Reports of elevated plasma catecholamine levels and augmented responses to autonomic blockade suggest increased sympathetic tone in borderline hypertension. It is not known if this reflects greater sympathetic neural outflow. We directly recorded muscle sympathetic nerve activity (microneurography) in 15 normotensive and 12 borderline hypertensive age-matched men to determine whether borderline hypertensive individuals have elevated sympathetic nerve activity. Supine heart rate, blood pressure, plasma norepinephrine, and efferent muscle sympathetic nerve activity (peroneal nerve) were measured after 6 days of both low and high dietary sodium intake (10 and 400 meq sodium/24 hr). Sympathetic nerve activity was elevated significantly in borderline hypertensive individuals on both low (37 +/- 1 in borderline hypertensive individuals vs. 29 +/- 1 bursts/min in normotensive individuals; p less than 0.01) and high (25 + 1 in borderline hypertensive individuals vs. 16 +/- 1 bursts/min in normotensive individuals; p less than 0.01) sodium diets. The borderline hypertensive group had higher systolic (p less than 0.01) and diastolic (p less than 0.05) blood pressures independent of sodium intake. Across both groups, high sodium intake reduced muscle sympathetic nerve activity (p less than 0.001), plasma norepinephrine (p less than 0.001), diastolic blood pressure (p less than 0.02), heart rate (p less than 0.002), and increased weight (p less than 0.005). A significant (p less than 0.05) group-by-diet interaction was observed for plasma norepinephrine levels. Specifically, compared with the normotensive group, plasma norepinephrine levels in the borderline hypertensive group tended to be higher on low sodium diet (p = 0.08) and lower on high sodium diet (p = 0.23).(ABSTRACT TRUNCATED AT 250 WORDS)

Elevated sympathetic nerve activity in borderline hypertensive humans. Evidence from direct intraneural recordings.

The renin-angiotensin system plays an important role in the regulation of arterial blood pressure and in the development of some forms of clinical and experimental hypertension. It is an important blood pressure control system in its own right but also interacts extensively with other blood pressure control systems, including the sympathetic nervous system and the baroreceptor reflexes. Angiotensin (ANG) II exerts several actions on the sympathetic nervous system. These include a central action to increase sympathetic outflow, stimulatory effects on sympathetic ganglia and the adrenal medulla, and actions at sympathetic nerve endings that serve to facilitate sympathetic neurotransmission. ANG II also interacts with baroreceptor reflexes. For example, it acts centrally to modulate the baroreflex control of heart rate, and this accounts for its ability to increase blood pressure without causing a reflex bradycardia. The physiological significance of these actions of ANG II is not fully understood. Most evidence indicates that the actions of ANG to enhance sympathetic activity do not contribute significantly to the pressor response to exogenous ANG II. On the other hand, there is considerable evidence that the actions of endogenous ANG II on the sympathetic nervous system enhance the cardiovascular responses elicited by activation of the sympathetic nervous system.

Interactions between ANG II, sympathetic nervous system, and baroreceptor reflexes in regulation of blood pressure

https://www.annalsthoracicsurgery.org/article/0003-4975(95)00718-0/pdf

Atrial fibrillation after coronary artery bypass grafting is associated with sympathetic activation.

Background:

The association between sympathetic activation and mortality in chronic heart failure and the favorable effect of beta blocking drugs has raised the possibility of therapeutic efficacy for central sympathetic inhibition with sustained-release (SR) moxonidine, an imidazoline receptor agonist.



Methods:

A randomized double-blind, placebo-controlled trial was initiated in 425 centers in 17 countries with a plan to enter 4533 patients with New York Heart Association class II–IV heart failure and a reduced ejection fraction. Moxonidine SR or matching placebo was titrated to a target dose of 1.5 mg BID. The trial was powered to detect a 20% reduction in mortality, which required a total of 724 deaths.



Findings:

An early increase in death rate and adverse events in the moxonidine SR group led to premature termination of the trial because of safety concerns after 1934 patients were entered. Final analysis revealed 54 deaths (5.5%) in the moxonidine SR group and 32 deaths (3.4%) in the placebo group during the active treatment phase. Survival curves revealed a significantly (P=0.012) worse outcome in the moxonidine SR group. Hospitalization for heart failure, acute myocardial infarction and adverse events were also more frequent in the moxonidine SR group. Plasma norepinephrine was significantly decreased by moxonidine SR (−18.8% from baseline) vs. placebo (+6.9%).



Interpretation:

Early termination of the trial limited conclusions regarding the long-term effects of central sympathetic inhibition. Nonetheless, the excess early mortality and morbidity suggest the likelihood of an adverse effect of moxonidine SR and raise concerns regarding the efficacy of generalized sympathetic inhibition in heart failure.

Adverse mortality effect of central sympathetic inhibition with sustained-release moxonidine in patients with heart failure (MOXCON).

The sections in this article are:


1
Cutaneous Vascular Responses to Heat Stress
1.1
Microvascular Anatomy

1.2
Efferent Neural Control of Skin Blood Flow

1.3
Local Thermal Control of Cutaneous Blood Vessels




2
Hemodynamic Responses to Heat Stress
2.1
Cardiac Output during Heat Stress

2.2
Cardiovascular Pressures




3
Regional Circulatory Responses to Heat Stress
3.1
Splanchnic Circulation

3.2
Renal Circulation

3.3
Skeletal Muscle Circulation

3.4
Circulation to Brain and Spinal Cord

3.5
Coronary Circulation




4
Modifiers of Control by Internal Temperature
4.1
Thermoregulatory Modifications

4.2
Nonthermoregulatory Modifications




5
Conclusion

Cardiovascular Adjustments to Heat Stress

For circulating norepinephrine (NE) to reflect sympathetic activity validly, plasma NE should show an intensity-dependent increase during sympathetic stimulation and decrease during sympathetic inhibition, and circulating NE should correlate with more directly obtained measures of sympathetic activity. Review of published evidence indicates that NE in peripheral plasma satisfies these criteria. However, models used to explain the relationship between circulating NE and sympathetic activity must take into account processes intervening between the synaptic cleft and free NE in the circulation and, since sympathetic outflow is regionalized, the contributions of specific vascular beds to circulating NE. In this report a model is presented where removal processes for NE are viewed as acting in series to produce a gradient in NE concentrations from synapse to plasma, and where the relative contributions of specific vascular beds are calculated from the arteriovenous difference in plasma NE across those beds and the percentage of cardiac output distributed to them. In general, venous plasma NE provides a useful estimation of average sympathetic outflow.

R J Polinsky

Papers

Relationship between plasma norepinephrine and sympathetic neural activity