Actions of bretylium and guanethidine on the uptake and release of [3H]-noradrenaline

Mechanism of the initial adrenergic effects of bretylium and guanethidine

Treatment with phenoxybenzamine and dichloroisoprenaline prevented the rise of blood pressure, contraction of the nictitating membrane and increase in cardiac contractile force produced by intravenous injections of bretylium and guanethidine in anaesthetized or spinal cats. Treatment with cocaine, imipramine or reserpine reduced the sensitivity to bretylium and guanethidine of the spinal cat. In the spinal cat treated with reserpine, sensitivity to the drugs could be restored by an infusion of noradrenaline. Chlorpromazine also blocked the pressor and nictitating membrane responses to bretylium and guanethidine. The drug effects were unaltered 1 hr after bilateral adrenalectomy. During the intravenous infusion of noradrenaline into spinal cats the pressor responses to bretylium and guanethidine were increased, whilst those to adrenaline and to noradrenaline were decreased. Guanethidine (3 to 5 mg/kg) injected intravenously into the cat caused a sudden relaxation of the rat isolated stomach-strip bathed in blood. In seven similar experiments bretylium (3 to 5 mg/kg) relaxed the strip only once; in the other six experiments there was either no effect (four experiments) or an increase in the tone of the strip (two experiments). It is concluded that the initial adrenergic effects of bretylium and guanethidine are mediated, at least in part, through a release of catechol amines from stores in the effector organ.

Adrenergic neurone blockade and other acute effects caused by N-benzyl-N'N"-dimethylguanidine and its ortho-chloro derivative

N-Benzyl-N'N"-dimethylguanidine sulphate (BW 467C60) and its ortho-chloro derivative (BW 392C60) had adrenergic neurone blocking and sympathomimetic effects resembling those of bretylium and guanethidine in cats, dogs and monkeys, but they were more potent in blocking adrenergic mechanisms in the cat. BW 467C60 was more active than its chloro derivative. Each compound inhibited release of noradrenaline during stimulation of the splenic nerve of cats, and increased smooth muscle responses to adrenaline and noradrenaline. Pressor responses to standard doses of tyramine were also increased except when large doses of BW 467C60 or BW 392C60 were given. The adrenergic neurone block by BW 467C60 was inhibited by dopamine, cocaine and amphetamine in situations in which these amines inhibit the effects of bretylium and guanethidine. In contrast to guanethidine, BW 467C60 and BW 392C60 did not lower the pressor amine content of the iris of cats 24 hr after administration of single doses of the compounds. BW 467C60 depressed the slope of curves relating the frequency of stimuli applied to the cervical sympathetic nerves and the resulting contraction of the nictitating membrane, but the effects of the lower rates of stimulation were preferentially inhibited. Large intravenous doses of BW 467C60 and BW 392C60 blocked autonomic cholinergic mechanisms and caused neuromuscular paralysis of voluntary muscle. These effects were brief, in contrast to the adrenergic neurone blockade. Both BW 467C60 and BW 392C60 were well absorbed from the alimentary tract. In contrast to guanethidine, BW 467C60 did not cause diarrhoea in guinea-pigs.

Mechanism of action of guanethidine

The early hypotensive action of intravenous guanethidine in rabbits, rats and cats anaesthetized with urethane is reversed after pretreatment with iproniazid. The fall in blood pressure following injection of guanethidine in rabbits is reduced after previous administration of reserpine. Reserpine, like adrenalectomy and splenectomy, suppresses the early pressor effect of guanethidine in cats anaesthetized with chloralose. Guanethidine inhibits the action of tyramine and nicotine, but potentiates the effect of noradrenaline on isolated rabbit atria. Guanethidine is also a weak inhibitor of monoamine oxidase activity. The results are discussed and compared with those shown by reserpine. It is concluded that the early effects of guanethidine are mainly due to the release of endogenous catechol amines.

Comparison of bretylium and guanethidine: tolerance, and effects on adrenergic nerve function and responses to sympathomimetic amines

Bretylium depresses the slope of regression lines relating frequency of sympathetic nerve stimulation to magnitude of contractions of the cat nictitating membrane. In contrast, guanethidine and reserpine preferentially abolish responses to low rates of nerve stimulation and cause a roughly parallel shift of the regression lines. The hypersensitivity of the nictitating membranes of cats to intravenous adrenaline or noradrenaline is far greater after a series of small daily doses of bretylium or guanethidine than after single large doses. The maximal sensitivity produced was similar to that after postganglionic sympathetic nerve section and exceeded that produced by ganglion blockade. The development of hypersensitivity to catechol amines is accompanied by some return of responses of the nictitating membranes to sympathetic nerve stimulation despite continued daily administration of bretylium or guanethidine. In cats given bretylium daily, responses to low rates of nerve stimulation become greater than in controls unless the dose of bretylium given subcutaneously is 50 mg/kg or more. When marked hypersensitivity to catechol amines has been produced by giving bretylium or guanethidine daily for 7 or 14 days, the sympathomimetic effects of these compounds are greater. Responses to intravenous dimethylphenylpiperazinium are also increased and the results suggest that even large daily doses of adrenergic neurone blocking agents do not appreciably impair the functioning of the adrenal medulla. The pressor effects of intravenous adrenaline, noradrenaline and dimethylphenylpiperazinium iodide increase less than the corresponding nictitating membrane responses. These results are discussed in relation to tolerance to adrenergic neurone blockade, and differences between the effects of bretylium and guanethidine found in man. Bretylium and guanethidine depress the slopes of the dose-response curves for the pressor and nictitating membrane contracting effects of tyramine. When single doses or a short series of daily doses were given, guanethidine caused more depression of the slopes than did bretylium, but nevertheless large depressions of slope were found after giving bretylium daily for several weeks. The magnitude of the responses can be greater or less than in controls depending on the dose of the sympathomimetic amine, the dose of the adrenergic neurone blocking agent and the duration of its administration. The results suggest that injection of tyramine produces a progressively smaller release of adrenaline or noradrenaline during the daily administration of bretylium (or guanethidine) but that in some test situations this is more than compensated for by the development of hypersensitivity to the catechol amine released. Some corresponding changes in responses to amphetamine and ephedrine are also described.

Mechanism of the positive inotropic responses to bretylium and guanethidine

Isolated, atropinized, rat atria exhibited positive inotropic responses to bretylium, guanethidine and tyramine. These responses were prevented by treatment of the animal with reserpine, or by addition of dichloroisoprenaline to the organ bath. The positive inotropic effects of these compounds on atria from reserpinized animals were restored by incubation of the tissue with noradrenaline. On the basis of these findings it is concluded that the cardiac stimulation by bretylium, guanethidine and tyramine involves the release of catechol amines. The usually reported increase in sensitivity of the myocardium from reserpinized animals to noradrenaline was not observed. The influence of bretylium and guanethidine on cardiac uptake and release of noradrenaline was also studied with the rat. Guanethidine decreased the concentration of catechol amines and inhibited the uptake of exogenous noradrenaline, while bretylium had no effect on either. The decrease in concentration of cardiac catechol amines produced by guanethidine was prevented by treatment of the animal with bretylium or with 1-phenyl-2-hydrazinopropane (pheniprazine), a monoamine oxidase inhibitor.

Evidence for a competitive antagonism of guanethidine by dexamphetamine

After guanethidine had blocked the response of the cat nictitating membrane to sympathetic nerve stimulation, dexamphetamine restored the responses to all frequencies of stimulation. Dexamphetamine antagonized the sympathetic nerve block by guanethidine in the isolated sympathetically innervated rabbit ileum; the evidence suggests that the antagonism was competitive. Dexamphetamine antagonized the sympathetic nerve block by guanethidine in the isolated hypogastric nerve-vas deferens preparation of the guinea-pig. Doses of dexamphetamine, larger than those required to antagonize the blocking action of guanethidine, abolished the responses of the nictitating membrane, ileum and vas deferens to nerve stimulation. Dexamphetamine did not influence the depletion of noradrenaline by guanethidine in the heart and spleen of rabbits. The hypothesis is advanced that both dexamphetamine and guanethidine act on the store of noradrenaline at sympathetic nerve endings.

THE EFFECTS OF DRUGS ON THE UPTAKE OF AMINES BY MAST CELLS

Neoplastic mast cells, taken from an ascitic tumour in mice and incubated in vitro, took up (14)C-labelled 5-hydroxytryptamine and histamine from the medium. Uptake during the first hour gave an approximate measure of the initial rate. The amount of each amine taken up in this time was determined by bioassay and by radioactivity, the two methods giving similar results. The curves obtained by plotting initial rate of uptake against concentration in the medium suggested that the uptake of 5-hydroxytryptamine was by an active process and also by diffusion, whereas uptake of histamine was by diffusion only. The cells also took up (14)C-labelled (+/-)-noradrenaline and tryptamine, apparently by diffusion. The active uptake of 5-hydroxytryptamine was inhibited by lowering the temperature to 25 degrees C or by increasing the pH to 8.9, procedures which had little effect on histamine uptake. The effects of cocaine, imipramine, chlorpromazine, mepyramine, promethazine, phenoxybenzamine, lysergic acid diethylamide, bromolysergic acid diethylamide, methysergide, guanethidine, dichloroisoprenaline and pronethalol on the uptake of amines were examined. In general, any antagonist which inhibited uptake of 5-hydroxytryptamine had little effect on uptake of histamine, and vice versa. Possible ways in which these antagonists produce their effects on amine uptake are discussed. A high concentration of 5-hydroxytryptamine, of tryptamine or of noradrenaline inhibited uptake of histamine, but only tryptamine decreased uptake of 5-hydroxytryptamine. These results, together with those from experiments with antagonists, suggest that there are specific binding sites for 5-hydroxtryptamine in these cells.

AN ELECTROPHYSIOLOGICAL INVESTIGATION OF THE ACTIONS OF SOME AUTONOMIC BLOCKING DRUGS ON TRANSMISSION IN THE GUINEA-PIG VAS DEFERENS

Membrane potentials have been recorded from the guinea-pig isolated vas deferens with intracellular and sucrose-gap electrodes during stimulation of the hypogastric nerve and of intramural nerve fibres. Atropine had no detectable effect on the excitatory junction potentials in response to nerve stimulation or on the spontaneous discharge of small potentials. High concentrations of adrenolytic drugs, acting on alpha-receptors were needed to block the response to nerve stimulation and the spontaneous discharge. During the onset and recovery from yohimbine blockade, junction potentials in response to repetitive stimulation were not sustained. Bretylium initially reduced both the junction potentials and the spontaneous discharge. However, after 30 min exposure, the spontaneous discharge increased in frequency although the response to nerve stimulation was abolished. Block of the junction potentials by procaine was rapid in onset compared with that by bretylium and guanethidine, but the spontaneous discharge was not abolished. These results are discussed in relation to the mechanism of transmission from sympathetic nerve to smooth muscle.

The measurement of norepinephrine clearance and spillover rate into plasma in conscious spontaneously hypertensive rats

The clearance of norepinephrine from plasma and the spillover rate of norepinephrine into plasma were determined in conscious unrestrained spontaneously hypertensive rats by measuring the concentrations of 3H-norepinephrine and norepinephrine in arterial plasma after 90 min of i.v. infusion with 3H-norepinephrine. In 50 conscious spontaneously hypertensive rats treated with saline (control animals), the following basal values were obtained: plasma norepinephrine concentration = 149 +/- 5 pg/ml; plasma epinephrine concentration = 61 +/- 4 pg/ml; norepinephrine clearance = 188 +/- 4 ml min-1 kg-1; and norepinephrine spillover rate = 27.5 +/- 0.8 ng min-1 kg-1. A significant portion of infused 3H-norepinephrine appeared to be cleared from the plasma by the uptake1 process, since desipramine decreased norepinephrine clearance by 32%. The vasodilating agents hydralazine and minoxidil produced dose-related increases in norepinephrine spillover rate and plasma norepinephrine concentration, but the percent increases in norepinephrine spillover rate exceeded the percent increases in plasma norepinephrine concentration because of concomitant increases in norepinephrine clearance, particularly after treatment with minoxidil. The increase in norepinephrine clearance caused by hydralazine and minoxidil probably resulted from the increase in cardiac output and resultant increase in hepatic and/or pulmonary blood flow. Adrenal secretion of norepinephrine did not appear to contribute to the elevation in norepinephrine spillover rate elicited by hydralazine and minoxidil. Chlorisondamine suppressed norepinephrine spillover rate by 77%, in association with a 70% decline in plasma epinephrine concentration, whereas bretylium lowered norepinephrine spillover rate by only 41%, with no change in plasma epinephrine concentration. The decrements in norepinephrine clearance caused by chlorisondamine (-23%) and bretylium (-15%) were more or less proportional to the magnitude of the vasodepression caused by these drugs. Both norepinephrine spillover rate and clearance fell in a dose-related fashion after treatment with clonidine. After treatment with the sympathoinhibitory agents chlorisondamine, bretylium and clonidine, the percent decreases in norepinephrine spillover rate always exceeded the percent decreases in plasma norepinephrine concentration. Based on these observations, we conclude that norepinephrine spillover rate provides a more accurate measurement of the activity of the peripheral sympathetic nervous system than does plasma norepinephrine concentration in conscious spontaneously hypertensive rats.

Release of neuropeptide Y (NPY) induced by tyramine in the isolated vas deferens of rat

The effect of tyramine on the isolated vas deferens of rats was investigated. Tyramine induced a dose-dependent contraction which was blocked by phentolamine and disappeared in adrenergic denervated tissues. In the presence of an antiserum to neuropeptide Y (NPY), the contraction induced by concentrations of tyramine greater than 10 microM was markedly increased. In addition to inducing the release of 3H-norepinephrine (NE), tyramine evoked a concentration-dependent efflux of NPY-like immunoreactivity (NPY-LI) from synaptosomal preparations. This action was not modified either by the removal of calcium ion from the medium or by the pretreatment with tetrodotoxin (0.5 microM). Desipramine suppressed the NPY-LI release induced by tyramine apparently by the inhibition of the uptake of tyramine is suggested by the significant positive correlation between the reduction of 14C-tyramine uptake and the inhibition of NPY-LI release induced by desipramine (r = 0.946). Therefore, we suggest that tyramine does induce the release of NPY from rat vas deferens, in addition to effecting NE secretion.

The noradrenaline rate in the anaesthetized rabbit: facilitation by adrenaline

1. 3H-Noradrenaline was infused intravenously into pentobarbitone anaesthetized rabbits to reach a steady-state plasma 3H-noradrenaline level, from which the noradrenaline plasma clearance was calculated. The plasma level of endogenous noradrenaline was determined simultaneously and the rate of noradrenaline release was then derived. 2. Pargyline, amezinium, desipramine and guanethidine all reduced the noradrenaline plasma clearance. The noradrenaline release rate was decreased by desipramine, guanethidine and clonidine. 3. Adrenaline (6 nmol/kg i.v. twice) enhanced the noradrenaline release rate by 53%. This effect was apparent after the plasma adrenaline has returned to basal levels. The adrenaline levels in sympathetically innervated tissues were elevated at this time. 4. When the rabbits were pretreated with either propranolol HCl (1 mg/kg i.p.) to block beta-adrenoceptors, or desipramine HCl (1 mg/kg i.v.) to block neuronal uptake, the facilitatory effect of adrenaline was abolished. Noradrenaline (6 nmol/kg i.v. twice) had no effect on the noradrenaline release rate. 5. These findings suggest that if the sympathetic transmitter stores contain sufficient adrenaline neuronally released adrenaline may modulate noradrenaline release in vivo by activating facilitatory presynaptic beta-adrenoceptors.

Responsiveness to vasoactive agents of cerebral and mesenteric arteries isolated from control and reserpine-treated dogs

1 Pretreatment of dogs for 20 to 24 h before the start of experiments with reserpine (0.5 mg/kg) depleted noradrenaline from cerebral and mesenteric arteries, the diminution being greater in the latter arteries. 2 Contractile responses of helically-cut strips of cerebral and mesenteric arteries to noradrenaline were unaffected by pretreatment with reserpine. Tyramine-induced contractions of mesenteric arteries were markedly attenuated by reserpine-pretreatment, whereas the contraction of cerebral arteries was not influenced. The contractile response of mesenteric arteries to transmural nerve stimulation or nicotine was abolished by reserpine-pretreatment, but the relaxation induced by nicotine of cerebral arteries contracted with prostaglandin F2 alpha was not affected. Pretreatment with reserpine attenuated the contractions of mesenteric arteries induced by angiotensin II, but did not alter the response of cerebral arteries to 5-hydroxytryptamine. 3 In prostaglandin-contracted cerebral and mesenteric arterial strips, relaxant effects of acetylcholine, isoprenaline and K+ were not significantly influenced by reserpine-pretreatment. 4 It appears that tyramine and nicotine do not release noradrenaline from dog cerebral arteries in amounts sufficient to cause significant contractions. Attenuation of the response to angiotensin II by pretreatment with reserpine is not the result of depletion of noradrenaline from the mesenteric arterial wall but may be due to interference with the mechanism specific to actions of angiotensin II.

Prevention by guanethidine analogues of output of noradrenaline induced by sodium reduction in rabbit ventricular slices

1 The prevention by guanethidine and related agents of the output of noradrenaline induced by low sodium was investigated in rabbit ventricular slices. When external NaCl was reduced, the output of noradrenaline into the medium collected at 30 min intervals, increased and the endogenous levels decreased. These changes induced by replacing sodium with sucrose or choline were not affected either by the omission of calcium and addition of 0.5 mM ethylene glycol-bis(aminoethylether)N,N,N',N' tetra-acetic acid (EGTA) or by an increase in the calcium concentration to 10 mM 30 min before sodium deprivation.2 Guanethidine 4 x 10(-6) and 4 x 10(-5) M and 4-7-exo-methylene-hexahydroisoindoline-ethyl guanidine (No. 865-123) 4 x 10(-5) to 8 x 10(-4) M inhibited, in a dose-dependent manner, increases in output of noradrenaline induced by reduction of sodium to 18 mM, while guanethidine 8 x 10(-5) M and high doses of bretylium produced no inhibition: the latter two released noradrenaline.3 The inhibitory actions of guanethidine 4 x 10(-5) M and No. 865-123 4 x 10(-4) M were prevented by tetracaine 3.3 x 10(-4) M, which per se did not modify the output of noradrenaline induced by 18 mM sodium.4 Accumulation of guanethidine and No. 865-123 in ventricular slices was greater than that noted in striated muscle slices and was dose-, time- and temperature-dependent. Tetracaine 3.3 x 10(-4) M did not prevent the accumulation of guanethidine 4 x 10(-5) M and No. 865-123 1.1 x 10(-6) to 4 x 10(-4) M.5 The guanidine derivatives appear to increase the permeability of adrenergic nerve endings to sodium ions.

Interaction of bretylium and guanethidine on the relaxations of the rat isolated fundal strip preparation, evoked by indirect stimulation

1. Isolated rat stomach fundal strip bathed in Krebs solution containing atropine (1 mug/ml), responded to indirect stimulation by a relaxation which was frequency dependent. These responses were blocked by phenoxybenzamine (6 mug/ml) or phentolamine (8 mug/ml).2. Strips obtained from rats previously treated with reserpine did not show relaxation to indirect stimulation. These responses were therefore adrenergic in nature.3. Bretylium (0.1-100 mug/ml) failed to block the relaxations produced by indirect stimulation, in fact relaxations were potentiated by the drug.4. Guanethidine (10 mug/ml) blocked the relaxations induced by indirect stimulation.5. Guanethidine may be taken up by adrenergic nerves actively since its action is not seen at 12 degrees C.6. Bretylium (10 mug/ml) prevented the actions of guanethidine at 37 degrees C.

Protection against induction of supersensitivity to catecholamines by cocaine

1. Adrenaline, noradrenaline, isoprenaline, tyramine, phentolamine, pronethalol, histamine and acetylcholine were each tested for their ability to prevent cocaine from causing supersensitivity to catecholamines in cat spleen strips in vitro. A high concentration of one of these drugs was added to the bath 5 min before cocaine hydrochloride (10 mug/ml). The effect on subsequent responses to catecholamines was compared with the effect of cocaine in control strips in the absence of an interfering drug.2. Phentolamine completely abolished the potentiating effect of cocaine. Large doses of adrenaline or noradrenaline reduced, but did not completely prevent, potentiation. Tyramine, isoprenaline, pronethalol, histamine and acetylcholine did not prevent potentiation.3. The ability of these drugs to interfere with potentiation does not correlate well with their ability to interfere with uptake of noradrenaline. Interference with uptake by cocaine is therefore unlikely to account fully for potentiation.

The effect of bretylium on intracellular cardiac action potentials in relation to its anti-arrhythmic and local anaesthetic activity

1. The initial effect of bretylium tosylate on isolated rabbit atria was to increase conduction velocity, contraction heights, spontaneous frequency and maximum driven frequency, and to reduce electrical threshold. At concentrations of 200 mg/l. or less, these were the only effects, and were consistent with the known sympathomimetic actions of bretylium.2. At extremely high concentrations, 1,200 and 2,400 mg/l., the initial actions were succeeded by weak quinidine-like effects; reduced conduction velocity, spontaneous and maximum driven frequencies, and rate of rise of action potential. The electrical threshold was raised, but contraction heights were not reduced.3. The local anaesthetic activity of bretylium, measured by reductions in the frog nerve action potential, was 1/90 that of procaine and 1/300 that of propranolol, on a molar basis.4. Acute pretreatment with bretylium, 20 mg/kg intravenously, significantly increased the amount of infused ouabain required before the appearance of the first signs of atrial arrhythmia in anaesthetized guinea-pigs, but did not prevent ventricular arrhythmias.5. Pretreatment with bretylium 30 mg/kg subcutaneously 24 hr, and again 4 hr before ouabain infusion, increased the dose of ouabain inducing atrial irregularity and slightly but significantly reduced the incidence of ventricular fibrillation.

Modulation of noradrenaline incorporation by nerve activities in the rat submaxillary gland

1. Rats were anaesthetized with chloralose and prepared for electrical stimulation of the cervical sympathetic trunk. The effect of such stimulation was studied on the incorporation of intravenously infused [(3)H]noradrenaline, [(3)H]adrenaline or [(3)H]dopamine by the submaxillary gland.2. A train of 200 stimuli every min for 30 min at 20/sec increased the total incorporation of [(3)H]noradrenaline by 92% over the unstimulated side of the gland, at 50/sec by about 200% and at 3.3/sec by about 30%. The incorporation of [(3)H]adrenaline and of [(3)H]dopamine was also increased by sympathetic stimulation.3. The increases in unaltered [(3)H]noradrenaline and its metabolites were proportional to the increase in total radioactivity.4. Mean venous outflow from the gland was decreased by 15-30% by the sympathetic stimulation.5. After elimination of extraneuronal binding of noradrenaline by ligation of the common excretory ducts, nerve stimulation still increased incorporation.6. Inhibition of normetanephrine production by pyrogallol did not antagonize the nerve impulse-mediated increase of [(3)H]noradrenaline incorporation. alpha-Methyltyrosine was also without effect.7. Desmethylimipramine and bretylium completely abolished the nerve impulse-mediated increase of [(3)H]noradrenaline incorporation in concentrations which did not affect the control uptake or abolish adrenergic transmission.8. Reserpine, guanethidine and phenoxybenzamine antagonized both the control and nerve impulse-mediated incorporations of [(3)H]noradrenaline.9. The results indicate that nerve impulses modulate the incorporation of transmitter amine at the neuronal membrane. Possible mechanisms for this modulation are discussed.

Recent advances in the treatment of hypertension

The most exciting recent advances in the treatment of hypertension have been in our appreciation of the probable involvement of the kidney in all forms of hypertensive disease, in our improved understanding of the basic underlying mechanisms in some forms of the disease, and in the clarification of mechanisms of action of some of the most useful drugs. This permits us to diagnose more accurately and to administer therapy more rationally. Treatment of the mild and moderate stages of the disease, although not yet proved to reduce mortality and morbidity, would appear to be justified since pressure control is readily achieved with the relatively non-toxic drugs currently available.

A calcium requirement for release of 3H-guanethidine by sympathetic nerve stimulation

Cat colons were labelled with 3H-guanethidine by close intra-arterial injection. Two hr later the colons were removed and the vascular bed was perfused with Krebs solution containing 0, 2·5 or 5 mM calcium. The spontaneous efflux of 3H-guanethidine was not changed by alterations in the calcium concentration, but stimulation of the post-ganglionic sympathetic nerves failed to enhance the output of 3H-guanethidine unless calcium was present in the perfusion fluid. It is concluded that extracellular calcium is essential for release of 3H-guanethidine by sympathetic nerve stimulation and that the neural release of the inactive transmitter substitute is related to the pharmacological action of the drug.