Enhanced release of dopamine- -hydroxylase from sympathetic nerves by calcium and phenoxybenzamine and its reversal by prostaglandins

Catecholamines: Knowledge and understanding in the 1960s, now, and in the future

The late 1960s was a heyday for catecholamine research. Technological developments made it feasible to study the regulation of sympathetic neuronal transmission and to map the distribution of noradrenaline and dopamine in the brain. At last, it was possible to explain the mechanism of action of some important drugs that had been used in the clinic for more than a decade (e.g. the first generation of antidepressants) and to contemplate the rational development of new treatments (e.g. l-dihydroxyphenylalanine therapy, to compensate for the dopaminergic neuropathy in Parkinson's disease, and β1-adrenoceptor antagonists as antihypertensives). The fact that drug targeting noradrenergic and/or dopaminergic transmission are still the first-line treatments for many psychiatric disorders (e.g. depression, schizophrenia, and attention deficit hyperactivity disorder) is a testament to the importance of these neurotransmitters and the research that has helped us to understand the regulation of their function. This article celebrates some of the highlights of research at that time, pays tribute to some of the subsequent landmark studies, and appraises the options for where it could go next.

Effects of vagus nerve stimulation on cognitive functioning in rats with cerebral ischemia reperfusion

Background

Vagus nerve stimulation (VNS) has become the most common non-pharmacological treatment for intractable drug-resistant epilepsy. However, the contribution of VNS to neurological rehabilitation following stroke has not been thoroughly examined. Therefore, we investigated the specific role of acute VNS in the recovery of cognitive functioning and the possible mechanisms involved using a cerebral ischemia/reperfusion (I/R) injury model in rats.

Methods

The I/R-related injury was modeled using occlusion and reperfusion of the middle cerebral artery (MCAO/R) in Sprague–Dawley rats. VNS was concurrently applied to the vagus nerve using a stimulation intensity of 1 mA at a fixed frequency of 20 Hz with a 0.4-ms bipolar pulse width. The stimulation duration and inter-train interval were both 3 s. Next, Morris water maze and shuttle-box behavioral experiments were conducted to assess the effects of VNS on the recovery of learning, memory, and inhibitory avoidance following I/R injury. Intracerebroventricular injection of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4), a selective neurotoxin for noradrenergic neurons, was used to evaluate the role of norepinephrine (NE) as a mediator of therapeutic effects of VNS on cognitive recovery.

Results

Compared with the MCAO/R group, the VNS+MCAO/R group had improved spatial memory as indicated by swimming path lengths and escape latencies in the Morris water maze, and fear memory, as indicated by the avoidance conditioned response rate, mean shock duration, and avoidance time in shuttle-box behavior experiments. Compared with the VNS+MCAO/R group, the DSP-4+VNS+MCAO/R group, which had reduced NE levels in cortical and hippocampal brain regions, showed a reversal of the VNS-induced benefits on spatial and fear memory performance.

Conclusions

VNS improves spatial and fear memory in a rat model of MCAO/R injury. However, a reduction in NE from the administration of DSP-4 blocks these protective effects, suggesting that NE may contribute to the influence exhibited by VNS on memory performance in rats with cerebral I/R-related injury.

Structure and function of the epididymis

Testicular spermatozoa are functionally immature in that they cannot fertilize ova. It was first demonstrated by Young that spermatozoa undergo certain changes as they migrate through the epididymis. He proposed that spermatozoa ripen during epididymal transit. It is now known that specific maturational changes occur in spermatozoa during epididymal transit which result in their developing the ability to fertilize ova. Concomitant with this functional maturity are changes in spermatozoal morphology, motility, chemistry, permeability, density and metabolism. It is apparent that in some way not understood these changes are necessary for sperm to achieve the ability to complete the fertilization process. When these mechanisms are understood, we may be able to effectively treat conditions such as necrospermia or abnormally low sperm motility. Furthermore, with the development of the hamster-egg penetration test a "new" type of male infertility has become evident in recent years; the inability of otherwise normal sperm to penetrate an ovum. It is during epididymal transit that this ability is normally acquired. Thus, any insight into how sperm attain the capacity to penetrate an ovum could lead to an effective treatment of patients whose sperm do not have this ability. In addition, the epididymis holds significant promise as the site of action for a male contraceptive. Thus, it is the purpose of this review to describe the structure and function of the mammalian epididymis with particular emphasis on the factors regulating sperm maturation.

Neuromuscular transmission and smooth muscle membrane properties in the guinea-pig ear artery

Effects of noradrenaline and alpha-adrenoceptor blocking agents on neuromuscular transmission of the guinea-pig ear artery were assessed using the micro-electrode method.1. The mean membrane potential, and length and time constants of the longitudinally oriented muscle cells were -64.5 +/- 5 mV (n = 150), 1.03 +/- 0.16 mm (n = 15) and 410 +/- 40 msec (n = 7) respectively. From the current-voltage relationship, weak outward current pulses produced an anomalous rectification of the membrane while stronger intensities produced a normal rectification of the membrane with a depolarization over 10-15 mV.2. Brief stimulation (0.1-0.5 msec) of the tissue produced an excitatory junction potential (e.j.p.). Facilitation produced by repetitive stimulation was evident only on very rare occasions. Higher stimulus intensities caused a stepwise increase of the amplitude of e.j.p.s.3. Spontaneously generated miniature excitatory junction potentials (m.e.j.p.s) were recorded from the muscle membrane. In many cells, the interval and amplitude histograms of m.e.j.p.s showed skew curves. On rare occasions, a bell-shaped amplitude distribution (quantal release of packeted noradrenaline (NA)) was observed.4. NA (> 3 x 10(-7)m) depolarized the membrane and increased the membrane resistance, as measured from the amplitude of the electrotonic potential. Phentolamine suppressed the NA-induced depolarization. However, high concentrations of phentolamine (> 10(-5)m) depolarized the membrane and increased the membrane resistance.5. NA (10(-8)m) caused no change in membrane potential though it suppressed the amplitude of an e.j.p. produced by a single stimulus, but did not suppress the amplitude of the subsequent e.j.p.s evoked by repetitive stimulation (0.2-2.0 Hz). A higher concentration of NA (3 x 10(-7)m) depolarized the membrane and markedly suppressed the amplitude of e.j.p.s. On the other hand, NA (10(-8) or 2 x 10(-8)m) generated burst discharges of m.e.j.p.s between silent periods or random generation.6. Phentolamine (10(-6)m) markedly enhanced the amplitude of e.j.p.s and caused a smooth facilitation in response to a train of stimuli with no effect on the membrane potential. A stepwise change in the amplitude of e.j.p.s was no longer observed at any given stimulus frequency and intensity.7. Phenoxybenzamine (10(-7)m) suppressed the amplitude of e.j.p.s with no change in the membrane potential.8. The results led to the conclusion that, at a concentration which has no effect on the post-junctional muscle membrane, NA and phentolamine seemed to be more effective on prejunctional adrenoceptors, while phenoxybenzamine seemed to have a greater effect on post-junctional adrenoceptors. NA depresses adrenergic transmission by negative feed-back, while the enhancing action of phentolamine can be explained partly by blocking of prejunctional adrenoceptors and, in addition, by an increase of NA release.

Prostaglandin synthesis inhibitors reverse alpha-adrenergic inhibition of acute insulin response to glucose

Prostaglandin E (PGE) has several effects on glucose homoeostasis and insulin secretion. The same events can be induced by alpha-adrenergic stimulation, which is known to stimulate PGE synthesis. To evaluate the hypothesis that PGE may be one intracellular mediator for certain alpha-adrenergic events, we examined the effects of a known PG synthesis inhibitor Sodium salicylate (SS) (40 mg/min iv) on the alpha-adrenergic effects of epinephrine (Epi) at two doses (3 and 6 micrograms/min) in normal male subjects. The lower dose of epinephrine diminished the acute insulin response (AIR) after a 20-g intravenous glucose pulse (control, 463 +/- 149; epinephrine, 97 +/- 38% of basal insulin, mean +/- SE, n = 6, P < 0.02); SS markedly augmented the AIR during epinephrine towards control values (339 +/- 137%; P < 0.02). In 12 subjects, the higher dose of Epi abolished the AIR. When similar studies were performed during a SS infusion, the AIR was partially restored (96 /+- 27% of basal insulin, n = 12, P < 0.01). Similarly, partial reversal of this alpha-adrenergic effect of Epi was observed with indomethacin, another inhibitor of PG synthesis. At both doses of Epi, SS augmented the glucose disappearance rate (KG) after the glucose pulse (P < 0.001). Sodium salicylate also increased basal glucagon levels (P < 0.05). In contrast, SS did not affect the glycemic response, the suppression of basal insulin levels, or the hemodynamic responses induced by adrenergic stimulation. We conclude that two prostaglandin synthesis inhibitors partially reverse the alpha-adrenergic inhibition of the AIR to glucose caused by Epi, without affecting other adrenergic events. The data are compatible with a role for prostaglandins in alpha-adrenergic events selectively in the pancreatic islet.

Interaction between adenosine triphosphate and noradrenaline in the isolated vas deferens of the guinea-pig

1 An interaction between adenosine triphosphate (ATP) and noradrenaline (NA) was investigated in the isolated vas deferens of the guinea-pig. 2 ATP (1.7 to 50 microM) enhanced contractions due to transmural electrical stimulation (ES; parameters: 30 Hz, 0.2 to 0.3 ms, 10 to 30 V; 1 s train duration). Responses to exogenous NA (12 microM) were also potentiated by ATP while contractions to acetylcholine (6 microM) were inhibited. 3 NA (1.2 microM) potentiated ATP-induced contractions and prevented the development of tachyphylaxis to ATP (510 microM). 4 Phentolamine (12.5 microM) prevented the potentiation by NA of ATP-induced contractions; these contractions were insensitive to phentolamine (up to 25 microM). 5 Removal of potassium chloride from the Tyrode solution for 10 min abolished the potentiating actions of both ATP and NA. 6 The present results suggest that the effect of ATP may be functionally closely related to that of NA at alpha-adrenoceptors in the vas deferens of the guinea-pig.

Dopamine beta-hydroxylase in health and disease

DBH is a copper-containing oxygenase that catalyzes the hydroxylation of the beta carbon of a wide variety of phenylethylamine derivatives using molecular oxygen ascorbate as cofactors. It is a glycoprotein with a molecular weight of 290,000 and consists of four identical subunits, each with a single copper atom and 5% carbohydrate by weight. The enzyme is a constituent of catecholamine storage vesicles in chromaffin cell and adrenergic neurons in the peripheral and central nervous system where it functions to synthesize noradrenaline from dopamine. Although endogenous inhibitors have been isolated, they have not been demonstrated to have a physiological function, and the kinetics of the enzyme in vitro and in vivo suggest that the enzyme is not a rate limiting step in catecholamine synthesis under normal conditions. DBH exists in both a soluble form within vesicles and as a constituent of their membranes with its active site directed inward. The significance of the partition of the enzyme into soluble and membrane forms is not understood, although the soluble form has a fivefold greater homospecific activity. DBH has been one of the most intensively investigated enzymes in neurochemistry for several reasons. It is a readily assayable constitutent of catecholamine storage vesicles and, as such, provides a convenient biochemical marker for subcellular fractionation work and studies of the cellular regulation of catecholamine synthesis, storage, and release. The adrenal medulla is a rich source of the enzyme for purification, and the purified enzyme is highly antigenic, thereby enabling the use of several immunological techniques to study the cellular dynamics of the enzyme and the organelles in which it is located. These include radioimmunoassay, immunohistochemistry, and cytochemistry. This review firstly summarizes the present state of knowledge concerning the molecular properties of DBH. It then describes the tissue, cellular, and subcellular localization of the enzyme and its physiological regulation. The remainder of the review concentrates on those aspects of research on DBH in which the authors have participated that have led to general advances such as the development of the concept of homospecific activity, the introduction of immunohistochemistry for the localization of enzymes involved in transmitter metabolism, the release of macromolecules from synaptic vesicles during the process of exocytosis, the use of antibodies to DBH administered in vivo to study the fate of synaptic vesicle membranes and to produce specific immunological lesions of noradrenergic nerves in the peripheral and central nervous system, the genetic, environmental, and physiological determinants of serum DBH activity as an index of sympathetic function in animals and man, and the question of its diagnostic value in disease.

Single pulse stimulation of guinea-pig vas deferens and the presynaptic receptor hypothesis

1 The effect of phenoxybenzamine on the efflux of [3H]-noradrenaline and the mechanical response to single pulse excitation of superfused guinea-pig vas deferens was determined to examine the validity of the currently accepted hypothesis of a presynaptic negative feedback system on adrenergic nerve terminals. 2 The adrenoceptor antagonist enhanced both the outflow of tritium and the mechanical response to single pulse stimulation. The efflux of labelled material and the responses to 4 pulses were also enhanced, as expected. 3 Blockade of neuronal and extraneuronal uptake did not by itself increase nerve-induced outflow or the mechanical response nor dated within the framework of a hypothesis that proposes that the enhancement of response and tritium efflux by phenoxybenzamine results from blockade of a feedback system whereby noradrenaline released by previous impulses inhibits its own subsequent release.

Tissue and plasma catecholamines and dopamine beta-hydroxylase activity of various animal species after neurogenic sympathetic stimulation

1. The effects of reserpine and exposure to a stressing atmosphere containing 20% CO2 for 5 hr on catecholamine (CA) levels and dopamine beta-hydroxylase (DBH) activity of adrenal, spleen and plasma of cat, rabbit, rat and guinea-pig were studied. 2. Twenty-four hr after an I.P. injection of reserpine, the CA contents of the adrenals and spleens were markedly reduced in all the four species studied. Adrenal and splenic DBH activity was unchanged in reserpine treated cats and rats. The enzyme activity decreased by 28 and 36% in adrenals of rabbits and guinea-pigs, and a 33% diminution was observed in the spleens of both species. 3. A marked rise in guinea-pig plasma DBH activity was obtained 24 hr after reserpine, while plasma DBH activity was unchanged in the rat. 4. Exposure of rats to a stressing atmosphere containing 20% CO2 for 5 hr increased the circulating noradrenaline (NA) levels by 600%, but plasma DBH activity remained unchanged. CO2 exposure caused both, an increase in plasm NA levels and DBH activity in the guinea-pig (950% and 100%, respectively). 5. The combined treatment with reserpine plus immediate exposure to CO2 produced a 230% rise in rat plasma NA with no concomitant change in DBH activity. A similar treatment caused a 75% fall of NA levels and an increase in DBH activity of 600% in the guinea-pig. 6. The ratios of total DBH activity to CA in the spleen and adrenal were much higher in the guinea-pig than in the rat. The fraction of the total DBH activity that can be solubilized by osmotic shock of purified adrenomedullary chromaffin granules was 28% in the rat and 71% in the guinea-pig. If one makes the assumption that 'soluble' and 'releasable' DBH may be equated, the amount of 'releasable' DBH into the circulation is much greater in the guinea-pig than in the rat. 7. The results suggest that the guinea-pig is a better model than the rat to study circulating DBH activity as an index of exocytotic CA release from adrenergic neurones and adrenal medulla, and therefore of sympathetic activity. The results also indicate that previous data trying to correlate sympathetic activity with circulating DBH activity carried out in the rat have to be reconsidered in the light of data obtained in the guinea-pig.

Changes in plasma dopamine beta-hydroxylase activity induced by stimulation of the complete sympathetic outflow in the pithed rat

1. Plasma dopamine beta-hydroxylase (DBH) activity, noradrenaline tissue levels and blood pressure were monitored in the pithed rat following electrical stimulation of the complete sympathetic outflow of the spinal cord. Stimulation at 10 or 30 Hz evoked marked increases in mean blood pressure which averaged 121 mmHg and were abolished by phenoxybenzamine. 2. Stimulation for 1 hr at 10 or 30 Hz did not change the noradrenaline content of the spleen nor the catecholamine content of the adrenals, but the heart noradrenaline content was doubled. 3. Plasma DBH activity was increased by 27% after 1 hr stimulation at 30 Hz, but remained unchanged after stimulation at 10 Hz, or at 30 Hz in phenoxybenzamine-treated rats. 4. We conclude that the pressor responses evoked by sympathetic nerve stimulation are due to the release of noradrenaline probably from adrenergic nerve terminals supplying the entire vasculature, and that acute alterations of circulating DBH activity are not dependent on the rate of catecholamine release evoked by direct electrical stimulation of sympathetic nerves in the whole pithed rat. The rat seems not to be a good model to study circulating noradrenaline and DBH levels as an index of exocytotic noradrenaline release from adrenergic neurones, and therefore of sympathetic activity.

Effect of parathyroidectomy on cardiovascular reactivity in rats with mineralocorticoid-induced hypertension

Previous work has shown that parathyroidectomy protected Sprague-Dawley rats against mineralocorticoid hypertension. 2 In order to explain this protection, we studied vascular reactivity to noradrenaline and angiotensin II in several groups of rats with and without their parathyroid and thyroid glands. Work was performed in vagotomized, anaesthetized rats after ganglionic blockade with pentolinium, and atropine sulphate. 3 The reactivity to noradrenaline was significantly lower in parathyroidectomized rats, especially at the beginning of mineralocorticoid treatment. 4 Autotransplantation of parathyroid glands in thyroparathyroidectomized rats re-established normal cardiovascular reactivity and development of hypertension. 5 Cardiovascular reactivity to angiotensin II was not affected in parathyroidectomized rats and was lowered in thyroparathyroidectomized thyroxine-treated rats.

Effect of atropine and acetylcholine on nerve stimulated output of noradrenaline and dopamine-beta-hydroxylase from isolated rabbit and guinea pig hearts

The role of a cholinergic muscarinic inhibitory mechanism in sympathetic neurotransmission was investigated in isolated rabbit and guinea pig hearts with intact sympathetic nerves. The effect of varying frequencies of stimulation (2.5, 5 and 10 Hz) on the concentration of noradrenaline (NA) and dopamine-beta-hydroxylase (DBH) released into the perfusate was investigated. Stimulation in the presence of atropine sulfate (3.4 micrometer) resulted in an augmented outflow of NA at all three frequencies while DBH outflow was significantly incrreased only at 5 and 10 Hz. d-Tubocurarine (2.0 micrometer) attenuated the augmenting effect of atropine on NA release at all frequencies of stimulation whereas it negated the significant effect of atropine on DBH release. Nerve stimulation in the presence of acetylcholine (0.55 micrometer) resulted in a significant decrease in the concentrations of NA and DBH in the perfusate. It is suggested that atropine augments NA outflow in part by blocking an "intrinsic" muscarinic inhibitory mechanism. Acetylcholine's inhibitory effect on NA release is reflected in a similar decrease in DBH release and, therefore, may function in vivo via an effect on exocytosis at the adrenergic nerve ending.

Correlation between increased dopamine-beta-hydroxylase activity and catecholamine concentration in plasma: determination of acute changes in sympathetic activity in man

In 11 healthy untrained volunteers the increase in plasma dopamine-beta-hydroxylase (DBH) activity during graded physical exercise has been examined as a true measure of increased activity of the sympathetic nervous system. The correlation between DBH activity, catecholamine concentration (CA) in plasma and heart rate was studied. When work on an electrically braked bicycle ergometer was gradually increased from 12.5 to 100, 200 and 300 watts there was a linear increase in DBH activity and heart rate; the increase in CA concentrations followed an exponential function. The peak values for DBH activity and CA concentration in plasma after the 300 watt work load (as percentages of the resting levels) were 130+/-3% and 820+/-71%, respectively; the adrenaline concentration in plasma increased only to 150+/-19% (p less than 0.05). There were significant correlations between heart rate and work load, DBH and work load and log CA and work load. The data imply direct correlations between heart rate and DBH, heart rate and log CA and DBH and log CA. The exponential increase in noradrenaline concentration in plasma might be due either to a greater net ""overflow'' from sympathetic nerve endings, and/or to increased secretion by the adrenal medulla. In the latter case, the release of noradrenaline would not be accompanied by secretion either of adrenaline or DBH. After work ceased there were sharp falls in heart rate and CA concentration, which indicate an immediate drop in sympathetic activity. DBH activity in plasma returned to normal very slowly; it reached half maximum values after 20-22 min. It is concluded that increased sympathetic activity in man can be estimated in vivo as changes in DBH and/or CA concentration in plasma. In contrast, a rapid decrease in sympathetic activity is directly reflected only by a rapid fall in the plasma concentrations of CA.

Some physiological and pharmalogical characteristics of the stimulus induced release of norepinephrine from the rabbit superior cervical ganglion

The cervical sympathetic nerve stimulated release of norepinephrine (NE) from the rabbit superior cervical ganglion (SCG) was characterized. The quantity of NE released per impulse declined with increasing stimulation frequency over the range of 3/sec to 15/sec. Nerve stimulated NE release was potentiated by 1 micromolar desmethylimipramine and 25 micromolar phenoxylbenzamine and inhibited by 1 micromolar bretylium, 25 micromolar methoxamine and 1 micromolar prostaglandin E1. These results suggest that nerve stimulated NE release in the rabbit SCG occurs from sympathetic fibers which are subject to the same neurosecretory control mechanisms as sympathetic fibers elsewhere in the autonomic nervous system.

Transmission to the longitudinal muscle of the guinea-pig vas deferens: The effect of pretreatment with guanethidine

1 Tissue was taken from guinea-pigs that had been injected with guanethidine (100 mg/kg, i.p.) 24 h before they were killed, and from paired control animals.2 Pretreatment with guanethidine caused a significant, substantial, and sometimes complete reduction of the nerve-mediated contractions of the vas deferens. There were no significant changes in the responses of the ileum to stimulation of cholinergic nerves or of the distal colon to stimulation of intrinsic (non-adrenergic) inhibitory nerves. Responses of the vas deferens and ileum to acetylcholine were unchanged, but contractions of the vas deferens elicited by exogenous noradrenaline were enhanced.3 The nerve-mediated contractions of the vas deferens were restored by exposing it to (+)-amphetamine followed by noradrenaline in vitro.4 It is concluded that noradrenaline is the transmitter released from motor nerves to the longitudinal muscle of the guinea-pig vas deferens. Possible explanations for the ineffectiveness of receptor blocking agents in antagonizing transmission are discussed.

Evidence for recycling of synaptic vesicle membrane during transmitter release at the frog neuromuscular junction

When the nerves of isolated frog sartorius muscles were stimulated at 10 Hz, synaptic vesicles in the motor nerve terminals became transiently depleted. This depletion apparently resulted from a redistribution rather than disappearance of synaptic vesicle membrane, since the total amount of membrane comprising these nerve terminals remained constant during stimulation. At 1 min of stimulation, the 30% depletion in synaptic vesicle membrane was nearly balanced by an increase in plasma membrane, suggesting that vesicle membrane rapidly moved to the surface as it might if vesicles released their content of transmitter by exocytosis. After 15 min of stimulation, the 60% depletion of synaptic vesicle membrane was largely balanced by the appearance of numerous irregular membrane-walled cisternae inside the terminals, suggesting that vesicle membrane was retrieved from the surface as cisternae. When muscles were rested after 15 min of stimulation, cisternae disappeared and synaptic vesicles reappeared, suggesting that cisternae divided to form new synaptic vesicles so that the original vesicle membrane was now recycled into new synaptic vesicles. When muscles were soaked in horseradish peroxidase (HRP), this tracerfirst entered the cisternae which formed during stimulation and then entered a large proportion of the synaptic vesicles which reappeared during rest, strengthening the idea that synaptic vesicle membrane added to the surface was retrieved as cisternae which subsequently divided to form new vesicles. When muscles containing HRP in synaptic vesicles were washed to remove extracellular HRP and restimulated, HRP disappeared from vesicles without appearing in the new cisternae formed during the second stimulation, confirming that a one-way recycling of synaptic membrane, from the surface through cisternae to new vesicles, was occurring. Coated vesicles apparently represented the actual mechanism for retrieval of synaptic vesicle membrane from the plasma membrane, because during nerve stimulation they proliferated at regions of the nerve terminals covered by Schwann processes, took up peroxidase, and appeared in various stages of coalescence with cisternae. In contrast, synaptic vesicles did not appear to return directly from the surface to form cisternae, and cisternae themselves never appeared directly connected to the surface. Thus, during stimulation the intracellular compartments of this synapse change shape and take up extracellular protein in a manner which indicates that synaptic vesicle membrane added to the surface during exocytosis is retrieved by coated vesicles and recycled into new synaptic vesicles by way of intermediate cisternae.

Vascular responses and noradrenaline overflows in the isolated blood-perfused cat spleen: some effects of cocaine, normetanephrine and -blocking agents

1. Vascular responses and noradrenaline overflows have been studied in the isolated blood-perfused cat spleen in response to electrical stimulation of the splenic nerve with trains of 200 supramaximal pulses at a frequency of 10 Hz given at 10 min intervals.2. In the absence of blocking agents the spleen gave well-defined responses and a mean maximum overflow of 436 +/- 96 pg noradrenaline per stimulus (n = 4) at the third train of stimuli.3. Cocaine (2 x 10(-5)M) increased the response nearly threefold and raised the overflow to 840 +/- 131 pg/stim (n = 4). At later stimulations the responses remained stable, but the overflows rose progressively to 1076 +/- 51 pg/stim after five trains of stimuli.4. Normetanephrine (10(-4)M) had no significant effect on response, but elevated the overflow to a mean maximum of 868 +/- 169 pg/stim (n = 4).5. Cocaine (2 x 10(-5)M) and normetanephrine (10(-4)M) given together increased the response by a factor of 6.5 and raised the overflow to 1258 +/- 247 pg/stim (n = 4).6. Hydergine (approx. 10(-5)M) almost completely abolished the response and raised the overflow to 859 +/- 173 pg/stim (n = 4).7. Cocaine (2 x 10(-5)M), normetanephrine (10(-4)M) and hydergine (approx. 10(-5)M) given in combination abolished the response and raised the overflow to 4089 +/- 1148 pg/stim (n = 4).8. Phenoxybenzamine (10(-4)M) abolished the response and elevated the overflow to 4215 +/- 738 pg/stim (n = 4).9. These results are interpreted in terms of selective and combined block of ;uptake I', ;uptake II' and alpha-adrenergic receptors. Facilitation of transmitter release by alpha-blocking drugs, and the possible existence of an uptake process associated with the alpha-receptors are discussed.

Enhancement of noradrenaline depletion in the cat spleen by phenoxybenzamine and phentolamine

The noradrenaline content of the microsomal fraction of cat spleen was reduced by nerve stimulation. The reduction was greater in the presence of phenoxybenzamine and phentolamine, but was not affected by the presence of cocaine. The results suggest that phenoxybenzamine and phentolamine are able to increase the impulse-evoked release of noradrenaline.

Radioimmunoassay and clearance of circulating dopamine- -hydroxylase

Release of the enzyme dopamine-β-hydroxylase (DBH) into the circulation by activity of the sympathetic nervous system was demonstrated by enzymatic assay of serum DBH. The development of a sensitive and specific solid-phase radioimmunoassay suitable for routine assay of serum DBH is described. It is sensitive to 1 ng of purified sheep DBH and is reproducible with a coefficient of variation of 4%. The mean of serum values for five normal sheep was 0.59 ± 0.02 (SE) µg/ml. The assay was sufficiently sensitive to measure DBH in 10 µliters of human serum even though cross-reactivity with antibodies against sheep DBH was incomplete. Clearance of circulating DBH was measured by the decay of radioactivity due to 125 I-DBH after steady state intravenous infusion in lambs. The estimated half-life of circulating DBH after correction for labeled metabolites was approximately 3 hours, and it was cleared from blood at 250 ml/hour. Under resting conditions, the rate of release of DBH into the circulation was about 10 µg DBH/hour kg -1 body weight. The distribution of radioactivity in various organs 7 to 48 hours after infusion showed that only the kidneys, lungs, and liver were significantly labeled. Since circulating DBH does not appear to be selectively taken up by sympathetic nerves nor rapidly metabolized, radioimmunoassay of serum DBH may be useful in measuring sympathetic activity in a wide range of physiological and disease states.

Inhibition of release of dopamine- -hydroxylase and norepinephrine from sympathetic nerves by colchicine, vinblastine, or cytochalasin-B (hypogastric nerve stimulation-exocytosis-microtubules-microfilaments-guinea pig)

Stimulation of the hypogastric nerve to the guinea pig vas deferens in the presence of phenoxybenzamine produced an enhanced release of both dopamine-beta-hydroxylase (EC 1.14.2.1) and norepinephrine. Addition of colchicine, vinblastine, or cytochalasin-B to the incubation medium caused an almost complete inhibition of release of both the amine and the enzyme. Colchicine and cytochalasin-B produced a blocking effect but did not, however, modify the ratio of dopamine-beta-hydroxylase to norepinephrine recovered in the bath fluid. The findings indicate that norepinephrine and dopamine-beta-hydroxylase are released from sympathetic nerve terminals by exocytosis and that this process is dependent upon the integrity of both microtubules and microfilaments.