The effect of cocaine and imipramine on tyramine-induced release of noradrenaline-3H from the rat vas deferens in vitro

6-Nitrodopamine potentiates contractions of rat isolated vas deferens induced by noradrenaline, adrenaline, dopamine and electric field stimulation

6-Nitrodopamine (6-ND) is a novel endogenous catecholamine that is released from the rat isolated vas deferens, and has been characterized as a major modulator of the contractility of rat isolated epididymal vas deferens (RIEVD). Drugs such as tricyclic antidepressants, α1 and β1β2 adrenoceptor blockers, act as selective antagonists of the 6-ND receptor in the RIEVD. In the rat isolated atria, 6-ND has a potent positive chronotropic action and causes remarkable potentiation of the positive chronotropic effects induced by dopamine, noradrenaline, and adrenaline. Here, whether 6-ND interacts with the classical catecholamines in the rat isolated vas deferens was investigated. Incubation with 6-ND (0.1 and 1 nM; 30min) caused no contractions in the RIEVD but provoked significant leftward shifts in the concentration-response curves to noradrenaline, adrenaline, and dopamine. Pre-incubation of the RIEVD with 6-ND (1 nM), potentiated the contractions induced by electric-field stimulation (EFS), whereas pre-incubation with 1 nM of dopamine, noradrenaline or adrenaline, did not affect EFS-induced contractions. In tetrodotoxin (1 μM) pre-treated (30 min) RIEVD, pre-incubation with 6-ND (0.1 nM) did not cause leftward shifts in the concentration-dependent contractions induced by noradrenaline, adrenaline, or dopamine. Pre-incubation of the RIEVD with the α2A-adrenoceptor antagonist idazoxan (30 min, 10 nM) did not affect dopamine, noradrenaline, adrenaline, and EFS-induced contractions. However, when idazoxan (10 nM) and 6-ND (0.1 nM) were simultaneously pre-incubated (30 min), a significant potentiation of the EFS-induced contractions of the RIEVD was observed. 6-nitrodopamine causes remarkable potentiation of dopamine, noradrenaline, and adrenaline contractions on the RIEVD, due to activation of adrenergic terminals, possibly via pre-synaptic adrenoceptors.

Examination of the paradoxical lack of responsiveness of the guinea-pig vas deferens to tyramine

1. The isolated superfused vas deferens from the guinea-pig was more sensitive to the contractile action of noradrenaline than that from the rat. Tyramine had about 10% of the potency of noradrenaline on the rat vas deferens, but had no contractile activity on the guinea-pig vas deferens (potency less than 0.1% of noradrenaline). 2. The response of the guinea-pig vas deferens to noradrenaline was not affected by tyramine in a concentration that produced a clear contractile response of the rat vas deferens. 3. Noradrenaline and tyramine released less radioactivity from guinea-pig than from rat vasa deferentia in which noradrenergic transmitter stores had been labelled with 3H-noradrenaline. In vasa deferentia from either species, tyramine released less radioactivity than noradrenaline. 4. Pretreatment with the monoamine oxidase inhibitor iproniazid increased the amount of radioactivity released by tyramine from vasa deferentia of both species, increased the contractile response of the rat vas deferens to tyramine, and resulted in the appearance of a contractile response to tyramine in the guinea-pig vas deferens. 5. alpha-methyltyramine, which is not a substrate for monoamine oxidase, had about the same potency as tyramine in releasing radioactivity from vasa deferentia of both species, and in contracting the rat vas deferens, but was virtually without contractile activity on the guinea-pig vas deferens. 6. Pretreatment with iproniazid resulted in the appearance of a contractile response to alpha-methyltyramine in the guinea-pig vas deferens. 7. Pretreatment with iproniazid resulted in change in the composition of the radioactivity released by tyramine, with an increase in the proportion of noradrenaline and a decrease in the proportion of deaminated products. There was also an increase in the amount of endogenous noradrenaline released by tyramine. 8. It was concluded that the main factor accounting for the lack of reactivity of the guinea-pig vas deferens to tyramine is intraneuronal metabolism of the noradrenaline displaced by it.

The mechanism of the 3H-noradrenaline releasing effect of various substrates of uptake1: role of monoamine oxidase and of vesicularly stored 3H-noradrenaline

The mechanism of action of indirectly acting sympathomimetic amines was studied in vasa deferentia of unpretreated rats (COMT inhibited), preloaded with 3H-noradrenaline. 1. Concentration-release curves were obtained for 12 unlabelled indirectly acting amines. From differences between these results and those in an accompanying report (involving tissues from rats pretreated with reserpine and pargyline), it is concluded that a "mobilisation" of vesicular 3H-noradrenaline is required for high and sustained rates of outward transport of 3H-noradrenaline from intact adrenergic varicosities. 2. Experiments with a reserpine-like compound (Ro 4-1284) supported the view that a "mobilisation" of vesicular 3H-noradrenaline is required for substantial release. 3. An atypical time course of release and abnormally high rates of release were observed in the presence of excessive concentrations of (+)-amphetamine. Such atypical effects are ascribed to the of basic amines to increase the intravesicular pH. 4. Analysis of the ratio NA/DOPEG (rate of efflux of 3H-noradrenaline/rate of efflux of 3H-DOPEG) indicated that the inward transport (by uptake) of substrates of MAO fails to achieve axoplasmic concentrations which saturate MAO. Inhibition (or saturation) of MAO is not a prerequisite for the initiation of outward transport. 5. A larger fraction of vesicular 3H-noradrenaline is accessible to equireleasing concentrations of (+)-amphetamine (an inhibitor of MAO) than of tyramine (a substrate of MAO). 6. From the present and the accompanying report it is concluded that "substantial and sustained indirect sympathomimetic effects" are to be expected for substrates of uptake which additionally mobilise vesicular noradrenaline. However, this "mobilisation" does not seem to involve a change in intravesicular pH, except at excessive concentrations.

Sympathomimetic effects of pancuronium bromide on the cardiovascular system of the pithed rat: a comparison with the effects of drugs blocking the neuronal uptake of noradrenaline

1. The effects of pancuronium bromide on the cardiovascular system of the pithed rat were examined. Pancuronium had two effects, a short-lasting cardiovascular stimulation following injection and a longer-lasting potentiation of responses to sympathetic nerve stimulation. 2 The initial effect of pancuronium was compared with that of tyramine. The cardioaccelerator but not the pressor responses to both pancuronium and tyramine were significantly reduced following sympathectomy with 6-hydroxydopamine (6-OHDA). 3 The action of pancuronium in potentiating sympathetic nerve responses was compared with that of known blockers of the neuronal uptake of noradrenaline (NA). Pancuronium (1 mg/kg) and cocaine (0.5 mg/kg) potentiated cardioaccelerator and pressor responses to sympathetic stimulation. These effects of pancuronium could be obtained following adrenalectomy and during neuromuscular blockade with gallamine. Pancuronium and uptake blockers potentiated the cardioaccelerator response to NA, reduced the response to tyramine, but did not affect the response to isoprenaline. Pancuronium and uptake blockers potentiated the pressor response to NA, but did not affect the response to tyramine or clonidine. 4 Following sympathectomy with 6-OHDA, pancuronium failed to potentiate cardioaccelerator and pressor responses to NA. 5 These results are discussed in relation to two main cardiovascular effects of pancuronium; an indirect sympathomimetic action and blockade of the neuronal uptake of NA.

Effects of antidepressant drugs on noradrenaline accumulation and contractile responses in the rat anococcygeus muscle

1 The effect of a series of antidepressant drugs on noradrenaline accumulation was studied in the isolated anococcygeus muscle of the rat. 2 The most potent inhibitors of noradrenaline accumulation were nortriptyline, desipramine and protriptyline. Opipramol, trimipramine and iprindole were active only in high concentrations. 3 Contractions of the anococcygeus muscle produced by noradrenaline were strongly potentiated by nortriptyline, desipramine and protriptyline. Other uptake inhibitors were less active in potentiating the noradrenaline response. 4 Nortriptyline, in concentrations that potentiated the action of noradrenaline, reduced or abolished the response to tyramine.

Uptake and release of catecholamines in sympathetic nerve fibres in the spleen of the cod, Gadus morhua

The effects of drugs known from mammalian experiments to interfere with uptake and release of adrenergic transmitters in the sympathetic nerve terminals have been investigated on perfused spleen and isolated spleen strips from the cod. Cocaine and desipramine inhibit the accumulation of 3H-noradrenaline in the perfused spleen, probably by interfering with uptake mechanisms. Both drugs also shift to the left the dose-response curves for noradrenaline and adrenaline on isolated strips, cocaine being most potent. The maximal contraction force of isolated strips also increases after cocaine when noradrenaline is the agonist, but not when methacholine is used or on chronically denervated strips. This effect of cocaine is therefore not due to a general postsynaptic effect. Tyramine and amphetamine both release stored 3H-noradrenaline from the perfused spleen, and tyramine also contracted the spleen strips. In contrast to the situation in mammals, the dose-response curve for tyramine is not affected by cocaine, and tyramine (up to 10(-3) M) present in the bath does not potentiate the dose-response curve for noradrenaline on isolated strips. Different uptake mechanisms for tyramine and noradrenaline into the nerve terminal are therefore suggested for the cod sympathetic fibres. The alpha-adrenoceptor blocking agent phentolamine produces an increase in overflow of label during nerve stimulation at 5Hz in the perfused spleen preloaded with 3H-noradrenaline. An alpha-adrenoceptor-mediated control of the release of catecholamines, similar to that in mammals, appears to be present in the cod.

Nature of neuronal uptake and noradrenaline--adrenaline effects in the isolated rat vas deferens

Uptake mechanisms in the rat vas deferens have not yet been thoroughly investigated. In this study we show that uptake inhibition by cocaine, amantadine and desmethylimipramine induces increases in sensitivity that are greater for adrenaline than for noradrenaline. However, this does not occur when guanethidine is used. These results are clearly in disagreement with the neuronal uptake hypothesis. Uptake mechanisms in the rat vas deferens may be different from those of other tissues.

The effect of ethacrynic acid on the guinea-pig and rat isolated vas deferens

1 The effect of ethacrynic acid (EA) was studied on guinea-pig and rat vas deferens in vitro.2 EA contracted the guinea-pig but not the rat vas deferens in a dose-dependent manner (50-800 mug/ml). Tyramine caused contraction in 10 out of 18 guinea-pig vas deferens; EA caused contraction in 17 of the preparations which did not respond to tyramine. Repeated doses of EA produced tachyphylaxis, but there was no cross tachyphylaxis to tyramine.3 The contractions produced by EA were prevented by phentolamine or reserpine pretreatment and potentiated by cocaine. A low concentration of desipramine (3 ng/ml) potentiated and higher concentrations (0.6 and 3.0 mug/ml) inhibited the response of vas deferens to EA.4 Hexamethonium (100 mug/ml) or atropine (0.1 mug/ml) did not inhibit the effect of EA, excluding the nicotinic and muscarinic receptors as the sites of action.5 The effect of noradrenaline (NA) on the guinea-pig and rat vas deferens was enhanced by EA pretreatment, which may be due to inhibition of NA uptake.6 It is concluded that EA releases NA from guinea-pig vas deferens. The mechanism of release seems to be different from that of tyramine.