Extra-vesicular binding of noradrenaline and guanethidine in the adrenergic neurones of the rat heart: a proposed site of action of adrenergic neurone blocking agents.
1 The binding and efflux characteristics of [14C]-guanethidine and [3H]-noradrenaline were studied in heart slices from rats which were pretreated with reserpine and nialamide. 2 Binding of both compounds occurred at extra-vesicular sites within the adrenergic neurone. After a brief period of rapid washout, the efflux of [14C]-guanethidine and [3H]-noradrenaline proceeded at a steady rate. The efflux of both compounds appeared to occur from a single intraneuronal compartment. 3 (+)-Amphetamine accelerated the efflux of [14C]-noradrenaline; this effect was inhibited by desipramine. 4 Unlabelled guanethidine and amantadine also increased the efflux of labelled compounds. Cocaine in high concentrations increased slightly the efflux of [14C]-guanethidine but not that of [3H]-noradrenaline. 5 Heart slices labelled with [3H]-noradrenaline became refractory to successive exposures to releasing agents although an appreciable amount of labelled compound was still present in in these slices. 6 It is suggested that [14C]-guanethidine and [3H]-noradrenaline are bound at a common extravesicular site within the adrenergic neurone. Binding of guanethidine to the extra-vesicular site may be relevant to its pharmacological action, i.e., the blockade of adrenergic transmission. (+info
Long-term effects of N-2-chlorethyl-N-ethyl-2-bromobenzylamine hydrochloride on noradrenergic neurones in the rat brain and heart.
1 N-2-Chlorethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP 4) 50 mg/kg intraperitoneally, produced a long-term decrease in the capacity of brain homogenates to accumulate noradrenaline with significant effect 8 months after the injection. It had no effect on the noradrenaline uptake in homogenates from the striatum (dopamine neurones) and on the uptake of 5-hydroxytryptamine (5-HT) in various brain regions. 2 In vitro DSP 4 inhibited the noradrenaline uptake in a cortical homogenate with an IC50 value of 2 muM but was more than ten times less active on the dopamine uptake in a striatal homogenate and the 5-HT uptake in a cortical homogenate. 3 DSP 4 (50 mg/kg i.p.) inhibited the uptake of noradrenaline in the rat heart atrium in vitro but this action was terminated within 2 weeks. 4 DSP 4 (50 mg/kg i.p.) cuased a decrease in the dopamine-beta-hydroxylase (DBH) activity in the rat brain and heart. The onset of this effect was slow; in heart a lag period of 2-4 days was noted. In brain the DBH-activity in cerebral cortex was much more decreased than that in hypothalamus which was only slightly affected. A significant effect was still found 8 months after the injection. The noradrenaline concentration in the brain was greatly decreased for at least two weeks, whereas noradrenaline in heart was only temporarily reduced. 5 The long-term effects of DSP 4 on the noradrenaline accumulation, the DBH activity and noradrenaline concentration in the rat brain were antagonized by desipramine (10 mg/kg i.p.). 6 It is suggested that DSP 4 primarily attacks the membranal noradrenaline uptake sites forming a covalent bond and that the nerve terminals, as a result of this binding, degenerate. (+info
Studies on the mechanism of action of amantadine.
1 The effect of amantadine hydrochloride on various aspects of catecholamine metabolism in the rat brain has been investigated. 2 Amantadine failed to have any significant effect on brain concentrations of dopamine or noradrenaline even when administered daily for 9 days. 3 Amantadine had no effect on the rate of decline of noradrenaline and dopamine concentrations after alpha-methyl-p-tyrosine. 4 In vitro amantadine inhibited dopamine uptake into synaptosomes only at high concentrations, and caused little release of dopamine from synaptosomes. 5 There is no evidence from these results to suggest that the anti-Parkinsonian effect of amantadine is related to an action on dopaminergic mechanisms. (+info
Further evidence that prostaglandins inhibit the release of noradrenaline from adrenergic nerve terminals by restriction of availability of calcium.
1 Guinea-pig vasa deferentia were continuously superfused after labelling the transmitter stores with [3H](-)-noradrenaline. Release of [3H]-(-)-noradrenaline was induced by transmural nerve stimulation. 2 Prostglandin E2 (14 nM) drastically reduced the release of [3H]-(-)-noradrenaline, while tetraethylammonium (2 mM), rubidium (6 mM), phenoxybenzamine (3 muM) each in the presence or absence of Uptake 1 or 2 blockade, and prolonged pulse duration (from 0.5 to 2.0 ms) all significantly increased the release of [3H]-(-)-noradrenaline per nerve impulse. 3 The inhibitory effect of prostaglandin E2 on evoked release of [3H]-(-)-noradrenaline was significantly reduced by tetraethylammonium, rubidium and prolonged pulse duration, whilst it was actually enhanced by phenoxybenzamine. This indicates that increased release of noradrenaline per nerve impulse does not per se counteract the inhibitory effect of prostaglandin E2. 4 It is concluded that tetraethylammonium, rubidium and prolonged pulse duration counteracted the inhibitory effect of prostaglandin E2 on T3H]-(-)-noradrenaline release by promoting calcium influx during the nerve action potential. The results are consistent with, and add more weight to the view that prostaglandins inhibit the release of noradrenaline by restriction of calcium availability. (+info
Modulation of long-term synaptic depression in visual cortex by acetylcholine and norepinephrine.
In a slice preparation of rat visual cortex, we discovered that paired-pulse stimulation (PPS) elicits a form of homosynaptic long-term depression (LTD) in the superficial layers when carbachol (CCh) or norepinephrine (NE) is applied concurrently. PPS by itself, or CCh and NE in the absence of synaptic stimulation, produced no lasting change. The LTD induced by PPS in the presence of NE or CCh is of comparable magnitude with that obtained with prolonged low-frequency stimulation (LFS) but requires far fewer stimulation pulses (40 vs 900). The cholinergic facilitation of LTD was blocked by atropine and pirenzepine, suggesting involvement of M1 receptors. The noradrenergic facilitation of LTD was blocked by urapidil and was mimicked by methoxamine, suggesting involvement of alpha1 receptors. beta receptor agonists and antagonists were without effect. Induction of LTD by PPS was inhibited by NMDA receptor blockers (completely in the case of NE; partially in the case of CCh), suggesting that one action of the modulators is to control the gain of NMDA receptor-dependent homosynaptic LTD in visual cortex. We propose that this is a mechanism by which cholinergic and noradrenergic inputs to the neocortex modulate naturally occurring receptive field plasticity. (+info
Allyl-containing sulfides in garlic increase uncoupling protein content in brown adipose tissue, and noradrenaline and adrenaline secretion in rats.
The effects of garlic supplementation on triglyceride metabolism were investigated by measurements of the degree of thermogenesis in interscapular brown adipose tissue (IBAT), and noradrenaline and adrenaline secretion in rats fed two types of dietary fat. In Experiment 1, rats were given isoenergetic high-fat diets containing either shortening or lard with or without garlic powder supplementation (8 g/kg of diet). After 28 d feeding, body weight, plasma triglyceride levels and the weights of perirenal adipose tissue and epididymal fat pad were significantly lower in rats fed diets supplemented with garlic powder than in those fed diets without garlic powder. The content of mitochondrial protein and uncoupling protein (UCP) in IBAT, and urinary noradrenaline and adrenaline excretion were significantly greater in rats fed a lard diet with garlic powder than in those fed the same diet without garlic. Other than adrenaline secretion, differences due to garlic were significant in rats fed shortening, also. In Experiment 2, the effects of various allyl-containing sulfides present in garlic on noradrenaline and adrenaline secretion were evaluated. Administration of diallyldisulfide, diallyltrisulfide and alliin, organosulfur compounds present in garlic, significantly increased plasma noradrenaline and adrenaline concentrations, whereas the administration of disulfides without allyl residues, diallylmonosulfide and S-allyl-L-cysteine did not increase adrenaline secretion. These results suggest that in rats, allyl-containing sulfides in garlic enhance thermogenesis by increasing UCP content in IBAT, and noradrenaline and adrenaline secretion. (+info
Sympathetic nerve alterations assessed with 123I-MIBG in the failing human heart.
Norepinephrine (NE) reuptake function is impaired in heart failure and this may participate in myocyte hyperstimulation by the neurotransmitter. This alteration can be assessed by 123I-metaiodobenzylguanidine (MIBG) scintigraphy. METHODS: To determine whether the impairment of neuronal NE reuptake was reversible after metoprolol therapy, we studied 18 patients (43+/-7 y) with idiopathic dilated cardiomyopathy who were stabilized at least for 3 mo with captopril and diuretics. Patients underwent, before and after 6 mo of therapy with metoprolol, measurements of radionuclide left ventricular ejection fraction (LVEF), maximal oxygen consumption and plasma NE concentration. The cardiac adrenergic innervation function was scintigraphically assessed with MIBG uptake and release measurements on the planar images obtained 20 min and 4 h after tracer injection. To evaluate whether metoprolol had a direct interaction with cardiac MIBG uptake and release, six normal subjects were studied before and after a 1-mo metoprolol intake. RESULTS: In controls, neither cardiac MIBG uptake and release nor circulating NE concentration changed after the 1-mo metoprolol intake. Conversely, after a 6-mo therapy with metoprolol, patients showed increased cardiac MIBG uptake (129%+/-10% versus 138%+/-17%; P = 0.009), unchanged cardiac MIBG release and decreased plasma NE concentration (0.930+/-412 versus 0.721+/-0.370 ng/mL; P = 0.02). In parallel, patients showed improved New York Heart Association class (2.44+/-0.51 versus 2.05+/-0.23; P = 0.004) and increased LVEF (20%+/-8% versus 27%+/-8%; P = 0.0005), whereas maximal oxygen uptake remained unchanged. CONCLUSION: Thus, a parallel improvement of myocardial NE reuptake and of hemodynamics was observed after a 6-mo metoprolol therapy, suggesting that such agents may be beneficial in heart failure by directly protecting the myocardium against excessive NE stimulation. (+info
Influence of vesicular storage and monoamine oxidase activity on [11C]phenylephrine kinetics: studies in isolated rat heart.
[11C]Phenylephrine (PHEN) is a radiolabeled analogue of norepinephrine that is transported into cardiac sympathetic nerve varicosities by the neuronal norepinephrine transporter and taken up into storage vesicles localized within the nerve varicosities by the vesicular monoamine transporter. PHEN is structurally related to two previously developed sympathetic nerve markers: [11C]-meta-hydroxyephedrine and [11C]epinephrine. To better characterize the neuronal handling of PHEN, particularly its sensitivity to neuronal monoamine oxidase (MAO) activity, kinetic studies in an isolated working rat heart system were performed. METHODS: Radiotracer was administered to the isolated working heart as a 10-min constant infusion followed by a 110-min washout period. Two distinctly different approaches were used to assess the sensitivity of the kinetics of PHEN to MAO activity. In the first approach, oxidation of PHEN by MAO was inhibited at the enzymatic level with the MAO inhibitor pargyline. In the second approach, the two hydrogen atoms on the a-carbon of the side chain of PHEN were replaced with deuterium atoms ([11C](-)-alpha-alpha-dideutero-phenylephrine [D2-PHEN]) to inhibit MAO activity at the tracer level. The importance of vesicular uptake on the kinetics of PHEN and D2-PHEN was assessed by inhibiting vesicular monoamine transporter-mediated storage into vesicles with reserpine. RESULTS: Under control conditions, PHEN initially accumulated into the heart at a rate of 0.72+/-0.15 mL/min/g wet. Inhibition of MAO activity with either pargyline or di-deuterium substitution did not significantly alter this rate. However, MAO inhibition did significantly slow the clearance of radioactivity from the heart during the washout phase of the study. Blocking vesicular uptake with reserpine reduced the initial uptake rates of PHEN and D2-PHEN, as well as greatly accelerated the clearance of radioactivity from the heart during washout. CONCLUSION: These studies indicate that PHEN kinetics are sensitive to neuronal MAO activity. Under normal conditions, efficient vesicular storage of PHEN serves to protect the tracer from rapid metabolism by neuronal MAO. However, it is likely that leakage of PHEN from the storage vesicles and subsequent metabolism by MAO lead to an appreciable clearance of radioactivity from the heart. (+info