with tilt and had significant increases in MSNA and heart rate baroreflex slopes. These data challenge the view that excessive generalized sympathetic activation is the precursor of the hemodynamic abnormality underlying recurrent neurally mediated syncope.

Neural circulatory control in vasovagal syncope

The orthostatic volume displacement associated with the upright position necessitates effective neural cardiovascular modulation. Neural control of cardiac chronotropy and inotropy, and vasomotor tone aims at maintaining venous return, thus opposing gravitational pooling of blood in the lower part of the body. The present concept of the vasovagal response or "common faint" implicates the development of inappropriate cardiac slowing due to sudden augmentation of efferent vagal activity, and arteriolar dilatation by sudden reduction or cessation of sympathetic activity. The venous pooling associated with lasting orthostatic stress results in development of central hypovolemia. At a certain point during the ongoing reflex adaptation to the hypovolemia in progress, a depressor reflex is set in train. The depressor reflex input along this second "peripheral" afferent pathway is postulated to originate from various sites in the cardiovascular system but remains uncertain. The common faint in humans is of both vaso- and vagal origin; the pure vagal response is less common than its vasodepressor variant. There is strong evidence for an early loss of vasomotor tone in the majority of fainting subjects. Blocking the vagus nerve or cardiac pacing is not of much help in preventing vasovagal syncope; though atropine or pacing may prevent bradycardia in vasovagal fainting, they have never been proven to prevent hypotension. Baroreflex modulation of autonomic outflow remains present during the presyncopal stages until it becomes offset by an opposing depressor reflex with relative bradycardia and relaxation of arterial resistance vessels. The nature of the vasodilatation associated with the vasovagal response has still not been settled.

Effect of continuous vagal enhancement on concealed conduction and refractoriness within the atrioventricular node

This study examined the complex interaction between vagal enhancement and how a concealed atrial impulse alters atrioventricular (AV) nodal function. In theory, vagal augmentation could increase or decrease the effect that a premature atrial beat has on the subsequent beat. In 10 patients we established the AV nodal effective refractory period (ERP) without and with a conditioning atrial stimulus (Sc); the stimulation protocol was then repeated after enhancing reflex vagal tone with a continuous phenylephrine infusion. During phenylephrine infusion, the sinus cycle length prolonged from 827 +/- 99 to 1,029 +/- 223 ms (p < 0.001) and AV nodal ERP increased from 331 +/- 51 to 425 +/- 64 ms (p < 0.005). At control, AV nodal ERP in the presence of Sc prolonged to 536 +/- 69 ms (p < 0.001), and during phenylephrine infusion increased to 579 +/- 57 ms (p < 0.01), a change significantly less than during control (58 +/- 14% vs 31 +/- 14%, respectively, p < 0.01). Further experiments suggest that the effect of Sc was reduced because it occurred earlier relative to the vagally prolonged AV nodal ERP. In conclusion, this study demonstrates a complex relation between the timing of a premature atrial beat causing concealed conduction and the degree of vagal tone. The concealed beat, as related to the AV node ERP, has a substantial effect on subsequent AV nodal conduction. These data give insights into clinical AV nodal function.

Autonomic dysfunction in short-term experimental diabetes

Previous data showed that diabetes induced by streptozotocin for 5 days causes changes in arterial pressure control and baroreflex regulation of heart rate in male Wistar rats. The impairment of baroreflex may be related to autonomic neuropathy as described by several investigators. The aim of this study was to identify autonomic changes in short-term experimental diabetes in rats (induced for 5 days with streptozotocin 65 mg IP). Intra-arterial blood pressure signals were obtained from 6 control group and 7 diabetic group rats and processed in a data acquisition system (CODAS, 1 kHz). Both vagal and sympathetic function were assessed through intravenous injections of methylatropine and propranolol. Streptozotocin induced hyperglycemia (18.9 +/- 1.8 versus 5.8 +/- 0.2 mmol/L) and reductions in mean arterial pressure (102 +/- 2 versus 117 +/- 3 mm Hg) and resting heart rate (298 +/- 14 versus 332 +/- 2 beats per minute). Sodium and potassium levels were not different between groups. The intrinsic heart rate was reduced in the diabetic group (302 +/- 10 versus 398 +/- 6 beats per minute). This group also exhibited depressed vagal and sympathetic tone (50% and 22%, respectively), reduction of vagal effect (42%), and no change in sympathetic effect. In conclusion, early autonomic dysfunction in short-term streptozotocin-induced diabetes seems to be related to changes in arterial pressure and baroreflex control.

Baroreflex function in streptozotocin (STZ) induced diabetic rats

To determine whether the renal sympatho-inhibition and bradycardia in responses to acute increases in arterial pressure are altered in the diabetic state, the renal nerve discharge and heart rate were measured in streptozotocin (STZ) induced diabetic (DIA) rats. Integrated renal sympathetic nerve activity and heart rate were measured before and during an acute increase in blood pressure in anesthetized (Inactin 0.1 g/kg, i.p.) control (vehicle) and DIA rats (Sprague Dawley rats injected with STZ 65 mg/kg i.p.). Blood glucose levels were significantly elevated in the DIA group compared with the control group. Baroreflex changes in renal nerve activity and heart rate were not significantly different in the DIA rats compared with control rats at a time when the renal sympatho-inhibition in response to acute volume expansion was blunted in the diabetic rats. In addition, blocking the effect of elevated angiotensin II in diabetic rats with the converting enzyme inhibitor enalapril did not change the baroreflex function in DIA rats compared with control rats. However, administration of vasopressin failed to potentiate the baroreflex in diabetic rats as it did in normal control rats. This study demonstrates that (1) the baroreflex function is normal in STZ induced diabetic rats unlike the volume reflex during the early phase of the disease, (2) blockade of the AII system does not alter baroreflex function in diabetic rats and (3) vasopressin fails to potentiate the baroreflex in diabetic rats as it does in the euglycemic normal rats.