Mechanism of efflux of noradrenaline from adrenergic nerves in rabbit atria

Release of 3H-noradrenaline from the rat vas deferens under various in vitro conditions

The release of 3H-(-)-noradrenaline (NA) from rat vas deferens in vitro was examined under various experimental conditions. It was found that in normal and reserpinized vas deferens the release of NA evoked by (+)-amphetamine (5 X 10(-6) M) or low external Na+ (26 mM) was antagonized by imipramine methiodide and desipramine, inhibitors of the NA uptake, but was not dependent on the presence of Ca2+ in the medium and was not antagonized by the potent local anaesthetic agent bethoxycaine. The release evoked by veratridine in reserpinized tissue was antagonized by the uptake inhibitors but was in normal tissue only partially inhibited in presence of Ca2+ but almost completely in absence of Ca2+. The release by high K+ (117 mM)+low Na+ (26 mM) in normal tissue was dependent on the presence of Ca2+ and was antagonized by the muscarinic agonists carbacholine and metacholine and by high concentrations of desipramine. In the reserpinized vasa the corresponding release was not dependent on Ca2+ and was not antagonized by the muscarinic agents but was inhibited by high concentrations of desipramine.

Release of noradrenaline by labetalol in the rat anococcygeus muscle

The effects of labetalol on the accumulation and spontaneous release of (--)-[3H]noradrenaline, and on contractile responses to exogenously applied (--)-noradrenaline were studied in the isolated anococcygeus muscle of the rat. 1. Labetalol (3 x 10(-7)--10(-4)M) inhibited the accumulation of (--)-[3H]noradrenaline. 2. Labetalol (10(-6)--10(-4)M) and guanethidine (6 x 10(-6) M) increased the spontaneous release of [3H] following incubation of the muscle with (--)-[3H]noradrenaline. Nortriptyline (10(-6) M) had no effect on the spontaneous release of [3H], antagonised the increased release of [3H] produced by 6 x 10(-6) M guanethidine but not that observed with 10(-5) M labetalol. Labetalol (5 x 10(-6) M) markedly increased the loss of tritiated deminated metabolites with little change in the loss of (--)-[3H]noradrenaline. 3. Labetalol (10(-7)--10(-5)M), alone or in the presence of 10(-6) M nortriptyline, had no effect on contractile responses to (--)-noradrenaline. 4. Following pretreatment with 6-hydroxydopamine (10(-3)M for 3 h) to deplete endogenous noradrenaline stores, labetalol (10(-7)--10(-5) M) inhibited responses to exogenously applied (--)-noradrenaline. 5. These results suggest that, in the rat anococcygeus muscle, labetalol is a noradrenaline releasing agent and an alpha-adrenoceptor antogonist.

Effect of labetalol on the accumulation and release of noradrenaline in rat ventricle

In rat ventricular tissue, labetalol inhibited the accumulation of (-)-[3H]noradrenaline and released [3H] following preloading with (-)[3H]noradrenaline. Cocaine (30 micrometer) inhibited the release observed with tyramine (5 micrometer) and beta-phenethylamine (5 micrometer) but not that observed with labetalol (5 micrometer). Reserpine pretreatment of the animals abolished the release observed with labetalol (5 and 50 micrometer). Labetalol primarily increased the loss of deaminated metabolites of noradrenaline. It is suggested that labetalol may release (-)-noradrenaline from the vesicles.

Tachyphylaxis to ethacrynic acid in the isolated atrium of guinea-pig and its relation to noradrenaline stores

1 The isolated electrically-paced atrium of the guinea-pig developed a dose-dependent increase in the force of contraction in response to ethacrynic acid (12-100 microgram/ml) which was blocked by pretreatment of the animals with reserpine but was unaffected by desipramine or colchicine added to the bathing medium. 2 There was a rapidly developing tachyphylaxis to repeated doses of ethacrynic acid which was not reversed by rest or incubation of the tissue with noradrenaline. 3 There was no cross tachyphylaxis between ethacrynic acid and tyramine, amphetamine or nicotine. 4 Ethacrynic acid (200 microgram/ml) decreased the noradrenaline content of the atria by 32%. 5 It is concluded that ethacrynic acid exerts its effects indirectly through the release of endogenous noradrenaline and that the mechanism of release seems to be different from that of other known indirect sympathomimetic drugs.

Neuronal norepinephrine as mediator for ouabain-induced smooth muscle contraction

With some latency, ouabain (10(-6)--10(-5) M) induced a long-lasting contractile response of the isolated vas deferens of the guinea-pig. The ouabain-induced contraction was potentiated by pretreatment with tropolone, whereas it was prevented by pretreatment with cocaine, bretylium, reserpine or phentolamine. The response was unaffected byatropine, methysergide and tetrodotoxin. Ouabain, perfused into the isolated tissue, enhanced dose-dependently the release of norepinephrine into the medium. However, this enhancement of norepinephrine release was prevented by reserpine or by removal of Ca from the medium. These findings support the hypothesis that the ouabain-induced contraction of vas deferens might be due to norepinephrine released from the adrenergic nerve ending granules through a Ca2+-dependent process.

On the reduced retention of extravesicular noradrenaline in rabbit atria produced by potassium

The effects of KC1 on the retention of extravesicular (--)-[3H]noradrenaline were investigated. Atria, from reserpine-pretreated rabbits, were exposed to tropolone and pargyline before incubation with (--)-[3H]noradrenaline. After efflux for 50-60 min, exposure of tissues to media containing 65 mM KC1 resulted in increased efflux of (--)-[3H]noradrenaline. This effect was not Ca2+-dependent and was not altered by oxymetazoline, phentolamine, indomethacin or methacholine. The KC1-induced reduction in retention of (-)-[3H]noradrenaline was, however inhibited by cocaine and desipramine, but not by lidocaine. Efflux was not increased by RbCl and CsCl. It is suggested that KCl may have accelerated the efflux of extravesicular (--)-[3H]noradrenaline by increasing a cocaine- and desipramine-sensitive carrier-mediated efflux process.

Efflux of 5-hydroxytryptamine from synaptosomes of rat cerebral cortex

The efflux of 3H-5-HT from a crude synaptosome preparation of the cerebral cortex of reserpinized rats was examined. The synaptosomes were loaded with 3H-5-HT by pre-incubation of the homogenate in presence of pargyline and desipramine in order to inhibit deamination of 5-HT and uptake into noradrenergic neurons. The synaptosomes were collected by centrifugation, washed and resuspended in 0.25 M sucrose. No spontaneous efflux of 5-HT was detectable at 0 degrees C but marked efflux was observed at 27 degrees C and 37 degrees C. 4-Chloroamphetamine, low external Na+ concentration, ouabain and the depolarizing agent veratridine markedly accelerated the initial (5 min) efflux. Inhibitors of the neuronal 5-HT uptake, e.g. chlorimipramine, H 102/09 and A 23189, antagonized the 5-HT efflux evoked by these means, whereas desipramine, which is a poor inhibitor of the 5-HT uptake, had only slight effect on the 5-HT efflux. It is suggested that 5-HT can be actively transported out from the synaptosomes by the reversed 5-HT uptake mechanism.

Effect of veratridine on the fluxes of 3H-noradrenaline and 3h-bretylium in the rat vas deferens in vitro

The effect of the depolarizing agent veratridine on the accumulation and release of of 3H-noradrenaline and 3H-bretylium in in the rat vas deferens in vitro was examined. Veratridine produced inhibition of the accumulation and induced marked release of the amines. In vas deferens from non-reserpinized rats the release of noradrenaline evoked by veratridine was partially antagonized by omission of Ca2+ in the incubation medium and partially inhibited by low concentrations of desipramine. In reserpinized vas deferens the release of noradrenaline like that of bretylium in normal vas was not affected by omission of Ca2+ but inhibited by low concentrations (3-5 X 10(-7) M) of desipramine. Tetrodotoxin and the local anesthetics millicaine and lidocaine antagonized the effect of veratridine on the accumulation and release of the amines, probably due to prevention of the depolarization. High concentration (3 X 10(-5) M) of desipramine had a similar effect on the release of noradrenaline in normal tissue in presence of external Ca2+. It is concluded that noradrenaline is released by veratridine from normal vas deferens by two mechanisms: 1) an exocytosis release, 2) a carrier mediated desipramine sensitive outward transport; In reserpinized tissue the second mechanism is solely responsible for the release of noradrenaline. Bretylium is only released by the second mechanism. It is suggested that the inhibiton of the amine accumulation by veratridine is due to an equilibrium of the influx at a low tissue to medium ratio.

Increase in the coronary vascular resistance by indomethacin in the isolated guinea pig heart preparation in the absence of changes in mechanical performance and oxygen consumption

Experiments were performed in isolated guinea pig hearts, perfused at constant volume. Left ventricular pressure, left ventricular dp/dtmax, coronary vascular pressure and coronary venous pO2 were measured continuously. Indomethacin (1.4 X 10(-6) M) increased coronary vascular resistance by 15% over control (p less than 0.01) without altering the myocardial contractile force. When PGE2 (1.4 X10(-8) M) was subsequently added, it abolished the indomethacin action and lowered the coronary vascular resistance below the pre-indomethacin control value. This increase in coronary vascular resistance caused by indomethacin was also seen when the animals were pretreated with reserpine and/or the adrenoceptor blocking agents phenoxybenzamine and propranolol, but was prevented completely by previous addition of PGE2 (1.4 X 10(-7) M). The results provide evidence for possible involvement of endogenous prostaglandin-like substances in the maintenance of coronary vascular resistance in the isolated guinea pig heart.

The potentiating effects of 4-aminopyridine on adrenergic transmission in the rabbit vas deferens

The effects of 4-aminipyridine on adrenergic transmission in rabbit vas deferens were investigated. 10(-5)-10(-3) M 4-aminopyridine markedly potentiated the sustained, secondary contractile response to transmural stimulation and significantly increased the amount of (+/-)-|3H]-metaraminol released at 5 Hz, but had little effect on responses to exogenous (-)-noradrenaline. 10(-5)-10(-4) M 4-aminopyridine did not significantly alter the accumulation or efflux of (-)-[3H]-noradrenaline in rabbit heart. It is suggested that 4-aminopyridine potentiated responses of rabbit vas deferens to transmural stimulation mainly by increasing transmitter release, possibly as a result of prolongation of the action potential. Responses to transmural stimulation were also potentiated by by 2- and 3-aminopyridine and by 3,4-diaminopyridine but not by pyridine, aniline, 4-dimethylaminopyridine, 4-aminoethylpyridine or 2-aminopyrimidine.

Ouabain-induced release of extraneuronal catecholamine in the isolated guinea-pig vas deferens

It was found that ouabain (10(-5) M) was effective in releasing the extraneuronal catecholamine which was taken up by uptake 2 process in the guinea-pig vas deferens. This result shows that a Na-K ATPase is essential for the storage of catecholamine in the extraneuronal site.

Active transport of 3H-bretylium in the rat vas deferens in vitro

The uptake and efflux of 3H-bretylium in slices of the rat vas deferens were examined. The uptake of bretylium was temperature, Na+ and Cl- dependent, sensitive to 6-hydroxydopamine but insensitive to reserpine. The uptake was inhibited by desipramine, (+)-amphetamine, imipramine methiodide and cocaine, whereas the local anaesthetics millicaine and lidocaine were poor inhibitors. The spontaneous efflux was very slow at 0 degrees C and slow at 37 degrees. Desipramine accelerated the efflux probably by inhibiting the re-uptake of bretylium. (+)-Amphetamine, low external Na+, high external K+ and ouabain caused a pronounced increase in the efflux at 37 degrees C but was almost without effect at 0 degrees C. The efflux evoked by (+)-amphetamine, low Na+ and ouabain was inhibited by desipramine, imipramine methiodide and cocaine, whereas millicaine and lidocaine had very poor effects. The increase of bretylium efflux by hypertonic K+ was only slightly inhibited by desipramine and partially antagonized by omission of Ca2+ in the medium. The results obtained indicate that bretylium was taken up in noradrenergic neurones by the noradrenaline transport mechanism and that the efflux of bretylium produced by (+)-amphetamine, low external Na+, low external Cl- and by ouabain occurred by the same transport mechanism as the uptake but now working in the outward direction whereas the efflux induced by high K+ seemed to occur mainly with another mechanism.

Evidence that the rapid binding of newly accumulated noradrenaline within synaptosomes involves synaptic vesicles

When rat brain synaptosomes were incubated with [3H]noradrenaline for 1 min and then exposed to osmotic shock, only about 20% of the newly accumulated [3H]noradrenaline was released. It would appear that most, but possibly not all of the newly accumulated [3H]noradrenaline is rapidly bound to some particulate cytoplasmic constituent within the synaptosome. [3H]Dopamine and [3H]5-hydroxytryptamine were also rapidly bound within synaptosomes but [3H]glycine and [3H]gamma-aminobutyric acid were not. Reserpinization (5 mg/kg, i.p., 24 h before preparation) only slightly reduced the initial rate of [3H]noradrenaline uptake by synaptosomes. However, when reserpinized synaptosomes were osmotically shocked, most of the newly accumulated radioactivity was released; this radioactivity was identified chromatographically as [3H]noradrenaline. On the basis of the findings with reserpinized preparations, it seems likely that (1) the rapid intrasynaptosomal binding involves synaptic vesicles and (2) the neuronal membrane transport system itself may be capable of driving the uptake of noradrenaline by nerve-terminals. The rapid vesicular binding observed may not be essential for the accumulation of the amine by presynaptic terminals during brief exposures.

The effect of cocaine on the distribution of labelled noradrenaline in rabbit aortic strips and on efflux of radioactivity from the strips

Reserpine-pretreated or normal rabbit aortic strips (whose noradrenaline-metabolizing enzymes were inhibited by treatment with 0.5 mM pargyline and by the presence of 0.1 mM U-0521) were incubated with 1.18 muM labelled noradrenaline for 30 min. Subsequently, strips were either analysed or washed out with amine-free solution for 240 to 250 min. From the efflux curves the original distribution of radioactivity was estimated by compartmental analysis. 1. The accumulation of radioactivity in the strips was reduced by 30 muM cocaine to that observed for nerve-free strips. In reserpine-pretreated strips this was due to a partial reduction of the filling of comartment IV (characterized by a half time of efflux of 10 to 20 min; earlier evidence had already shown that part of this compartment is of neuronal origin; Hensling et al., 1976) and a pronounced decrease of the filling of compartment V (characterized by the longest half time). In normal strips the same changes were observed as well as pronounced reduction of the "bound fraction". Thus, cocaine reduced the filing of those compartments which had been identified as neuronal ones (Henseling et al., 1976). 2. When 30 muM cocaine was added to the wash out solution only, the neuronal efflux of radioactivity from reserpine-pretreated strips was accelerated, while the efflux from extraneuronal or extracellular compartments remained unaffected. This effect of cocaine was the same for (-)- and (+)noradrenaline, and its was not significaltly decreased when paired strips exposed to an inhibitor of extraneuronal uptake (86 muM corticosterone) throughout the experiment. In normal strips, cocaine had very little or no effect on the efflux of radioactivity. 3. The results are consistent with the view that cocaine impairs the influx of amine into the neurone, while block of re-uptake of unchanged amine is one of the determinants of its effect on the rate of efflux from the axoplasm. Comparison of efflux curves with corresponding relaxation curves (determined in the absence and presence of cocaine) indicates that, when both noradrenaline-metabolizing enzymes are inhibited, relaxation of the strips is accounted for by a) the efflux of unchanged amine from neuronal (and extraneuronal) stores and b) the sensitivity of the preparation to noradrenaline.