Norepinephrine Appearance Research Articles

Regional extraction of circulating norepinephrine, DOPA, and dihydroxyphenylglycol in humans

Dihydroxyphenylglycol (DHPG) is the main intraneuronal metabolite of the sympathetic neurotransmitter, norepinephrine (NE), and dihydroxyphenylalanine (DOPA) the immediate product of the rate-limiting step in catecholamine biosynthesis. Simultaneous measurements of regional rates of appearance (spillovers) of NE, DOPA, and DHPG in plasma have the potential to provide unique information about aspects of sympathoneural function but have not actually been measured in humans. In the present study, spillovers of DHPG, DOPA, and NE in the heart, head, leg, and lungs, were estimated from regional extraction fractions of infused [ 3H]-1-NE, DHPG, and [ 13C 6]DOPA or unlabelled DOPA in humans during cardiac catheterization. There was little cardiac extraction of DHPG (7 ± SEM 2%) or DOPA (8 ± 4%) but substantial extraction of NE (69 ± 4%). Values for cardiac spillover of DHPG and DOPA therefore were similar to values for the arteriovenous increment times plasma flow (arteriovenous production rate), whereas the cardiac spillover of NE averaged about 7-times the NE arteriovenous production rate. Cardiac DHPG spillover (28 ± 3 ng/min) exceeded the spillovers of NE (9 ± 2 ng/min) and DOPA (15 ± 4 ng/min). In contrast, cranial DOPA spillover (159 ng/min) exceeded those of NE and DHPG by 8- and 2-fold and accounted for about 1/10 of the total spillover of DOPA into arterial plasma. In the femoral vascular bed, arteriovenous production rates of NE and DHPG were unrelated to femoral spillovers of NE and DHPG. Arterial and regional clearances of [ 13 C 6]DOPA were similar to those of unlabelled DOPA. The results suggest that (1) endogenous NE, DOPA, and DHPG all are released into the bloodstream by the heart, head, and limbs of humans; (2) DHPG and DOPA are not co-released with NE; (3) cardiac arteriovenous production rates of DOPA and DHPG can be used to indicate cardiac spillover of these catechols, whereas the cardiac NE arteriovenous production rate substantially underestimates cardiac NE spillover; and (4) estimates of limb spillover of NE and DHPG require concurrent measurements of the corresponding regional clearances.

Effects of age and level of physical activity on plasma norepinephrine kinetics

We examined the effects of age and physical activity on plasma norepinephrine (NE) kinetics in active and inactive older and younger men. NE kinetics were estimated by calculation of the rates of NE appearance and clearance during infusion of tritiated norepinephrine [( 3H]-NE). Older active men had the highest level of plasma NE (pg/ml) and NE appearance (microgram.min-1.FFW-1, where FFW is fat-free weight) relative to younger men and inactive older men. NE clearance (l.min-1.FFW-1) was not influenced by age or physical activity. No significant associations were noted between percent body fat, supine blood pressure with resting levels of NE, NE appearance, and NE clearance. We concluded that 1) chronic participation in physical activity by older men, but not younger men, is associated with higher resting levels of plasma NE and NE appearance and 2) the degree of adiposity does not influence plasma NE kinetics in healthy younger and older men. The level of physical activity is an influencing factor on plasma NE and NE appearance into circulation in healthy older men.

Preliminary evidence of benzodiazepine subsensitivity in panic disorder

Benzodiazepine receptors appear to mediate both the anxiolytic effects of benzodiazepine drugs and the anxiogenic effects of several benzodiazepine inverse agonists (Tallman and Gallagher 1985). Evidence that brain benzodiazepine receptor density is decreased in genetically fearful strains of rats (Robertson et al. 1978) suggests that human anxiety states might be associated with decreased functioning of the benzodiazepine system. This hypothesis has not been tested in humans. Recently, using a tritiated norepinephrine (NE) isotope dilution technique, we showed that a single oral dose of 15 mg of diazepam suppresses sympathetic nervous system (SNS) outflow as reflected by a fall in plasma NE appearance rate (NEA) and plasma epinephrine (EPI) (Roy-Byrne et al. 1988). This suggests that central nervous system (CNS) benzodiazepine pathways play a role in the regulation of

Suppression of norepinephrine appearance rate in plasma by diazepam in humans

Studies demonstrating benzodiazepine-induced reductions in plasma norepinephrine (NE) have assumed that changes in circulating plasma NE closely parallel changes in sympathetic nervous system (SNS) activity and that benzodiazepines suppress SNS outflow. However, decreases in plasma NE could also result from increased removal of NE from plasma via neuronal uptake or tissue metabolism. This study used a tritiated norepinephrine ([3H] NE) isotope dilution technique for measurement of plasma NE kinetics to determine if the fall in plasma NE induced by a single dose of diazepam orally administered to eight psychiatrically-healthy volunteers was due to a fall in plasma NE appearance rate or an increase in plasma NE removal. Diazepam decreased plasma NE appearance, but not clearance, and also decreased plasma epinephrine and mean arterial pressure, memory performance and alertness. Plasma levels of diazepam were correlated with drug effects on memory and alertness but not cardiovascular or SNS effects.

The importance of body composition to the increase in plasma norepinephrine appearance rate in elderly men.

Several studies have documented an increase in sympathetic nervous system (SNS) activity, as reflected by either plasma norepinephrine (NE) concentration or NE appearance rate, with aging. Because similar increases have been noted in young obese persons, and because adiposity increases with age, we hypothesized that body composition might be an important determinant of heightened SNS activity. Baseline SNS activity, energy expenditure, and responses to a standard formula "meal" were compared in 11 young (31.1 +/- 5 years) (m +/- SD) and 9 old (64.9 +/- 6.3 years) persons. Both baseline NE concentration, p less than .05, and the NE appearance rate, p less than .05, were increased in the elderly group. The percentage of body fat, p = .004, and age, p less than .02, were correlated independently with NE appearance rate but not with NE concentration. Although plasma NE increased after the meal in both groups, NE appearance increased in the young group only. We conclude that NE appearance rate is a better reflection of SNS activity than NE concentration. We also found that the percentage of body fat and age are independent determinants of baseline SNS activity, which together account for 52% of the variability in SNS activity, as reflected by NE appearance rate.

Age Differences in the Plasma Clearance Mechanisms for Epinephrine and Norepinephrine in Humans*

There is an age-related increase in plasma norepinephrine (NE) in humans that is due to both an increase in NE appearance into plasma and a decrease in plasma NE clearance. However, previous studies demonstrated no difference in plasma epinephrine (EPI) in young and old subjects, and the effect of aging on plasma EPI appearance and clearance is unclear. To study age differences in basal NE and EPI metabolism we infused eight young (aged 19-26 yr) and eight old (aged 64-74 yr) normal subjects with [3H]NE or [3H]EPI (15 microCi/m2 bolus dose plus 0.35 microCi/m2/min for 50 min) to achieve steady state conditions on separate days. The old subjects had higher arterialized plasma NE levels [mean, 217 +/- 13 (+/- SE) vs. 149 +/- 12 pg/mL; P less than 0.005] and plasma NE appearance. In contrast, neither plasma EPI levels (98 +/- 8 vs. 104 +/- 10 pg/mL; P = NS) nor EPI appearance rates were different in the old and young subjects. The plasma clearance rates of EPI and NE were nearly identical in the young subjects (1.63 +/- 0.14 vs. 166 +/- 0.09 L/min X m2; P = NS). Plasma NE clearance was lower in the old compared to the young subjects (1.38 +/- 0.06 vs. 1.64 +/- 0.10 L/min X m2; P less than 0.05) and was lower than EPI plasma clearance in the same subjects. Although NE and EPI can be removed by both neuronal and nonneuronal uptake mechanisms, and mean plasma clearance values for NE and EPI are the same in the young, the age-related decline in catecholamine clearance is specific for NE. This finding implies a differential effect of age on a catecholamine removal mechanism that is specific for NE.

Sympathetic nervous system activity in panic disorder

To assess sympathetic nervous system (SNS) activity in panic disorder, arterialized venous norepinephrine (NE) and epinephrine (EPI) were measured in 10 patients and 10 age- and weight-matched controls. In addition, arterialized plasma NE kinetics were determined using a tritiated NE isotope dilution technique. There were no significant differences between patients and controls for resting, supine plasma NE levels, plasma NE appearance rate, plasma NE clearance, or plasma cortisol. However, plasma EPI levels were significantly higher in panic patients (103 ± 23 vs. 33 ± 16 pg/ml). Furthermore, there was a significant correlation between anxiety ratings and plasma EPI levels in panic disorder patients. These findings suggest that during the resting state, panic disorder is associated with a selective activation of the adrenomedullary component of the SNS.

Afferent vagal stimulation, vasopressin, and nitroprusside alter cerebrospinal fluid kinin.

The effects of afferent vagal stimulation, cerebroventricular vasopressin, and intravenous nitroprusside on cerebrospinal fluid (CSF) kinin levels, mean arterial pressure (MAP), and heart rate (HR) were determined in anesthetized dogs in which a ventriculocisternal perfusion system (VP) was established. Following bilateral vagotomy, stimulation of the central ends of both vagi for 60 min significantly increased MAP and CSF perfusate levels of kinin and norepinephrine (NE). MAP was increased a maximum of 32 +/- 4 mmHg, and the rates of kinin and NE appearance into the CSF perfusate increased from 4.2 +/- 1.4 to 22.1 +/- 6.9 and from 28 +/- 5 to 256 +/- 39 pg/min, respectively. A significant correlation was found between CSF kinin and NE levels in these experiments. In other experiments the addition of arginine vasopressin to the VP system caused a significant increase in CSF perfusate kinin without affecting MAP or HR. Intravenous infusion of nitroprusside lowered MAP without affecting kinin levels in the CSF. However, on cessation of nitroprusside infusion, CSF kinin increased significantly in association with the return in MAP to predrug level. Collectively the data are consistent with the hypothesis that central nervous system kinins have some role in cardiovascular regulation, and furthermore that this role may involve an interaction between brain kinin and central noradrenergic neuronal pathways.

Age and alpha-2 adrenergic regulation of plasma norepinephrine kinetics in humans.

To investigate whether the age-related elevation of plasma norepinephrine (NE) is due to impaired alpha-2 adrenergic inhibition of sympathetic nervous system (SNS) outflow, arterialized plasma NE kinetics were measured before and 120 to 140 min after 1.5 and 5.0 micrograms m/kg oral clonidine in 6 old (57 to 78 years) and 8 young (25 to 39 years) normotensive male volunteers. Baseline plasma NE levels were higher in old compared with young men (M +/- SEM, 355 +/- 58 vs. 197 +/- 22 pg/ml, p less than .02). Clonidine produced significant (p less than .05) dose-related reductions in plasma NE, NE appearance rate, NE clearance, and mean arterial blood pressure (MAP) in both groups. There was no difference between old and young men in response to low dose clonidine. Following the higher dose, both groups had similar suppression of plasma NE (-51 +/- 7% vs. -58 +/- 2%, p greater than .05) and NE appearance (-60 +/- 6% vs. -62 +/- 2%, p greater than .05), but older men had a greater fall in NE clearance (-20 +/- 2% vs. -10 +/- 1%, p less than .003) and MAP (-28 +/- 3% vs. -10 +/- 4%, p less than .006). These findings suggest that sensitivity to alpha-2 receptor-mediated suppression of plasma NE and NE appearance is not diminished in elderly men.

Apparent turnover of norepinephrine in the intact rat brain estimated from its rate of appearance in the cerebroventricular compartment.

The apparent turnover of norepinephrine (NE) in the intact brain of urethane-anesthetized rats was estimated in vivo from the rate of appearance of endogenous NE in the cerebroventricular compartment. Ventriculocisternal perfusion, a radioenzymatic assay for endogenous NE and isotope dilution of 3H-NE were used to determine the rate of appearance (Ra) of NE. The effect of tranylcypromine, a monoamine oxidase (MAO) inhibitor and releasing agent on Ra of NE, was also studied. In control animals, the mean Ra value was 0.40 pmol/min. Pretreatment with tranylcypromine resulted in a threefold increase in the mean value for Ra, which was due to increased secretion of NE into the CSF. The results indicate that 90% of the NE entering the cerebroventricular compartment was removed. Pretreatment with tranylcypromine did not modify removal, thus suggesting that tranylcypromine has no effect on the physiological processes which regulate the removal of NE from the CSF.

Distribution of metaraminol and its relation to norepinephrine

A modified fluorometric procedure of high specificity and sensitivity is described for the analysis of metaraminol (MET) in biological material. With this method the distribution of MET was studied in the rat. Except for brain, peripherally administered MET was well distributed. Organs with a large population of norepinephrine (NE) storage vesicles such as the heart, spleen and adrenal, took up relatively large amounts of MET and retained it longer than those tissues with few vesicles such as the liver, lung and kidney. MET had a longer blood half-life than described for NE in other species. In part, this can be attributed to the lack of metabolism of MET as well as our finding that MET, unlike NE, was concentrated in the cellular fraction of blood. Under the conditions of these experiments there was no close correlation on a concentration or time basis with respect to the appearance and disappearance of MET and NE in various organs. This was particularly evident in the adrenal gland where MET resembled reserpine in that one unit of drug displaced many units of catecholamines. In this case, however, we did not rule out a possible contributing effect via the CNS.

Observations on the appearance of norepinephrine in the sympathetic nervous system of the chick embryo

The appearance of norepinephrine in developing autonomic structures of the chick embryo has been investigated. A highly sensitive and specific method for the cellular localization of certain catecholamines and tryptamines was used in combination with chemical determination of catecholamines. Norepinephrine was found in the sympathetic chain at the very stage when this appears as a well defined organ (embryos incubated 3.5 days). Outgrowing adrenergic nerves may contain norepinephrine from a very early stage of their development. Migrating norepinephrine-containing cells could be followed into the aortic plexus and to the interrenal buds, where they form the adrenal medulla. Up to the hatching stage, all medullary cells seem to store norepinephrine. Some data are given on the appearance of adrenergic nerves in the pancreas, duodenum, rectum, and heart. DOPA decarboxylase activity appeared shortly before norepinephrine could be histochemically demonstrated.