Further studies on renal nerve stimulation induced release of noradrenaline and dopamine from the canine kidney in situ

Sympathetic activity in obesity: a brief review of methods and supportive data

The increase in the prevalence of obesity and the concomitant rise in obesity-related illness have led to substantial pressure on health care systems throughout the world. While the combination of reduced exercise, increased sedentary time, poor diet, and genetic predisposition is undoubtedly pivotal in generating obesity and increasing disease risk, a large body of work indicates that the sympathetic nervous system (SNS) contributes to obesity-related disease development and progression. In obesity, sympathetic nervous activity is regionalized, with activity in some outflows being particularly sensitive to the obese state, whereas other outflows, or responses to stimuli, may be blunted, thereby making the assessment of sympathetic nervous activation in the clinical setting difficult. Isotope dilution methods and direct nerve recording techniques have been developed and utilized in clinical research, demonstrating that in obesity there is preferential activation of the muscle vasoconstrictor and renal sympathetic outflows. With weight loss, sympathetic activity is reduced. Importantly, sympathetic nervous activity is associated with end-organ dysfunction and changes in sympathetic activation that accompany weight loss are often reflected in an improvement of end-organ function. Whether targeting the SNS directly improves obesity-related illness remains unknown, but merits further attention.

Increased Efferent Cardiac Sympathetic Nerve Activity and Defective Intrinsic Heart Rate Regulation in Type 2 Diabetes

Elevated sympathetic nerve activity (SNA) coupled with dysregulated β-adrenoceptor (β-AR) signaling is postulated as a major driving force for cardiac dysfunction in patients with type 2 diabetes; however, cardiac SNA has never been assessed directly in diabetes. Our aim was to measure the sympathetic input to and the β-AR responsiveness of the heart in the type 2 diabetic heart. In vivo recording of SNA of the left efferent cardiac sympathetic branch of the stellate ganglion in Zucker diabetic fatty rats revealed an elevated resting cardiac SNA and doubled firing rate compared with nondiabetic rats. Ex vivo, in isolated denervated hearts, the intrinsic heart rate was markedly reduced. Contractile and relaxation responses to β-AR stimulation with dobutamine were compromised in externally paced diabetic hearts, but not in diabetic hearts allowed to regulate their own heart rate. Protein levels of left ventricular β1-AR and Gs (guanine nucleotide binding protein stimulatory) were reduced, whereas left ventricular and right atrial β2-AR and Gi (guanine nucleotide binding protein inhibitory regulatory) levels were increased. The elevated resting cardiac SNA in type 2 diabetes, combined with the reduced cardiac β-AR responsiveness, suggests that the maintenance of normal cardiovascular function requires elevated cardiac sympathetic input to compensate for changes in the intrinsic properties of the diabetic heart.

Renal sympathetic denervation: applications in hypertension and beyond

Renal afferent and efferent sympathetic nerves are involved in the regulation of blood pressure and have a pathophysiological role in hypertension. Renal sympathetic denervation is a novel therapeutic technique for the treatment of patients with resistant hypertension. Clinical trials of renal sympathetic denervation have shown significant reductions in blood pressure in these patients. Renal sympathetic denervation also reduces heart rate, which is a surrogate marker of cardiovascular risk. Conditions that are comorbid with hypertension, such as heart failure and myocardial hypertrophy, obstructive sleep apnoea, atrial fibrillation, renal dysfunction, and metabolic syndrome are closely associated with enhanced sympathetic activity. In experimental models and case-control studies, renal denervation has had beneficial effects on these conditions. Renal denervation could become a commonly used procedure to treat resistant hypertension and chronic diseases associated with enhanced sympathetic activation. Current work is focused on refining the techniques and interventional devices to provide safe and effective renal sympathetic denervation. Controlled studies in patients with mild-to-moderate, nonresistant hypertension and comorbid conditions such as heart failure, diabetes mellitus, sleep apnoea, and arrhythmias are needed to investigate the capability of renal sympathetic denervation to improve cardiovascular outcomes.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

Comparative effects of mibefradil and nifedipine gastrointestinal transport system on autonomic function in patients with mild to moderate essential hypertension

BACKGROUND

L-type dihydropyridine calcium channel blockers (CCBs) have been implicated in increased cardiovascular events in patients with hypertension, perhaps due to adverse effects on autonomic nervous system (ANS) function. Blockade of T-type calcium channels may limit ANS dysfunction by inhibition of T channel-mediated neuroendocrine effects.

OBJECTIVE AND DESIGN

This double-blind, parallel group study compared the effect of nifedipine gastrointestinal transport system (GITS) (L-type CCB) versus mibefradil (T-type CCB) on ANS function in patients with mild-moderate essential hypertension.

METHODS

Sixteen patients (10 male, 6 female; age 57.2 +/- 2.3 years), diastolic blood pressure (DBP) < 95 mmHg were randomized to nifedipine 30 mg daily or mibefradil 50 mg daily (2 weeks), then nifedipine 60 mg daily or mibefradil 100 mg daily (4 weeks). Sympathetic nervous system activity (SNSA) was assessed using norepinephrine kinetics. Parasympathetic nervous system activity (PSNA) was assessed from 24 h Holter recordings of heart rate variability (HRV). Non-invasive baroreflex sensitivity (BRS) provided integrated assessment of ANS.

RESULTS

Patient groups were well matched at baseline. Achieved DBP was lower in patients treated with mibefradil compared with nifedipine, (83.4 +/- 1.7 versus 95.25 +/- 3.3 mmHg). There were no significant differences in SNSA and BRS between groups, however the root mean square of successive differences and high frequency power (HFP) were increased in mibefradil compared with nifedipine-treated patients [(+ 1.07 +/- 1.6 versus -3.36 +/- 1.2 ms, P < 0.05) and (+ 0.28 +/- 0.1 versus -0.23 +/- 0.1 ms2, P < 0.01), respectively]. Furthermore, Ln HFP/Ln total power was increased from week 0 to week 6 in the mibefradil-treated group, (0.71 +/- 0.02 versus 0.74 +/- 0.03, P = 0.046).

CONCLUSION

No differences existed between effect of L- and T-type CCBs on SNSA and BRS. However, T-type CCBs increased PSNA, independent of achieved changes in heart rate.

Role of the sympathetic nervous system in human renovascular hypertension

The findings in humans as to whether elevated sympathetic nerve activity contributes to renovascular hypertension have been less consistent compared with the results obtained in experimental models of renovascular hypertension. Collectively, there are several lines of evidence to support the view that sympathetic nerve activity is elevated in patients with renovascular hypertension. It is uncertain, however, whether this adrenergic overactivity is specific for renovascular hypertension per se, or the cause of severe hypertension with target organ damage. Central or peripheral stimulation of sympathetic nerve activity by angiotensin II, or stimulation of central sympathetic outflow via afferent renal nerves of ischemic kidneys, are possible mechanisms to explain the elevated sympathetic nerve activity in renovascular hypertension. Therapy that diminishes the activity of the sympathetic nervous system and the renin-angiotensin system seems rational and could perhaps also improve the poor prognosis for these patients.

Presynaptic dopamine receptors involved in the inhibition of noradrenaline and dopamine release in the human gastric and uterine arteries

Electrical stimulation-induced depolarization releases both dopamine (DA) and noradrenaline (NA) from sympathetic neurones of the human gastric and uterine arteries. The overflow of catecholamines elicited by electrical stimulation was measured by using high performance liquid chromatography with electrochemical detection. The addition of yohimbine (0.01-10 microM), an alpha2-adrenoceptor antagonist, to the perfusion fluid increased, in a concentration-dependent manner, the electrically-evoked DA and NA overflow from gastric and uterine arteries. In the presence of sulpiride (0.01-10 microM), a dopamine D2-type receptor antagonist, the overflow of both amines was found to be increased in the uterine artery, but not in the gastric artery. Apomorphine (0.1-10 microM), a dopamine receptor agonist, produced a dose-dependent inhibition in the amount of DA and NA released from gastric and uterine arteries. SCH 23390 (0.1-10 microM), a dopamine D1 receptor antagonist, had no effect on the release of both amines in both preparations. The inhibitory effect of apomorphine was blocked by sulpiride in the gastric and uterine arteries but not by SCH 23390. The results presented suggest the existence of dopamine D2-type receptors in the human gastric and uterine arteries. They seem to have, in each artery, a different physiological importance.

How to assess sympathetic activity in humans

Sympathetic factors play a central role not only in cardiovascular homeostatic control but also in the pathogenesis and/or in the progression of several cardiovascular diseases, such as essential hypertension, myocardial infarction, cardiac arrhythmias and congestive heart failure. This explains why assessment of adrenergic neural function in humans has been, and certainly still remains, one of the major fields in cardiovascular research. The present paper will review in detail the haemodynamic, pharmacological, biochemical, neurophysiological, neurochemical and neural imaging techniques by which sympathetic activity is assessed in humans, highlighting the main advantages and limitations of each of them. Although plasma noradrenaline measurement represents a useful guide to assess sympathetic neural function, direct recording of sympathetic nerve traffic via microneurography and noradrenaline radiotracer methods have in recent years largely supplanted the plasma noradrenaline approach. This is because they allow (1) discrimination between the central or peripheral nature of increased plasma noradrenaline levels, and (2) precise estimation of the behaviour of regional sympathetic neural function both under physiological and pathological conditions. In contrast, the approach based on spectral analysis of heart rate and blood pressure signals has been shown to have important limitations which prevent the method from faithfully reflecting sympathetic cardiovascular drive. Neural imaging techniques, which require expensive technical support, allow direct visualization of sympathetic enervation of human organs, thus providing information on the 'in vivo' metabolism of noradrenaline in different cardiovascular districts. Although technical improvements have allowed a more precise assessment of human adrenergic function, no technique so far available can be viewed as a 'gold standard' with which the others might be compared. Limitations and disadvantages of the various techniques may be reduced if these methods are seen as being complementary and employed in combination, allowing more reliable information to be achieved on the sympathetic abnormalities characterizing cardiovascular diseases, and thus hopefully providing a stronger rationale for newer therapeutic approaches involving pharmacological modification of the sympathetic nervous system and adrenoreceptors.

Sympathetic drive to liver and nonhepatic splanchnic tissue during heavy exercise

The contribution of sympathetic drive and vascular catecholamine delivery to the splanchnic bed during heavy exercise was studied in dogs that underwent a laparotomy during which flow probes were implanted onto the portal vein and hepatic artery and catheters were inserted into the carotid artery, portal vein, and hepatic vein. At least 16 days after surgery, dogs completed a 20-min heavy exercise protocol (mean work rate of 5.7 +/- 1 miles/h, 20 +/- 2% grade). Arterial epinephrine (Epi) and norepinephrine (NE) increased by approximately 500 and approximately 900 pg/ml, respectively, after 20 min of heavy exercise. Because Epi is not released from the splanchnic bed and because Epi fractional extraction (FX) = NE FX, NE uptake by splanchnic tissue can be calculated despite simultaneous release of NE. Basal nonhepatic splanchnic (NHS) FX increased from a basal rate of 0.52 +/- 0.09 to a peak of 0.64 +/- 0.05 at 10 min of exercise. Hepatic Epi FX increased from a basal rate of 0.68 +/- 0.10 to 0.81 +/- 0.09 at 20 min of exercise. Even though NHS extraction of Epi reduced portal vein Epi levels by approximately 60%, the release of NE from NHS tissue maintained portal vein NE at levels similar to those in arterial blood. NHS NE spillover increased from a basal rate of 5.7 +/- 1.4 to 11.7 +/- 2.8 ng x kg(-1) x min(-1) at 20 min of exercise. Hepatic NE spillover increased from a basal rate of 5.0 +/- 1.2 ng x kg(-1) x min(-1) to a peak of 14.2 +/- 2.8 ng x kg(-1) x min(-1) at 15 min of exercise. These results show that 1) approximately two- and threefold increases in NHS and hepatic NE spillover occur during heavy exercise, demonstrating that sympathetic drive to these tissues contributes to the increase in circulating NE; 2) the high catecholamine FX by the NHS tissues results in an Epi level at the liver that is considerably lower than that in the arterial blood; and 3) circulating NE delivery to the liver is sustained despite high catecholamine FX due to simultaneous NHS NE release.

Mechanism of resistance to alpha-adrenergic receptor antagonists of renal nerve stimulation-induced vasoconstriction at low frequencies

To determine why renal vasoconstriction elicited by periarterial nerve stimulation (RNS) at lower frequencies (< 4 Hz) is resistant to alpha-adrenergic receptor blockade in the rat kidney, we reevaluated the effect of alpha-receptor antagonists on the vasoconstrictor response to norepinephrine (NE) and to RNS and on the release of adrenergic transmitter. The alpha-receptor antagonist prazosin (PZ) at 0.2 and 7 nM reduced the vasoconstrictor response to NE, and 2.4 microM PZ abolished it. PZ (0.2 or 7 nM) reduced RNS-induced vasoconstriction without altering the fractional tritium overflow. PZ (2.4 microM) enhanced fractional tritium overflow and reduced the vasoconstrictor response to RNS at 2-10 Hz, but not at 0.5 or 1 Hz. The effect of 0.2 nM PZ to reduce RNS-induced vasoconstriction was reversed by increasing the concentration to 2.4 microM. Corynanthine (COR; 2.6 microM), a preferential alpha-receptor blocker, or phenoxybenzamine (PBZ; 30 nM) abolished the vasoconstrictor response to NE but only partially reduced response to RNS and enhanced the fractional tritium overflow. Rauwolscine (RW; 2.5 nM), a preferential alpha 2-receptor antagonist, did not alter the vasoconstrictor response to NE but potentiated RNS-induced vasoconstriction and fractional tritium overflow. RW (7.7 microM) inhibited NE-induced vasoconstriction but potentiated the vasoconstrictor response to RNS and fractional tritium overflow. PZ (7 nM) abolished the potentiation by RW and reduced the vasoconstrictor response to RNS. These data suggest that a component of RNS-induced vasoconstriction in the rat kidney is attributable to co-release of a nonadrenergic transmitter with NE. The diminished effect of alpha-receptor antagonists at higher concentrations (e.g., PZ 2.4 microM) to reduce RNS-induced vasoconstriction is caused by their prejunctional action to enhance co-release of the nonadrenergic transmitter.

Renal noradrenaline spillover correlates with muscle sympathetic activity in humans

1. To study the relationship between indices of resting sympathetic traffic in nerves to skeletal muscles and the kidneys, simultaneous measurements were made of muscle sympathetic activity in the peroneal nerve and renal noradrenaline spillover in ten healthy normotensive males aged 18-69 years (mean 42 years). 2. Group mean levels (+/-S.D.) of muscle sympathetic activity and renal spillover were 22 +/- 17 bursts min-1 and 105 +/- 49 ng min-1, respectively. There were significant positive correlations between individual values of muscle sympathetic activity and renal noradrenaline spillover (r = 0.76, P < 0.01) and similarly between muscle sympathetic activity and renal venous plasma concentration of noradrenaline(r = 0.79, P < 0.007). 3. The results indicate that, although the sympathetic system has the capacity for selective activation of different subdivisions, in healthy human subjects resting traffic is similar or proportional in sympathetic nerves to skeletal muscles and the kidney.

Increased central nervous system monoamine neurotransmitter turnover and its association with sympathetic nervous activity in treated heart failure patients

BACKGROUND

Congestive heart failure is a debilitating disease characterized by impaired cardiac function with accompanying activation of a variety of neural and hormonal counter-regulatory systems. Abnormal activity of the sympathetic nervous system and renin-angiotensin-aldosterone axis and a predisposition to the generation of fatal ventricular arrhythmias are often associated with the development of the disease. Although the underlying cause of sudden death in these patients remains to be unequivocally elucidated, abnormally increased cardiac sympathetic nervous activity may be involved.

METHODS AND RESULTS

Twenty-two patients with severe congestive heart failure (New York Heart Association functional class III or IV with left ventricular ejection fraction of 18 +/- 1%) and 29 healthy male volunteers participated in this study. By combining direct sampling of internal jugular venous blood via a percutaneously placed catheter with a norepinephrine and epinephrine isotope dilution method for examining neuronal transmitter release, we were able to quantify the release of central nervous system monoamine and indoleamine neurotransmitters and investigate their association with the increased efferent sympathetic outflow that is variably present in treated patients with this condition. Mean cardiac norepinephrine spillover was 145% higher in treated heart failure patients than in healthy subjects (P < .05), with norepinephrine release from the heart in 6 of 22 patients being more than the highest control value. Raised internal jugular venous spillover of epinephrine (26 +/- 12 versus 2 +/- 4 pmol/min, P < .05) and of norepinephrine and its metabolites (2740 +/- 480 versus 875 +/- 338 pmol/min, P < .05), indicative of increased central nervous system turnover of both catecholamines, occurred in cardiac failure and was quantitatively linked to the degree of activation of the cardiac sympathetic nervous outflow, as was the jugular overflow of the principal serotonin metabolite, 5-hydroxyindoleacetic acid.

CONCLUSIONS

An association between the degree of activation of central monoaminergic neurons and the level of sympathetic nervous tone in the heart was identified in treated patients with heart failure. Epinephrine neurons in the brain may contribute to the sympathoexcitation that is seen in this condition, with the activation of sympathoexcitatory noradrenergic neurons, most likely those of the forebrain, playing an accessory role.

Whole body and renal noradrenaline release during acute infusion of endothelin-1 in conscious sheep

1. The present study investigated in conscious sheep the response of the sympathetic nervous system to a systemic infusion of 20 nmol/h endothelin-1 (ET-1), using a tritiated-noradrenaline (NA) tracer dilution technique. 2. Mean arterial pressure increased from 79 +/- 3 mmHg to a maximal level of 102 +/- 12 mmHg by 30 min of ET-1 infusion. 3. Total and renal NA kinetics were measured during this time. Total NA spillover was not affected by infusion of ET-1. In contrast, renal NA spillover decreased from a control level of 81 +/- 5 to 30 +/- 14 ng/min (P < 0.01) after 20 min and to 27 +/- 7 ng/min (P < 0.01) after 30 min of ET-1 infusion. 4. The present findings are consistent with the proposal that a direct vasoconstrictor action of ET-1 results in a paroreflex mediated reduction in renal sympathetic vasoconstrictor activity.

Effects of isotonic saline loading on renal tubular and neurogenic dopamine release in conscious rabbits

1. This study was designed to investigate the effects of isotonic saline loading on renal tubular and neurogenic dopamine (DA) in conscious rabbits. 2. Isotonic saline loading did not affect mean arterial pressure, heart rate or renal blood flow but markedly increased urine volume, sodium excretion and DA excretion. 3. Renal DA spillover was not affected by venous emptying, while renal noradrenaline (NA) spillover tended to decrease during saline loading. The ratio of % renal DA spillover to % renal NA spillover increased to 2.3 +/- 0.6 (P < 0.05) 3 h after saline loading. 4. Isotonic saline loading increased renal tubular DA production but had little effect on neurogenic DA release.

The effect of metformin and insulin on sympathetic nerve activity, norepinephrine spillover and blood pressure in obese, insulin resistant, normoglycemic, hypertensive men

To evaluate the effect of metformin on insulin sensitivity and to further examine the relationship between insulin resistance, sympathetic nerve activity and blood pressure, 6 obese insulin resistant, normoglycemic hypertensive men were investigated (age 49 +/- 2 years, BMI 27.6 +/- 1.2, mean +/- SEM). The study had a placebo controlled, double blind, cross over design with 6 weeks' metformin treatment (850 mg b.i.d) vs placebo. Blood pressure was measured weekly. At the end of each treatment period, glucose infusion rate (GIR), muscle sympathetic nerve activity (MSA) and renal and total body norepinephrine (NE) kinetics (radioisotope dilution) were examined during euglycemic hyperinsulinemic clamp. Fasting insulin was 13 +/- 3 and 10 +/- 2 mU/l and fasting glucose 5.3 +/- 0.2 and 5.1 +/- 0.1 mmol/l after placebo and metformin treatment, respectively (ns). GIR during the last hour of the insulin clamp was 3.7 +/- 0.6 vs 3.6 +/- 0.6 mg/kg x min (ns). Resting MSA, total body and right renal NE spillover did not differ significantly after placebo and metformin treatment. Systolic and diastolic blood pressures were 151 +/- 10/95 +/- 5 mmHg after placebo and 146 +/- 5/94 +/- 5 mmHg after metformin treatment (ns). Thus metformin treatment did not have any significant effect on insulin sensitivity, blood pressure or sympathetic activity in this small group of patients. Renal plasma flow and MSA increased significantly during the insulin clamp, whereas renal NE and total body NE spillover remained unchanged, suggesting nonuniform regional sympathetic nerve responses to acute hyperinsulinemia.

Clinical application of noradrenaline spillover methodology: delineation of regional human sympathetic nervous responses

The proportionality which in general exists between rates of sympathetic nerve firing and the overflow of noradrenaline into the venous drainage of an organ provides the experimental justification for the use of measurements of noradrenaline in plasma as a biochemical measure of sympathetic nervous function. Static measurements of noradrenaline plasma concentration have several limitations. One is the confounding influence of noradrenaline plasma clearance on plasma concentration. Other drawbacks include the distortion arising from antecubital venous sampling (this represents but one venous drainage, that of the forearm), and the inability to detect regional differentiation of sympathetic responses. Clinical regional noradrenaline spillover measurements, performed with infusions of radiolabelled noradrenaline and sampling from centrally placed catheters, and derived from regional isotope dilution, overcome these deficiencies. The strength of the methodology is that sympathetic nervous function may be studied in the internal organs not accessible to nerve recording with microneurography. Examples of the regionalization of human sympathetic responses disclosed include the preferential activation of the cardiac sympathetic outflow with mental stress, cigarette smoking, aerobic exercise, cardiac failure, coronary insufficiency, essential hypertension and in ventricular arrhythmias, and the preferential stimulation or inhibition of the renal sympathetic nerves with low salt diets and mental stress, and with exercise training, respectively. By application of the same principles, regional release of the sympathetic cotransmitters neuropeptide Y and adrenaline can be studied in humans. Cotransmitter release, however, is detected only with some difficulty. In restricted circumstances we find evidence of regional cotransmitter release to plasma, such as the release of neuropeptide Y from the heart at the very high rates of sympathetic nerve firing occurring with aerobic exercise, and cardiac adrenaline release also with exercise and after loading of the neuronal adrenaline pool by intravenous infusion of adrenaline.

Variable effects of beta-adrenoceptor blockade on muscle blood flow during exercise

The role of beta-adrenoceptors in exercise-induced muscle hyperaemia was investigated. Exercise was performed with a small and a large muscle mass: knee extension (KE) and bicycle exercise (BE). Seven healthy subjects performed light and maximal KE and eight subjects performed stepwise dynamic BE to exhaustion before and after acute i.v. administration of propranolol (0.15 mg kg-1). Leg blood flow was measured by a bolus dye dilution technique. During KE at low and high power leg blood flow was reduced by 8.7 and 10.5% after propranolol was administered, mean arterial blood pressure (MAP) was reduced at low, but not at high power resulting in increased leg vascular resistance (LVR) during high intensity. During BE propranolol reduced leg blood flow and increased LVR at low power, but not at high power. At high BE intensity LVR did not change with increasing power and was slightly decreased after propranolol was administered. In this situation oxygen uptake was close to maximum and the concentration of catecholamines was 3-5 times higher compared with KE. There was no significant effect of propranolol on lactate release or arterial-femoral venous (a-fv) differences for adrenaline or noradrenaline. We conclude that beta-adrenoceptors modulate local vasodilation in skeletal muscles during exercise independently of local muscle energy demand, but that the effect is highly dependent on active muscle mass since alpha-adrenergic activity during maximal BE seemed to disguise any effect of propranolol on LVR.

The dopamine prodrug, gludopa, decreases both renal and extrarenal noradrenaline spillover in conscious rabbits

1. Renal and total noradrenaline (NA) spillover rates were examined under control conditions and during graded infusions of gludopa (gamma-L-glutamyl-L-dopa) in conscious rabbits. 2. Gludopa infusion at 25 and 100 micrograms/kg per min did not alter mean arterial pressure (MAP) and heart rate (HR), but had significant dose-related effects on the renal dopamine (DA) system. At the high dose there were pronounced increases in urinary DA excretion (> 6000-fold) and renal DA content (> 100-fold); renal NA content doubled. 3. Renal venous DA increased after gludopa infusion, but arterial plasma DA concentrations were not significantly changed. Mean arterial plasma gludopa and L-dopa concentrations reached 890, 3190 ng/mL and 3, 10 ng/mL at low and high doses, respectively. 4. Gludopa resulted in a pronounced dose-dependent fall in renal NA spillover, which at 100 micrograms/kg per min accounted for almost half of the reduction in overall NA spillover rate. 5. The significant falls in renal and extrarenal NA spillover rate during gludopa infusion are consistent with suppression of renal and overall sympathetic activity. Gludopa-induced inhibition of renal NA spillover is likely to be due to the actions of DA generated in the kidney on presynaptic DA-2 and alpha-2 receptors. A central sympathoinhibitory mechanism may explain the reduced total NA spillover.

Plasma catecholamines--analytical challenges and physiological limitations

Catecholamines in plasma may be measured to assess sympathoadrenal activity. Numerous assay methodologies have been published, illustrating the fact that there are many analytical problems. Different methodologies are discussed briefly. A plea for better validation, especially with regard to specificity (which should not be confused with sensitivity or reproducibility), is made. Plasma NA is a frequently used marker for sympathetic nerve activity in humans, but the data obtained are often misinterpreted due to lack of appreciation of the physiological determinants of the NA concentration measured. NA overflow from an organ gives a good reflection of nerve activity in that organ. However, sympathetic nerve activity is highly differentiated, particularly during stress, and conventional plasma NA levels (usually forearm venous samples) cannot be taken as an indication of 'sympathetic tone' in the whole individual. NA is rapidly removed from plasma, resulting in meaningless net veno-arterial concentration differences over organs unless its removal from arterial plasma is taken into account. In the forearm, for example, 40-50% of catecholamines are removed during one passage; about half of the NA in a venous sample is derived from the arm and half from the rest of the body. Therefore, conventional venous sampling overemphasizes local (mainly skeletal muscle) nerve activity. Whole-body sympathetic nerve activity may be monitored in arterial or mixed venous (i.e. pulmonary arterial) samples, which reflect NA overflow from all organs in the body. NA levels are determined both by overflow to plasma and clearance from plasma. NA turnover studies with 3H-NA infusions may be needed to assess clearance, but the simpler concentration measurements usually yield adequate information if the sampling site is relevant. NA overflow from an organ can be assessed (using 3H-NA or ADR as a marker for NA extraction in the organ) and provides valuable information on local sympathetic activity. Mental stress elicits marked circulatory responses, with mainly cardiorenal sympathetic activation and minor elevations of conventional venous plasma NA levels, thus illustrating the differentiated firing pattern of the sympathetic nerves. Circulating ADR is less important than neurogenic mechanisms in the responses to stress. Concentration-effect studies for infused catecholamines may be used for receptor sensitivity studies in vivo, but reflexogenic contributions to responses need to be determined. However, prejunctional mechanisms cannot be assessed without knowledge of the nerve activity present; for example, ADR infusion leads to increased nerve activity. When correctly sampled, measured and interpreted, plasma catecholamines can yield very valuable information on sympathoadrenal activity.

Catecholamines and essential hypertension

Given the ubiquitous distribution of catecholamines in mammals, and their importance in a range of physiological processes pivotal to blood pressure regulation, the subject of catecholamines and essential hypertension has a broader context than simply consideration of sympathetic nervous system and adrenal medullary dysfunction. These further matters are the likely involvement in hypertension pathogenesis of the CNS catecholaminergic neurones influencing peripheral sympathetic outflow, the possible pathogenetic significance of adrenaline released as a cotransmitter in sympathetic nerves, and the natriuretic renal tubular dopamine mechanisms for regulating body sodium balance which appear to be impaired in patients with essential hypertension. The central consideration, however, remains the important issue of the causes and consequences of the now well-documented sympathetic nervous overactivity which characterizes the early developmental phases of essential hypertension.

A study of the neuronal and non-neuronal stores of dopamine in rat and rabbit kidney

The present study has examined the effects of 6-hydroxydopamine (6-OHDA) alone and in combination with pargyline, desipramine and GBR 12909 and denervation as induced by occlusion of the renal artery (RAO) on the endogenous dopamine (DA) and noradrenaline (NA) contents in rat and rabbit renal tissues; the effects of chemical denervation on catecholamine levels in the left ventricle were also studied. In rat and rabbit renal medulla and rat renal cortex, 6-OHDA and pargyline plus 6-OHDA selectively reduced NA (85-92% reduction) without a parallel decrease in DA tissue content (19-27% reduction). This 6-OHDA- and pargyline plus 6-OHDA-insensitive DA pool was found to be resistant to denervation as induced by RAO. The NA-depleting effect of 6-OHDA in these renal areas was found to be prevented by the previous administration of desipramine, but not with that of GBR 12909. In the rabbit renal cortex, 6-OHDA selectively reduced NA (90% reduction) without a parallel depletion of DA (20% reduction); previous treatment with pargyline abolished this selectivity. Again, only desipramine, but not GBR 12909, was found to prevent the NA and DA depleting effect of 6-OHDA in the rabbit renal cortex. Denervation induced by RAO was also found to produce a parallel depletion of DA and NA tissue levels in this renal area. In the left ventricle, 6-OHDA alone or in combination with pargyline produced a parallel depletion of DA and NA tissue levels (79-88% reduction) in both species. These results provide evidence against the presence of independent dopaminergic neurones in rat and rabbit kidney and suggest that in rat and rabbit renal medulla and rat renal cortex most of DA is stored in a non-neuronal compartment; in rabbit renal cortex some of the DA appears to be located in noradrenergic neurones, in a store different from that which contains NA.

Splanchnic and renal vasoconstrictor and metabolic responses to neuropeptide Y in resting and exercising man

The local clearance of neuropeptide Y (NPY) and whether NPY influences splanchnic and renal metabolism in man have not been investigated previously. The influence of NPY on splanchnic and renal blood flows at physiologically elevated levels has also not been investigated. The effects of a 40-min constant NPY infusion (3 pmol kg-1 min-1) at rest and during 130 min of exercise (50% of VO2max) were studied in six healthy subjects and compared with resting and exercising subjects receiving no NPY. Blood samples were drawn from arterial, hepatic and renal vein catheters for the determination of blood flows (indicators: cardiogreen and para-aminohippuric acid [PAH]), NPY, catecholamines, glucose, lactate and glycerol. NPY infusion was accompanied by: (1) significant fractional extraction of NPY-like immunoreactivity (NPY-Li) by splanchnic tissues at rest (58 +/- 5%) and during exercise (53 +/- 6%), while no arterial-venous differences could be detected across the kidney; (2) a reduction in splanchnic and renal blood flows of up to 18 and 13% respectively (P less than 0.01-0.001) at rest without any additional changes during exercise; and (3) metabolic changes as reflected in: (a) a more marked fall in arterial glucose during exercise compared to the reference group (P less than 0.05); (b) a 35% lower splanchnic glucose release (P less than 0.01) during exercise due to diminished glycogenolysis (P less than 0.01); and (c) a lower arterial lactate level (18% P less than 0.05) together with unchanged splanchnic lactate uptake during exercise, suggesting reduced lactate production by extrahepatic tissues. The disappearance of plasma NPY-Li after the infusions was biphasic with two similar half-lives at rest (4 and 39 min) and during exercise (3 and 43 min).

Exercise training lowers resting renal but not cardiac sympathetic activity in humans

Endurance exercise training has previously been shown to reduce the plasma concentration of norepinephrine. Whether reduction in sympathetic activity is responsible for the blood pressure-lowering effects of exercise training is unknown. Using a radiotracer technique, we measured resting total, cardiac, and renal norepinephrine spillover to plasma in eight habitually sedentary healthy normotensive men (aged 36 +/- 3 years, mean +/- SEM) after 1 month of regular exercise and 1 month of sedentary activity, performed in a randomized order. One month of bicycle exercise 3 times/wk (40 minutes at 60-70% maximum work capacity) reduced resting blood pressure by 8/5 mm Hg (p less than 0.01) and increased maximum oxygen consumption by 15% (p less than 0.05). The fall in blood pressure was attributable to a 12.1% increase in total peripheral conductance. Total norepinephrine spillover to plasma was reduced by 24% from a mean of 438.8 ng/min (p less than 0.05). Renal norepinephrine spillover fell by an average of 41% from 169.4 ng/min with bicycle training (p less than 0.05), accounting for the majority (66%) of the fall in total norepinephrine spillover. Renal vascular conductance was increased by 10% (p less than 0.05), but this constituted only 18% of the increase in total peripheral conductance. There was no change in cardiac norepinephrine spillover. The reduction in resting sympathetic activity with regular endurance exercise is largely confined to the kidney. The magnitude of the fall in renal vascular resistance, however, is insufficient to directly account for the blood pressure-lowering effect of exercise, although other effects of inhibition of the renal sympathetic outflow may be important.

Evidence of a selective increase in cardiac sympathetic activity in patients with sustained ventricular arrhythmias

BACKGROUND

Although enhanced efferent cardiac sympathetic nervous activity has been proposed as an important factor in the genesis of ventricular arrhythmias and sudden cardiac death, direct clinical evidence has been lacking.

METHODS

We measured the rates of total and cardiac norepinephrine spillover into the plasma, which reflect respectively overall and cardiac sympathetic nervous activity, in 12 patients who had recovered from a spontaneous, sustained episode of ventricular tachycardia or ventricular fibrillation outside the hospital 4 to 48 days earlier. The results were compared with those from three age-matched reference groups without a history of ventricular arrhythmias: 12 patients with coronary artery disease, 6 patients with chest pain but normal coronary arteries, and 12 healthy, normal subjects.

RESULTS

The patients who had had ventricular arrhythmias had reduced left ventricular ejection fractions, as compared with the patients with coronary artery disease or chest pain (mean [+/- SE], 46 +/- 3 percent vs. 58 +/- 4 percent and 69 +/- 5 percent, respectively; P less than 0.003). The rates of total norepinephrine spillover into the plasma were similar in the three reference groups, but 80 percent higher in the patients with ventricular arrhythmias (P less than 0.005). The rate of cardiac norepinephrine spillover was 450 percent higher in these patients (176 +/- 39 pmol per minute, as compared with 32 +/- 8 pmol per minute in the normal subjects; P less than 0.001), a disproportionate increase relative to the increase in total spillover, which indicated selective activation of the cardiac sympathetic outflow. This increase in cardiac norepinephrine spillover was probably caused by a reduction in left ventricular function.

CONCLUSIONS

These results suggest that in some patients major ventricular arrhythmias are associated with and perhaps caused by sustained and selective cardiac sympathetic activation. We speculate that depressed ventricular function was present before the ventricular arrhythmia occurred, and that this resulted in reflex cardiac sympathetic activation, which in turn contributed to the genesis of the arrhythmia.

Splanchnic release of neuropeptide Y during prolonged exercise with and without beta-adrenoceptor blockade in healthy man

Plasma levels of neuropeptide Y- (NPY-) like immunoreactivity (Li) and catecholamines in the brachial artery, femoral vein and hepatic vein were monitored during physical exercise in a total of 19 healthy men to detect any local release from the leg and splanchnic region. In addition, propranolol (0.15 mg kg-1 i.v.) was given during exercise to determine whether beta-adrenoceptor blockade influenced the increase in plasma NPY-Li and catecholamines. Leg and splanchnic blood flows were measured using indicator dilution techniques and indocyanine green dye. Graded arm exercise was associated with elevations of arterial plasma NPY-Li (two-fold) and noradrenaline (12-fold) comparable to those previously found during leg exercise. During prolonged leg exercise a significant vasoconstriction and release of NPY-Li and noradrenaline was observed in the splanchnic region while no net exchange was found in the exercising leg where marked vasodilatation occurred. Administration of propranolol during exercise produced a clear-cut additional increase in plasma NPY-Li as well as in noradrenaline and adrenaline. It is concluded that splanchnic vasoconstriction during exercise is associated with a local release of both NPY-Li and noradrenaline. The additional elevation in plasma NPY-Li and catecholamines after propranolol during exercise is probably due to increased nerve activity and/or decreased disposal.

A functional role for renal dopaminergic nerves in the dog

1. Efferent renal nerve stimulation at 5 Hz causes secretion of both dopamine (DA) and noradrenaline (NA) into renal venous plasma. DA comprises about 8% of the total catecholamine overflow; by contrast, DA efflux into femoral venous plasma following stimulation of the lumbar sympathetic nerves is 1% or less of total catecholamine. 2. Intact, but not denervated, kidneys of volume-loaded dogs also secrete both dopamine (DA) and noradrenaline (NA) into renal venous blood at rest, but the DA:NA ratio is considerably higher than that evoked by nerve stimulation. 3. Acute animal treatment with 6-hydroxydopamine (6-OHDA) abolishes stimulus-evoked catecholamine overflow and the usual fall in glomerular filtration and sodium and water excretion that accompanies renal nerve activation. 4. When 6-OHDA is administered in the presence of a selective inhibitor of UptakeDA (GBR 12909), stimulus-evoked DA overflow is selectively protected against the effect of 6-OHDA. Under these circumstances, nerve stimulation increases glomerular filtration and excretion of water, but not of sodium. These effects are abolished by DA1 receptor blockade. 5. These data indicate that DA is released from intrarenal dopaminergic nerve terminals in vivo, both in response to direct nerve stimulation and tonically under conditions of volume expansion. The main effects of dopaminergic nerve activation are juxtaglomerular vasodilatation and inhibition of distal tubular water reabsorption.

Effects on renal sympathetic axons in dog of acute 6-hydroxydopamine treatment in combination with selective neuronal uptake inhibitors

1. In anaesthetized dogs, we have investigated the effect on renal responses to sympathetic nerve stimulation of acute treatment with the catecholaminergic neurotoxin 6-hydroxydopamine (2 mg kg-1 i.v.), administered alone or after blockade of neuronal catecholamine uptake pathways for noradrenaline (NA) or dopamine with desmethylimipramine or benztropine, respectively. 2. Under control conditions, renal nerve stimulation caused renal vasoconstriction, reduced glomerular filtration and sodium and water excretion and caused net efflux of NA and dopamine into the renal venous plasma. Two h after administration of 6-hydroxydopamine alone, there was abolition of both functional responses and catecholamine efflux during nerve stimulation. 3. In animals pretreated with desmethylimipramine (1 mg kg-1), 6-hydroxydopamine had no significant effect on functional responses to renal nerve stimulation and nerve-evoked efflux of NA was only moderately reduced. Efflux of dopamine was still markedly reduced by 6-hydroxydopamine, but more variably than occurred without desmethylimipramine treatment. 4. In animals pretreated with benztropine (0.2 mg kg-1), nerve-evoked efflux of dopamine, but not that of NA, was protected against reduction by 6-hydroxydopamine. A higher dose of benztropine (1 mg kg-1) protected efflux of both NA and dopamine against 6-hydroxydopamine. 5. We conclude that acute treatment with a low dose of 6-hydroxydopamine is an effective method of inactivating peripheral sympathetic nerves. The differential effects of desmethylimipramine and benztropine in preserving nerve-evoked efflux of NA and dopamine after 6-hydroxydopamine support the view that these catecholamines originate predominantly from different intrarenal axons. However, neither uptake blocker appears to be completely specific in its actions.

Plasma noradrenaline and dopamine in renin-mediated hypertension

Noradrenaline (NA) and dopamine (DA) have opposite effects on the kidney; NA causes vasoconstriction and increased sodium reabsorption while DA promotes vasodilation and natriuresis. In 15 patients investigated for renin-mediated hypertension measurements of plasma renin activity (PRA), NA and DA concentrations were made in arterial and renal venous blood from both kidneys before and after acute stimulation of renin release by i.v. dihydralazine. Nine patients had unilateral renin secretion and were classified as renin-positive, while the remaining six patients were renin-negative. Renin-positive patients had higher arterial and renal venous PRA, NA and DA levels than the negative ones. In the renin-positive group V-A differences for NA and DA were present on both sides despite unilateral secretion of renin. NA but not DA levels were higher in the renin-secreting kidney, which can partly be explained by the reduced plasma flow to the involved kidney. After dihydralazine the arterial NA and DA rose similarly in renin-positive and renin-negative patients, while PRA rose only in the renin-positive cases. In the renin-positive patients where stimulation of renin secretion caused a marked increase of the PRA gradient on the affected side only, renal gradients for NA and DA increased bilaterally. The increase in DA was more pronounced than that of NA yielding a rise in DA/NA ratio on the affected side. Arterial PRA was positively correlated to the plasma concentrations of NA and DA. V-A differences for PRA and NA or DA were positively correlated on the involved renin-secreting side. In summary, patients with renin-dependent hypertension have elevated plasma NA and DA concentrations. Stimulation of renin release by dihydralazine increases the DA/NA ratio in arterial and renal venous blood indicating release of 'precursor dopamine' from noradrenergic fibres and/or activation of dopaminergic nerves. There seems to be a relationship between renal nerve activity and renin release in renin-dependent hypertension.

Release and vasoconstrictor effects of neuropeptide Y in relation to non-adrenergic sympathetic control of renal blood flow in the pig

The possible involvement of neuropeptide Y (NPY) in sympathetic control of renal blood flow was investigated in the pig in vivo. Exogenous NPY caused renal vasoconstriction with a threshold effect at an arterial plasma concentration of 164 pmol 6(-1). Stimulation of the renal nerves (0.59, 2 and 10 Hz) in control animals evoked rapid and frequency-dependent reduction in renal blood flow and overflow of NPY-like immunoreactivity (NPY-LI) and noradrenaline (NA) from the kidney, suggesting co-release from sympathetic nerves. Following the administration of the alpha- and beta-adrenoceptor antagonists phenoxybenzamine and propranolol, the vasoconstrictor response to exogenous NA was reduced by 98%, whereas that of NPY was unaltered. The response to nerve stimulation with 0.59 Hz was abolished, whereas relatively slowly developing reductions in renal blood flow by 7 and 28% were obtained upon stimulation with 2 and 10 Hz respectively. The nerve stimulation-evoked overflow of NA at 0.59 and 2 Hz, but not at 10 Hz and not that of NPY-LI, was enhanced after adrenoceptor blockade. Twenty-four hours after reserpine treatment (1 mg kg-1 i.v.) the contents of NPY-LI and NA in the renal cortex were reduced by 80 and 98% respectively. Sectioning of the renal nerves largely prevented the reserpine-induced depletion of NPY-LI, but not that of NA. Nerve stimulation of the denervated kidney with 2 and 10 Hz 24 h after reserpine treatment evoked slowly developing and long-lasting reductions in renal blood flow by 6 and 52% respectively. These responses were associated with overflow of NPY-LI, which was similar to and threefold higher than that observed in controls at 2 and 10 Hz respectively, while no detectable overflow of NA occurred. Repeated stimulation with 10 Hz resulted in a progressive fatigue of the vasoconstrictor response and the associated overflow of NPY-LI, giving a high correlation (r = 0.86, P less than 0.001) between the two parameters. It is concluded that NPY is a potent constrictor of the renal vascular bed. Furthermore, although NA is the likely transmitter mediating most of the responses to low to moderate nerve activation under control conditions, the data suggest that NPY may mediate the non-adrenergic reductions in renal blood flow evoked by high-frequency sympathetic nerve stimulation after reserpine treatment.

Comparison of resting and stimulus-evoked catecholamine release from the femoral and renal vascular beds of the dog

Plasma levels of dopamine (DA) and noradrenaline (NA) were measured in arterial and in femoral and renal venous blood of chloralose-anaesthetised dogs at rest, and during electrical stimulation of the femoral and renal sympathetic nerve supplies. In the femoral bed, sympathetic nerve stimulation elevated venous efflux of NA, but did not reproducibly elevate DA efflux: when this was increased, it comprised less than 1% of the stimulus-evoked catecholamine efflux. By contrast, renal nerve stimulation liberated both NA and DA from the kidney, and DA comprised about 8% of the total stimulus-evoked efflux. Comparison of efflux from intact and denervated kidneys indicated substantial neurogenic release of both NA and DA at rest, with DA comprising 20% of this efflux. The results extend previous evidence for dopaminergic sympathetic innervation of the dog kidney, and suggest that both dopaminergic and noradrenergic renal nerves are tonically active in anaesthetised animals.

Reflex activation of renal nerves in humans: differential effects on noradrenaline, dopamine and renin overflow to renal venous plasma

Using a thermodilution technique for renal venous blood flow measurements, renal sympathetic nerve activity was evaluated in 10 healthy volunteers by measurements of noradrenaline (NA) and dopamine (DA) overflow to renal venous plasma. Renin release was measured simultaneously. At rest, arterial adrenaline (ADR) levels were 0.24 +/- 0.03 nmol-1 and NA and DA levels were higher in renal venous than in arterial plasma (1.24 vs. 0.98 and 0.14 vs. 0.09 nmol l-1, respectively, P less than 0.01 for both). The renal extraction of ADR from arterial plasma was 40 +/- 4%. ADR extractions were used to correct for the renal removal of NA or DA from arterial plasma when calculating the renal overflow of NA or DA to renal venous plasma. At rest, the thus corrected renal venous overflows of NA and DA were 228 +/- 34 and 29 +/- 3 pmol min-1, respectively. Isometric handgrip exercise (IHG) increased renal vascular resistance (RVR) by 20% and NA overflow by 123%, without altering renin release or DA overflow. Vasodilatation induced by dihydralazine (HYDR) increased NA overflow by 63% (P less than 0.05) and elevated DA overflow by 107 +/- 59%. The renal DA/NA overflow ratio was reduced from 0.15 to 0.06 (P less than 0.01) during IHG, but was not altered by HYDR. Renin release increased by 377% after HYDR (P less than 0.001) and was correlated to the reduction of mean arterial pressure but not changes in NA overflow. Thus, both IHG and HYDR increased renal sympathetic nerve activity, although differential effects on renin release and DA overflow were observed. The dissociation of renal NA and DA responses suggests that the human kidney may have a subset of dopaminergic nerves.

Evidence for dopaminergic co-transmission in dog mesenteric arterial vessels

1. The overflow of dopamine and noradrenaline (NA) from the main trunk of the dog mesenteric artery and its proximal branches during prolonged depolarization (120 min) by K+ (52 mM) was quantified by high performance liquid chromatography with electrochemical detection. 2. K+-induced depolarization resulted in release of both dopamine and NA. The amount of NA released from both blood vessels declined progressively throughout the experiment. In the main trunk the same pattern of release was observed for dopamine, whereas in the proximal branches the overflow of dopamine increased throughout the experiment. 3. The addition of phentolamine (0.2 microM) to the perifusion fluid increased the overflow of both amines. In the presence of sulpiride (1 microM) the overflow of dopamine and NA was found to be increased in the proximal branches, but not in the main trunk. The addition of phentolamine to sulpiride caused a further increase in amine overflow in proximal branches, but not in the main trunk. 4. The addition of alpha-methyl-p-tyrosine (50 microM) to the perifusion fluid caused a decrease in the amounts of dopamine and NA released from both preparations. In alpha-methyl-p-tyrosine-treated preparations phentolamine increased amine overflow to the same extent as in experiments without tyrosine hydroxylase inhibition. The increasing effect of sulpiride on the overflow of dopamine and NA from the proximal branches was completely abolished after alpha-methyl-p-tyrosine. 5. The results presented suggest that in the proximal branches of the dog mesenteric artery, dopamine beta-hydroxylase represents a rate limiting step in the synthesis of NA; dopamine, through activation of prejunctional dopamine receptors acts like a prejunctional co-transmitter in the control of transmitter release, but only newly-synthesized dopamine appears to be responsible for this effect.

A comparison of sympathoadrenal activity and cardiac performance at rest and during exercise in patients with ventricular demand or atrial synchronous pacing

Cardiac sympathetic function was assessed by measuring the coronary sinus overflow of noradrenaline and dopamine at rest and during supine exercise in eight patients with high degree atrioventricular block treated with dual chamber pacemakers (DDD). Patients exercised (30-60 W) during both ventricular inhibited (VVI) and atrial synchronous (VAT) pacing. During exercise cardiac output increased less markedly in the VVI mode than in the VAT mode. The cardiac output response was entirely stroke volume dependent in the VVI mode and mainly heart rate dependent in the VAT mode. Coronary sinus noradrenaline concentrations were higher in the VVI mode at rest and during exercise. Noradrenaline overflow from the heart was enhanced during VVI pacing and increased from about 100 pmol/min (17 ng/min) at rest to 1087 pmol/min during exercise (60 W) in the VVI mode and 545 pmol/min in the VAT mode. Dopamine overflow from the heart was less than 5 pmol/at rest but increased 2-5 fold during exercise. Also arterial concentrations of catecholamine increased more during exercise in the VVI mode, but the differences between pacing modes were less pronounced. Circulating adrenaline seems to be of little importance for cardiac function under these conditions; in healthy individuals the arterial concentrations of adrenaline attained in this study have small effects. Cardiac noradrenaline overflow correlated with pulmonary capillary venous pressures and atrial rates in both pacing modes, indicating a relation between cardiac sympathetic activity and cardiac function. Enhanced cardiac release of noradrenaline may increase cardiac contractility and thereby partially compensate for the lack of heart rate responsiveness to exercise during VVI pacing.

Further evidence for a noradrenaline-independent storage of dopamine in the dog kidney

1. The endogenous dopamine (DA) and noradrenaline (NA) tissue levels in three different areas of the dog kidney, and their modification by 6-hydroxydopamine (6-OHDA), pargyline plus 6-OHDA, pargyline or reserpine were studied by means of high pressure liquid chromatography with electrochemical detection. 2. 6-OHDA alone or in combination with pargyline induced parallel decreases of DA and NA contents in the inner cortex. In the outer cortex and the medulla, 6-OHDA selectively reduced NA without a parallel decrease in DA tissue content. Previous treatment with pargyline abolished this selectivity in the outer cortex but not in the medulla. Five days after administration of pargyline alone, DA and NA tissue content was not different from that observed in controls. 3. Reserpine caused a marked decrease in the DA and NA tissue content in all the three renal areas studied, though in the medulla a reserpine-resistant DA-component was found. 4. On the basis that small dense cored vesicles (SDCV) in noradrenergic neurones are more susceptible to 6-OHDA than large dense cored vesicles (LDCV), and that this difference is abolished by previous administration of pargyline, our findings suggest that the 6-OHDA-insensitive store of DA in the outer cortex of the dog kidney is located in a neuronal compartment different from that which contains NA, but most probably in noradrenergic neurones.

Influence of renal denervation on vascular responsiveness of isolated rat intrarenal arteries

Microsurgical renal denervation of the rat has been reported to increase blood loss and bleeding time after a standardized kidney resection. To investigate the vascular effects of denervation, isolated intrarenal arteries were studied using sensitive 'isometric' recording equipment. Four pieces of evidence were obtained to indicate an effective functional denervation I week after renal nerve transection: (i) Phentolamine reduced the K+-induced contraction in controls but not in denervated arteries. (ii) The K+-induced contraction was significantly smaller in denervated than in control arteries. (iii) Noradrenaline (NA) was a significantly more potent vasoconstrictor (4 x) in denervated than in control arteries. (iv) Cocaine increased the NA sensitivity in control arteries (3 x), whereas it failed to do so in denervated vessels. Vasopressin, 5-hydroxytryptamine (5-HT), NA (in the presence of cocaine), prostaglandin F2 alpha (PGF2 alpha) and dopamine (DA) produced concentration-dependent contractions in the mentioned order of potency. Denervated arteries were found to be about two to three times more sensitive to the vasoconstrictors than control arteries. Angiotensin I and II had no contractile effect in any of the vessel segments examined. Indomethacin-pretreated arteries also failed to respond to angiotensin II. Neuropeptide Y produced only weak contractions and failed to influence the NA concentration-response relationship in either control or denervated arteries. In conclusion, renal denervation caused a general supersensitivity of the vascular smooth muscle cells to both circulating and non-circulating vasoconstrictors. Our results cannot explain the increased blood loss and bleeding time seen after denervation, but rather support the view that the enhanced bleeding was caused by an interrupted vasoconstrictor influence of the sympathetic nerves.

Physiological aspects on catecholamine sampling

Catecholamine (CA) determinations are valuable tools in studies of sympatho-adrenal activity. However, several methodological problems should be considered when designing experiments and interpreting plasma CA results. The commonly assessed antecubital venous noradrenaline (NA) concentrations reflect local nerve activity, since about half of this NA is derived from the forearm tissues. Sympathetic nerve activity is not uniform, but may vary considerably between organs. Overall sympathetic activity is best assessed by measurements of NA in arterial or mixed venous blood. Venous adrenaline (ADR) levels may also be unrepresentative due to marked and variable extraction in the peripheral tissues. Urinary NA and ADR excretion studies still provide valuable information. Regional studies of NA overflow from individual organs give good estimates of local nerve activity and will increase the understanding of the functional organization of the sympathetic nervous system.

Endogenous renal dopamine and control of blood pressure

Activation of specific receptors for dopamine in the renal vasculature and tubules leads to increases in glomerular filtration, and to diuresis and natriuresis. There is evidence for intrarenal production and release of dopamine, which may originate from two sources: tubular decarboxylation of plasma l-DOPA and a population of dopaminergic sympathetic neurons that innervate the renal cortex. Studies of plasma and urinary catecholamine levels indicate that dopamine is released within the kidney in response to sodium loading and to activation of sensory pathways related to nociception and chemoreception. There is also evidence for deficient renal release of dopamine in patients with renovascular or essential hypertension. Collectively, the available data suggest that intrarenal dopamine has a physiological function in control of blood volume and blood pressure, and that defects in this control may be implicated in the aetiology of some hypertensive states.

Circadian variation of catecholamine excretion in rats: correlation with locomotor activity and effects of drugs

Rats housed in metabolic cages were used to study the circadian variation in the urinary excretion of free catecholamines. Small samples of urine (25-100 microliter) were analyzed for adrenaline, noradrenaline and dopamine by high pressure liquid chromatography (HPLC) and electrochemical detection. Various ways in which the values for excretion of catecholamines can be expressed (per min; per ml; per mmol creatinine; as ratio over dopamine) were calculated and discussed. Correction for excretion of creatinine resulted in the lowest variations coefficient among the experimental data. The correction for creatinine removed the circadian rhythm present in the output of noradrenaline (NA) and dopamine (DA). Adrenaline corrected for creatinine still displayed a pronounced circadian variation which was related to the overall locomotor activity of the animals (as recorded by photocells). Collection of 1 hr samples instead of 3 hr samples resulted in a worsening of the relationship between the excretion of adrenaline and locomotor activity. Finally, the possibility that the DA antagonist haloperidol, the DA agonist dipropyl-5,6-2-amino-6,7-dihydroxytetrahydronaphtalene (dipropyl-5,6 ADTN) and the alpha-antagonist phentolamine, could modify the excretion of free urinary catecholamines was investigated. Haloperidol and 5,6-dipropyl-ADTN did not change the output of the catecholamines, but phentolamine induced a strong increase in the excretion of NA. The latter observation suggest that at least part of the excretion of NA may originate from peripheral noradrenergic neurotransmission.

Effect of hypotension induced by sodium nitroprusside on catecholamine overflow in the canine kidney

The overflow of noradrenaline (NA) and dopamine (DA) to plasma in the kidney in response to hypotension induced by sodium nitroprusside were studied in barbiturate-anaesthetized dogs in order to evaluate the possible existence of separately regulated renal noradrenergic and dopaminergic nerve fibres. When mean arterial blood pressure was lowered to 55 +/- 5 mmHg, arterial plasma NA, DA and adrenaline concentrations were increased and renal blood flow decreased. Renal sympathetic nerve activity was assessed by measuring the renal overflow of catecholamines to plasma. To obtain more accurate estimates of the renal contribution to catecholamines in renal venous plasma we corrected for the renal extraction of arterial catecholamines, assessed by the extraction of endogenous adrenaline. The corrected renal NA overflow to plasma increased from 164 +/- 52 to 419 +/- 137 pmol min-1 (P less than 0.05) during sodium nitroprusside induced hypotension. The renal overflow of DA to plasma was, however, not influenced significantly. The DA/NA ratio for renal venous plasma concentration as well as for renal overflow to plasma was decreased (P less than 0.05) by sodium nitroprusside induced renal nerve activation. In contrast, electrical renal nerve stimulation has previously been shown to enhance the overflows of DA and NA in parallel. One possible interpretation of these findings is that sodium nitroprusside selectively activated renal noradrenergic but not the putative dopaminergic nerve fibres while electrical stimulation activated both types of fibres.

Neuronal and nonneuronal contributions to renal catecholamine content in the dog

Endogenous noradrenaline and 3,4-dihydroxyphenylethylamine (dopamine) levels were measured in different zones of the dog kidney following chronic unilateral renal denervation. In outer and inner renal cortex, and in outer medulla, greater than 95% of the tissue content of both catecholamines was contributed by renal nerves, whereas in inner medulla only nonneuronal catecholamines were found. The amounts of neuronal dopamine present in outer renal cortex were greater than would be expected for a population of solely noradrenergic nerves.

Renal overflow of noradrenaline and dopamine to plasma during hindquarter compression and thoracic inferior vena cava obstruction in the dog

Efferent renal nerve activity was assessed by measurements of the overflow of endogenous noradrenaline (NA) and dopamine (DA) to plasma in the kidney. To obtain correct estimates of the renal contribution to the renal venous outflow of NA and DA, corrections for the renal extraction of catecholamines in arterial plasma were performed by use of tracer amounts of [3H]NA. Hindquarter compression (previously known to cause a neurogenically mediated blood pressure elevation) increased the concentrations of NA, adrenaline and DA in arterial and renal venous plasma. The renal overflow of NA increased from 83.7 +/- 32.0 to 361.3 +/- 119.4 pmol min-1 (P less than 0.05) during hindquarter compression. When compared to the renal NA overflow during electrical renal nerve stimulation, this corresponds to an increase in average renal nerve impulse activity from approximately 0.4 to 1.6 Hz. Hindquarter compression also increased the renal overflow of DA to plasma. When venous return to the heart was reduced by obstruction of the thoracic inferior vena cava, the mean arterial blood pressure fell and all catecholamines in plasma increased gradually during the first 10 min of obstruction. The renal overflow of NA was only slightly increased, indicating a minor increase in renal nerve activity. The overflow of DA to plasma was not altered by obstruction of the thoracic inferior vena cava. Neither maneuver substantially altered the DA/NA ratio for renal overflow rates or for renal venous plasma concentrations indicating that there was no preferential activation of either noradrenergic or putative dopaminergic nerve fibres.

Dopaminergic and noradrenergic sympathetic nerves of the dog have different DOPA decarboxylase activities

We have compared the pattern of neural catecholamine fluorescence with that of immunoreactivity for the catecholamine-synthesizing enzymes tyrosine hydroxylase (TH) and DOPA decarboxylase (DDC) in dog atrium, which is innervated by noradrenergic nerves, and in dog kidney, which is thought to be supplied by dopaminergic nerves as well. In both tissues the distribution of nerves containing catecholamine fluorescence was similar to that of nerves exhibiting TH-like immunoreactivity. By contrast, DDC-like immunoreactivity was present in some (but not all) of the nerves associated with the intrarenal blood vessels, but was not detectable in any atrial nerves. High DDC activity provides further confirmation of the existence of sympathetic dopaminergic neurons supplying the kidney.

Renal extraction of endogenous and radiolabelled catecholamines in the dog

Efferent renal nerve activity can be studied by measurements of the renal venous outflow of noradrenaline and dopamine. Accurate estimates of the intrarenal release to plasma of these catecholamines, however, require determinations of the net contribution of the catecholamines in arterial plasma to their renal venous outflow. We therefore studied the extractions of endogenous noradrenaline, dopamine and adrenaline, as well as 3H-labelled tracer amounts of noradrenaline, and dopamine in innervated and denervated canine kidneys. Approximately two-thirds of noradrenaline and dopamine in arterial plasma were extracted in the kidney, while 80-90% of adrenaline in arterial plasma was extracted. The fractional extractions of the three catecholamines were not substantially altered when the sympathetic nervous system was moderately activated by bilateral carotid occlusion or when the renal nerve activity was abolished by acute denervation. It is concluded that biochemical assessment of renal sympathetic nerve activity by studies of the renal venous noradrenaline and dopamine outflow requires some estimate of the net arterial contribution to the renal venous outflow. Ideally, catecholamine extraction by the kidney should be evaluated by studies of the renal extraction of 3H-labelled noradrenaline and dopamine, but the extraction of endogenous adrenaline may also be useful for this purpose.