Noradrenaline Efflux Research Articles

Noradrenergic mechanisms and the cardiovascular actions of nitroglycerin

In this article we review noradrenergic activities of nitroglycerin in the central and peripheral nervous systems. Nitroglycerin may cause paradoxical bradycardia and occasional life threatening hypotension in patients. Intracisternal injections and microinjections of nitroglycerin into nucleus tractus solitarii produce hypotension and bradycardia, effects which mimic the baroreflex and may involve central noradrenergic mechanisms. The drug also triggers an α 2-adrenoceptor-mediated sympatho-inhibition reflex through vagal afferents. Nitroglycerin mimics biological responses associated with sympathetic neuronal activity, e.g., increase in outflow of norepinephrine and its metabolites from perfused guinea pig atria, medulla-pons tissue and cerebrospinal fluid. The sympathomimetic effects of nitroglycerin are antagonized by pre-treatment with yohimbine or rauwolscine. Clinical studies and animal experiments show that hemodynamics of nitroglycerin and sodium nitroprusside are different. Nitroglycerin is lipophilic and the compounds readily enters cells to form nitric oxide, but sodium nitroprusside is very hydrophilic and the compound has difficulty crossing membranes. Thus, intravenous nitroglycerin-induced increases in central noradrenergic activation and inhibitory reflexes may account for at least some of the therapeutic actions and side effects of the drug. In contrast, minimal central responses are produced by intravenous administration of sodium nitroprusside.

Influence of age on control of norepinephrine release: Ca 2+ channels and dopamine D 2 receptors

Possible age-related changes in the roles of L- and N-type Ca 2+ channels and dopamine D 2 receptors in control of norepinephrine release were investigated in tail arteries of F-344 rats. Nifedipine had no effect on stimulation-evoked tritium efflux at 6 or 24 months of age; however, ω-conotoxin GVIA reduced efflux by 70 to 80% at both ages even when frequency of stimulation was altered. Activation of prejunctional dopamine D 2 receptors by the selective agonist N-0923 [( S)-(−)-2-( N-propyl- N-2-thienylethylamino)-5-hydroxytetralin] inhibited contractile responses to transmural nerve stimulation in a frequency and concentration-dependent manner. Effects were similar from 6 to 26 months. Furthermore inhibition by N-0923 of stimulation-evoked [ 3H]norepinephrine efflux was not different at 12 and 24 months. Thus, N-type Ca 2+ channels predominate in control of norepinephrine release, and this is unchanged with advancing age or stimulation intensity. Furthermore, D 2 receptor-mediated inhibition of norepinephrine release is not altered with advancing age.

Real-time monitoring of endogenous noradrenaline release in rat brain slices using fast cyclic voltammetry: 3. Selective detection of noradrenaline efflux in the locus coeruleus

Fast cyclic voltammetry (FCV) at carbon fibre microelectrodes was used to monitor ‘real time’ endogenous noradrenaline (NA) efflux in superfused slices of rat locus coeruleus (LC) following local electrical stimulation. When stimulated with a standard train (30 pulses, 100 Hz, 0.2 ms, 10 mA, every 5 min), efflux of monoamine was constant over the experimental period (2.5 h): Amine efflux declined by only 16 ± 5% while uptake half-life lengthened by only 9 ± 8%. When calibrated in solutions of NA, peak amine efflux corresponded to 0.31 ± 0.04 μM (mean±S.E.M., n = 28) and was removed by uptake with a half-life of 2.93 ± 0.28 s ( n = 16). The released compound was confirmed as NA on the basis of pharmacological and electrochemical criteria. Stimulated monoamine efflux was reversibly reduced by 78% by omission of Ca 2+ from the superfusate for 30 min ( P < 0.05). Ro 4-1284 (1 μM), a fast-acting reserpine-like drug, decreased amine efflux by 86% ( P < 0.05). The monoamine oxidase inhibitor pargyline (2 μM) increased efflux by 30% ( P < 0.05). Desipramine (0.05 μM), a selective NA uptake blocker, significantly increased amine efflux (by 96%, P < 0.05) and uptake half-life (by 314%, P < 0.05). Fluvoxamine (0.5 μM), the selective serotonin (5HT) uptake blocker, increased efflux by 59% ( P < 0.05) and the uptake half-life by 122% ( P < 0.05). Vanoxerine (GBR 12909: 0.3 μM), the dopamine (DA) uptake blocker, had no effect on amine efflux or uptake half-life. The voltammogram of the released amine had single oxidation and reduction peaks. Comparison of these redox characteristics with standards confirmed that the released substance generated voltammograms indistinguishable from those produced by the catecholamines NA and DA but distinct from that produced by the indoleamine 5HT. Taken together, the combined pharmacological and electrochemical data confirm that the amine detected in the LC was NA. This study demonstrates the ability of FCV to selectively monitor endogenous NA efflux and uptake in ‘real time’ and with high spatial resolution.

Neonatal guanethidine sympathectomy suppresses autotomy and prevents changes in spinal and supraspinal monoamine levels induced by peripheral deafferentation in rats

In the rat, sciatic and saphenous nerve section resulted in self-mutilation of the ipsilateral limb. Fifteen and 60 days after surgery, monoamine levels were altered not only in the spinal cord but also in supraspinal structures. Thus, in the ipsi- and contralateral sides of the spinal lumbar region, an increase in the levels of 5-hydroxyindoleacetic acid (5-HIAA) was observed 15 days after surgery and in the levels of serotonin (5-HT) and noradrenaline 60 days later. Changes in the content of 5-HT and its metabolite were also evident, at these time points, in periaqueductal gray matter and trigeminal nucleus. Chemical sympathectomy carried out by administering guanethidine to neonatal rats reduced the degree of autotomy and suppressed the changes in monoaminergic systems following peripheral neurectomy. This study supports the hypothesis that the local noradrenaline outflow from sympathetic fibers in the neuroma is one of the causal factors in autotomy and it indicates that autotomy is under the control of descending monoaminergic pathways originating in brain-stem nuclei.

Characterization of K(+)-evoked [3H]D-aspartate outflow in the rat hippocampus in vitro.

The characteristics of K(+)-evoked outflow of [3H]D-aspartate, a glutamate release marker, were systematically investigated in the rat hippocampus, using 35 mM K(+)-evoked [3H]noradrenaline outflow as a reference. Elevation of external K+ concentrations increased [3H]D-aspartate outflow in a concentration-dependent manner both in slices and synaptosomes. In the absence of external Ca2+, K(+)-evoked [3H]D-aspartate outflow was decreased by approx 60% in synaptosomes and 80% in slices. However, elimination of external Ca2+ in the presence of 2 mM EGTA significantly reduced only 100 mM K(+)-evoked outflow, both in slices and synaptosomes. In the absence of external Ca2+, 35 mM K(+)-evoked [3H]noradrenaline outflow was abolished even when EGTA was present in the solution. Furthermore, the Ca(2+)-channel blockers omega-conotoxin (10 nM) and nifedipine (0.5 microM) did not significantly reduce K(+)-evoked [3H]D-aspartate outflow; [3H]noradrenaline outflow, however, was reduced by more than one third by omega-conotoxin. Finally [3H]D-aspartate overflow was insensitive to tetrodotoxin (0.5 microM) both in synaptosomes and in slices, while that of [3H]noradrenaline was significantly reduced in slices. It is concluded that (1) [3H]D-aspartate outflow is partly Ca(2+)-dependent; (2) differences between K(+)-evoked [3H]D-aspartate and [3H]noradrenaline outflow include sensitivity to stimulation by EGTA, to Ca(2+)-channel blockers and to tetrodotoxin. Some of these discrepancies may be ascribed to the existence of a cytosolic, Ca(2+)-independent pool of releasable glutamate and [3H]D-aspartate. These observations pose some problems as to the experimental approach for the study of Ca(2+)-dependent [3H]D-aspartate release.

Influence of age and gender on brown adipose tissue norepinephrine turnover.

We hypothesized that the attenuated brown adipose tissue thermogenic capacity observed previously in cold-exposed 27-month-old male versus female Fischer 344 rats might result, in part, from blunted sympathetic signaling to the tissue. As an index of sympathetic activity to brown fat, norepinephrine (NE) turnover in this tissue was evaluated at rest (22-24 degrees C) and during 1.5 hr of cold exposure (6 degrees C) in male and female Fischer 344 rats, aged 6, 12, and 26 months. Resting NE turnover as well as the rate constant for NE efflux from brown fat, expressed as total and as per gram of tissue protein, did not, in general, differ from age or gender. During cold exposure, rate constants and NE turnover rates increased significantly from those at rest in all groups. Brown fat NE turnover in cold-exposed 26-month-old male rats was greater than that observed in age-matched females, suggesting greater, not less, sympathetic signaling in the males versus females. These data indicate that the attenuated brown fat thermogenic capacity as well as the blunted cold-induced thermogenic responses of cold-exposed older male versus female rats reported previously cannot be explained by diminished release of NE from sympathetic nerves innervating brown adipose tissue.

Early age-dependent changes in noradrenaline efflux in the bed nucleus of stria terminalis: Voltammetric data in rat brain slices

Fast cyclic voltammetry (FCV) was applied to the detection of stimulated noradrenaline (NA) efflux in the bed nucleus of stria terminalis (BSTV) of the superfused rat brain slice from rats of different ages. Three age groups were compared: young (5 weeks; 130 ± 8 g), adult (2 weeks; 308 ± 12 g), and mature rats (36 weeks; 575 ± 21 g). The effect of train duration (20 – 99 pulses at 100 Hz) and frequency of stimulation (50 pulses at 10 – 500 Hz) on NA efflux were examined. The effect of the α 2 antagonist yohimbine (1 μM) was also investigation. NA efflux was significantly ( p < 0.05) greater in young than adult or mature rats on all train durations and stimulus frequencies tested; Maximal NA efflux on the longest train (99 pulses, 100 Hz, 10 mA, 0.2 ms) was 423 ± 48 nM (young), 135 ± 24 nM (adult), and 155 ± 26 nM (mature). There were no significant differences between adult and mature rats. Yohimbine (1μM) elevated NA efflux to a greater extent at lower (10 & 20 Hz) than higher frequencies in all age groups. Yohimbine also potentiated NA efflux more ( p < 0.05) in young and adult rats than in mature animals. There were no significant differences between young and adult rats. The results indicate early maturational differences in both NA efflux and its control by α 2 adrenoceptors. Interestingly, the decreases in NA efflux and in yohimbine response were not temporally linked.

Involvement of α2-receptors in the analgesia induced by transient forebrain ischemia in rats

Transient forebrain ischemia induced in rats by the four-vessel occlusion method produced analgesic effects in the hotplate test that persisted for 2 weeks. Ischemia-induced analgesia was attenuated by low doses of α 2-agonist clonidine (0.01–0.10 mg/kg, IP) and enhanced by low doses of α 2-antagonists yohimbre (1–2 mg/kg, IP) and idazoxan (0.25–1.00 mg/kg, IP) administration 7 days after ischemia. Ischemia-induced analgesia was not affected by methysergide, naloxone, propranolol, or phenoxybenzamine administered 7 days after ischemia, when motor control and arousal level of rats recovered to normal conditions. The enhanced response to yohimbine was antagonized by pretreatment with clonidine (0.75 mg/kg, IP) and naloxone (10 mg/kg, IP), suggesting the involvement of endogenous opioid peptides. The enhanced response to yohimbine was still present 2 months after ischemia, when preischemic hotplate threshold was restored. As α 2-agonists reduce and α 2-antagonists increases the outflow of central noradrenaline, it is suggested that activation of acti central noradrenergic systems is involved in the mediation of ischemia-induced analgesia.

Methoxamine enhances the release of endogenous noradrenaline from rabbit ear artery: possible involvement of ATP.

The effect of methoxamine, an alpha 1-adrenoceptor agonist, on the electrically-evoked release of endogenous noradrenaline was examined in the isolated rabbit ear artery. Noradrenaline was quantified by high performance liquid chromatography-electrochemical detection. The release of adenine nucleotides and nucleosides by methoxamine was examined using high performance liquid chromatography-fluorescence detection. The release of noradrenaline evoked by electrical field stimulation (EFS) at 4 Hz was reduced by tetrodotoxin 0.3 mumol/l and clonidine 1 mumol/l by approximately 80% and 50%, respectively. On the other hand, methoxamine at 10 but not 1 mumol/l enhanced the release of noradrenaline to approximately twice the control, and the enhancement was prevented by prazosin 1 mumol/l. The facilitatory action of methoxamine was also abolished after desensitization of P2-purinoceptors by alpha,beta-methylene ATP 30 mumol/l as well as by the presumed P2-purinoceptor antagonist suramin given at 10 mumol/l. Exogenous ATP 10 mumol/l significantly enhanced the EFS-evoked release of noradrenaline, and the enhancement was abolished by alpha,beta-methylene ATP and suramin. None of the drugs changed the spontaneous outflow of noradrenaline. These results indicate that endogenous ATP, acting at prejunctional purinoceptors, may participate in the facilitatory effect of methoxamine. Indeed, methoxamine 10 mumol/l significantly enhanced the spontaneous outflow of ATP and, less so, ADP. The methoxamine evoked release of ATP and ADP was antagonized by prazosin 1 mumol/l. It is concluded that methoxamine releases endogenous ATP from postjunctional sites which then, via prejunctional purinoceptors, facilitates action potential-evoked release of noradrenaline in rabbit ear artery.

Developmental alterations in [formula omitted] stimulated [ 3H]norepinephrine release in rat brain cortex and hippocampus

Developmental alterations in N- methyl- d-aspartate (NMDA)-stimulated [ 3H]norepinephrine release from rat brain cortical and hippocampal slices were studied. NMDA (10–1000 μM) resulted in a concentration-dependent increase in [ 3H]norepinephrine efflux; maximal responses (% released) in the cortex were: (1.53 ± 0.12, 3.68 ± 0.20, 2.94 ± 0.20, 4.60 ± 0.28 and 5.28 ± 0.33) and the hippocampal responses were: (1.90 ± 0.18, 3.84 ± 0.23, 3.60 ± 0.28, 5.16 ± 0.38 and 5.81 ± 0.45) at varying postnatal ages (1, 7, 14, 21 and 90 days) respectively. Cortical tissue from 7-day-old pups exhibited a transient increase in maximal efflux and a decrease in EC 50. These results indicated that developmental alterations in the NMDA receptor appear to be translated into differences in NMDA stimulated [ 3H]norepinephrine release.

Sympathoadrenal responses to glucoprivation and lipoprivation in rats

The effects of glucoprivation and lipoprivation on sympathoadrenal outflow were investigated in rats with permanent intra-atrial catheters. Glucoprivation was induced by infusion of a hypoglycemic dose of insulin (3 U/kg) or by infusion of the glucose antimetabolite, 2-deoxy- D-glucose (2-DG, 200 mg/kg). Lipoprivation was induced by infusion of sodium mercaptoacetate (MA, 600 μmol/kg), which blocks beta oxidation of fatty acids. Stress-free blood samples for measurement of blood glucose, plasma nonesterified fatty acids (NEFA), and epinephrine (E) and norepinephrine (NE) concentrations were collected remotely before and after drug injection. Glucoprivation and lipoprivation differed significantly in their effects on the sympathoadrenal system. Both 2-DG- and insulin-induced glucoprivation appeared to increase adrenomedullary secretion selectively, leading to dramatically increased plasma E levels. Although plasma NE levels also rose during glucoprivation, other evidence suggests that this effect may be secondary to the rise in E. In contrast, MA-induced lipoprivation increased the outflow of NE from the sympathetic nerve endings without a significant effect on plasma E concentrations. Plasma E levels rose only late in the test, as blood glucose levels began to fall. Results indicate that glucoprivation and lipoprivation are distinct metabolic signals, each capable of selectively activating one branch of the sympathoadrenomedullary system and thereby facilitating the mobilization of metabolic fuels appropriate for the specific metabolic challenge.

Cardiovascular change and hypothalamic norepinephrine release in response to environmental stress.

The major objective of this study was to compare the magnitude and duration of cardiovascular (CV) responses to acute environmental stresses with the associated patterns of noradrenergic activity in the paraventricular nucleus (PVN) and posterior nucleus (PH) of the hypothalamus. Simultaneous microdialysis samples of extracellular norepinephrine (NE) were collected at 5-min intervals from PVN and PH and the CV responses were recorded before, during, and for 15 min after acute shaker (cage oscillation) stress or inhalation of ether vapor in freely moving rats. Five minutes of shaker stress, 60 and 150 cycles/min, elicited pressor responses coupled with increases in dialysate NE from both PVN and PH in a frequency-dependent manner. Tachycardia occurred at 150 but not 60 cycles/min. Ten minutes after 60 cycles/min and 15 min after 150 cycles/min, NE efflux in PH was still increased, whereas in PVN it returned to control as had arterial pressure and heart rate. Ether vapor elicited a transient CV response but a continuing efflux of NE in PH and PVN. Urethan anesthesia raised baseline values of dialysate NE in PH and PVN but significantly attenuated cardiovascular and dialysate NE responses to shaker stress. We conclude that acute environmental stress simultaneously elicits CV responses and the efflux of NE from PVN and PH but, during or after stress, CV values may return to control levels while NE efflux remains elevated in PVN and/or PH.

Real-time monitoring of endogenous noradrenaline release in rat brain slices using fast cyclic voltammetry. 2. Operational characteristics of the α 2 autoreceptor in the bed nucleus of stria terminalis, pars ventralis

Fast cyclic voltammetry (FCV) at carbon fibre microelectrodes was used to monitor stimulated noradrenaline (NA) efflux in slices of the ventral part of the rat bed nucleus of stria terminalis (BSTV) superfused with artificial cerebrospinal fluid at 32°C. NA efflux was evoked by local electrical stimulation (trains of 10–50 pulses, 0.2 ms duration, 10 mA constant current at 10–500 Hz). The effects of four α 2 antagonists (yohimbine, rauwolscine, parazosin and WB 4101) and three α 2 agonits (clonidine, oxymetazoline and UK 14304) were examined. All drugs (1 μM) were added via the superfusate. Yohimbine and rauwolscine increased NA efflux on the lower but not the higher frequency trains: maximum increases (on 20 Hz, 50 pulse stimulation) were to 392±63% (yohimbine) and 243±7% (rauwolscine). There was a threshold train duration for demonstration of autoreceptor antagonism of 500–1000 ms. Prazosin and WB 4101 did not increase NA efflux but caused a modest decrease at the higher (100–500 Hz) frequencies. The effects of the α 2 agonists were also affected by stimulus train duration. Longer trains reduced agonist (clonidine) effects. When tested on pseudo-one pulse (POP) stimulations (less than 100 ms duration), the α 2 agonists decreased NA efflux. UK 14304 reduced NA efflux on 20 pulse/200 Hz stimulation to a g reater degree ( 86±7%) than the partial agonists clonidine ( 39±3%) or oxymetazoline ( 40±8%). The present results demonstrate α 2 autoreceptors are a major mechanism in the control of NA efflux in the BSTV. The lack of potentiation of NA efflux by prazosin suggests that the autoreceptor is not of the α 2B or α 2C subtype while the clear inhibitory action of oxymetazoline suggests an α 2A or α 2D receptor. With appropriate selection of stimulation conditions, the actions, of both antagonists and agonists (partial and full) can be examined.

N omega-nitro-L-arginine, an inhibitor of nitric oxide synthesis, decreases noradrenaline outflow in rat isolated perfused mesenteric vasculature.

In the isolated perfused rat mesenteric vasculature with intestine attached N omega-nitro-L-arginine (L-NNA) (30 mumol/l), an inhibitor of nitric oxide (NO) synthesis from L-arginine, did not alter spontaneous noradrenaline outflow. Transmural field stimulation (2-10 Hz) caused a frequency-dependent increase in noradrenaline outflow. The evoked overflow was reduced by L-NNA. L-Arginine (0.3 mmol/l) attenuated the inhibition of noradrenaline overflow by L-NNA. These results suggest that NO increases the release of noradrenaline in rat mesenteric vasculature.

Attenuation of vasoconstriction by endogenous nitric oxide in rat caudal artery.

1. The effects of NG-nitro-L-arginine (L-NNA), NG-nitro-L-arginine methyl ester (L-NAME), haemoglobin and methylene blue have been examined on vascular reactivity in the rat isolated caudal artery. The effects of L-NNA and sodium nitroprusside were also investigated on the stimulation-induced (S-I) efflux of noradrenaline in the rat caudal artery. 2. L-NNA (10 microM) and L-NAME (10 microM) significantly attenuated the vasodilator responses to acetylcholine (1 nM-1 microM), but had no effect on vasodilator responses to papaverine (1-100 microM). 3. Vasoconstrictor responses to sympathetic nerve stimulation (3 Hz, 10 s), noradrenaline (0.01-1 microM), methoxamine (1-10 microM), 5-hydroxytryptamine (0.01-0.3 microM), phenylephrine (0.1-10 microM), endothelin-1 (10 nM) and KCl (40 mM) were significantly enhanced by 10 microM L-NNA. L-NAME (10 microM) caused a significant enhancement of vasoconstrictor responses to noradrenaline and sympathetic nerve stimulation in endothelium-intact, but not in endothelium-denuded tissues. 4. Haemoglobin and methylene blue (both 10 microM) enhanced the vasoconstrictor responses to sympathetic nerve stimulation and noradrenaline. The enhancements were absent in endothelium-denuded arterial segments. 5. In endothelium-denuded arterial segments precontracted with phenylephrine, the vasodilator responses to the nitric oxide donor, sodium nitroprusside (0.1-300 nM) were decreased by increasing the level of precontraction. 6. L-NNA (10 microM) had no effect on the S-I efflux of radioactivity from arteries in which transmitter stores had been labelled with [3H]-noradrenaline. 7. These results suggest that endothelial nitric oxide attenuates vasoconstrictor responses in the rat caudal artery through activation of soluble guanylate cyclase to decrease smooth muscle contractility. Therefore, the findings provide evidence that nitric oxide acts as a functional antagonist to oppose vasoconstriction.

First-line pharmacological treatment of hypertension.

Over the last two decades, the treatment of arterial hypertension has improved considerably. Several new well-tolerated antihypertensive drugs have be- come available which have enhanced the therapeutic possibilities available to the physician at the expense of other compounds such as the centrally acting sympatholytic agents. In most industrialized coun- tries. four classes of antihypertensive drugs are recommended for first-line therapy, i.e. diuretics, beta-blockers, angiotensin converting enzyme (ACE) inhibitors and calcium antagonists [ 1-31. These drugs have the advantage of lowering blood pressure by different mechanisms. Thus diuretics increase urinary sodium excretion and thereby reduce total body sodium which is often augmented in patients with essential hypertension. The antihypertensive efficacy of beta-blockers results in part from their ability to reduce cardiac output via their negative inotropic and chronotropic properties. Another im- portant effect of beta-blockers takes place at the nerve endings of the sympathetic nervous system. At the presynaptic level, beta-adrenergic stimulation by norepinephrine or epinephrine enhances the amount of norepinephrine released for a given nervous activity. Beta-blocking agents, by blocking the acti- vation of presynaptic receptors, thus reduce the efflux of norepinephrine. In addition, beta-blockers that are devoid of intrinsic sympathetic activity decrease renin secretion, an effect that may also contribute to the ability of these agents to lower blood pressure. In contrast to beta-blockers, ACE inhibitors interfere specifically with the renin-angio- tensin system. These agents decrease blood pressure because they inhibit the conversion of angiotensin I into the potent vasoconstrictor angiotensin TI. In addition to its vasoconstrictor properties, angiotensin

Effects of nerve stimulation and zymosan on glycogenolysis in perfused livers from cold-exposed rats.

The effects of sympathetic nerve stimulation and zymosan (cell wall particles from yeast) on glycogenolysis were studied in perfused livers from rats kept for 5 and 20 days at 4 degrees C. The rate of glycogenolysis induced by nerve stimulation decreased significantly without any decrease in norepinephrine outflow during cold exposure, and the rate induced by norepinephrine did not change. By contrast, the rate of zymosan-induced glycogenolysis increased markedly during cold exposure. The rats with denervated hepatic nerves did not show the increased response to zymosan. In cold-exposed rats, both mepacrine and ibuprofen inhibited the effects of zymosan and of nerve stimulation without any inhibition of the outflow of norepinephrine. Neither inhibitor had any effect on the effects of norepinephrine. The metabolic effects of nerve stimulation and zymosan were not additive in cold-exposed rats. These results suggest that cold exposure may modulate the metabolism of arachidonic acid in Kupffer cells via hepatic nerve and decrease the eicosanoid-dependent glycogenolysis by nerve stimulation.

Modulation of noradrenergic transmission in the rat isolated portal vein: Role of prejunctional α2-adrenoceptors and β-adrenoceptors

1. 1. The effect of several adrenoceptor agonists and antagonists on the spontaneous and stimulus-evoked release of [ 3H]noradrenaline was studied in rat isolated portal vein. 2. 2. Yohimbine (10 −6 M) increased the stimulus-evoked [ 3H]noradrenaline efflux. Adrenaline alone (3 × 10 −6 M) was without effect, while it increased the resting efflux when added together with yohimbine. 3. 3. Propranolol alone was without effect on the release of [ 3H]noradrenaline. When added (2 × 10 −6 M) at the same time as yohimbine, it reduced the stimulation-induced 3H efflux. When added before adrenaline and yohimbine, propranolol (10 −5 M) reduced both spontaneous and stimulus-evoked release of [ 3H]noradrenaline. 4. 4. The effect of several β-blocking drugs was measured on the enhancing effect of yohimbine on the stimulation-evoked 3H efflux. The β 1-adrenoceptor blocking drugs: atenolol (5 × 10 −6 and 10 −5 M), metoprolol (5 × 10 −6 and 10 −5 M), like the more selective bisoprolol (2 × 10 −6 and 4 × 10 −6 M) inhibited yohimbine activity. Likewise, propranolol (2 × 10 −6 and 5 × 10 −6 M) and the β 2-adrenoceptor blocker ICI 118551 exhibited an antagonistic effect. 5. 5. These results indicate the possibility for noradrenaline to activate presynaptic β-adrenoceptors in rat portal vein. They show an interaction between the presynaptic α 2- and β-adrenoceptor mediated systems in the release of noradrenaline. They suggest the presence and the activity of facilitatory β 1-adrenoceptors.

Real-time monitoring of endogenous noradrenaline release in rat brain slices using fast cyclic voltammetry: 1. Characterisation of evoked noradrenaline efflux and uptake from nerve terminals in the bed nucleus of stria terminalis, pars ventralis

Fast cyclic voltammetry (FCV) at carbon fibre microelectrodes was used to monitor endogenous noradrenaline (NA) efflux in superfused slices of bed nucleus of stria terminalis pars ventralis (BSTV) in ‘real time’. NA efflux was evoked by local electrical stimulation at bipolar tungsten stimulating electrodes. Confirmation of the identity of the released species as NA was made on the basis of anatomica, electrochemical and pharmacological proofs. Firstly, the signal matched the NA innervation density: efflux of monoamine was greater in BSTV than in the pars dorsalis of the nucleus. Secondly, the voltammogram of the released species was indistinguishable from those of the catecholamines NA and dopamine (DA) but dissimilar to that of the indoleamine serotonin (5-hydroxytryptamine, 5-HT). Thirdly, amine efflux was influenced in a predictable fashion by the drugs tested. Tetrodotoxin (10 −6 M) or omission of Ca 2+ from the superfusate reversibly reduced amine efflux by 90.2 and 88.0% respectively. Ro 4-1284 (10 −6 M) decreased amine efflux by 75.8%. Desipramine (5 × 10 −8 M), the selective NA uptake blocker, significantly increased amine efflux and uptake half-life (to 214.3 and 389.5% of control respectively). Fluvoxamine (5 × 10 −7 M) and GBR 12909 (3 × 10 −7 M), blockers of 5-HT and DA uptake respectively, had no effect on amine efflux, although fluvoxamine caused a modest (91.0%) increase in the uptake half-life. Pargyline (2 × 10 −6 M) affected neither efflux nor uptake. The combined anatomical, electrochemical and pharmacological data confirm that the monoamine detected in BSTV by local electrical stimulation was NA. Stimulated NA efflux was stable and reproducible over at least 2.5 h (longest period tested). This study demonstrates the ability of FCV to selectively monitor endogenous NA efflux and uptake in ‘real time’ and with high spatial resolution.

Relationship Between the Sympatholytic Action of Nebivolol and Hypotension

Nebivolol, a chemically novel beta 1-adrenoceptor antagonist, acutely lowers blood pressure in spontaneously hypertensive rats, anaesthetised normotensive dogs, and hypertensive patients. We have investigated the actions of dl-nebivolol in five conscious normotensive rabbits (sham, mean blood pressure (BP) of 82.2 +/- 4.1 mm Hg, mean +/- SEM) and four hypertensive rabbits (renal wrap hypertension) (wrap, mean BP of 117.6 +/- 1.5 mm Hg). Nebivolol (1 mg/kg i.v.) did not significantly lower the BP or heart rate in either group 30 min after injection. In the same rabbits, on another day, after autonomic blockade (mecamylamine), nebivolol (0.1, 0.3, and 1.0 mg/kg i.v.) right shifted the bolus i.v. isoproterenol tachycardia dose-response curves by dose ratios of 5, 18, and 90 in sham rabbits, respectively, and 5, 11, and 23 in wrap rabbits, respectively, indicating significant cardiac beta 1-adrenoceptor antagonism. In guinea pig isolated right atria pretreated with atropine (1 microM) and desipramine (DMI, 0.1 microM), norepinephrine concentration-response curves were antagonised competitively by nebivolol (3-100 nM), giving a pKb of 7.90. In separate atria without DMI pretreatment, neither nebivolol (100 nM) nor propranolol (100 nM) had any significant effect on the increase in the rate of norepinephrine efflux following electrical field stimulation (0.5-2 Hz, 3 min). These findings suggest that at concentrations of nebivolol that show substantial beta 1-adrenoceptor antagonism, there is no evidence of hypotension or bradycardia nor additional effects on cardiac norepinephrine release. Why nebivolol lowers blood pressure in some species but not in the conscious rabbit is not known.