Adrenergic transmitter changes and response to sympathetic nerve stimulation after differing pretreatment with alpha-methyldopa

Prevention of kidney haemorrhagic necrosis of choline-deficient rats by alpha-methyldopa treatment

In a previous report from our laboratory we have shown a marked increase in the levels of renal catecholamines in choline-deficient rats in comparison to choline-supplemented animals, while the content of acetylcholine remained unchanged. Since the changes in tissue catecholamines occurred before kidney lesions developed, we have suggested that an imbalance between sympathetic and parasympathetic systems plays an important role in the pathogenesis of renal injury in choline-deficient rates, this imbalance being the result of an excess of catecholamines in the kidneys. A series of experiments was then planned to explore this theory further by administering adrenergic blocking agents in an attempt to prevent the development of the renal injury in choline deficiency. We report here our results on the administration of α-methyldopa, a drug that depletes the tissue stores of catecholamines, to choline-deficient and choline-supplemented rats. Young male Wistar rats were divided at random into 4 groups: Group CS, fed a choline-supplemented diet; Group CS + D, fed a choline-supplemented diet and treated with α-methyldopa; Group CD, fed a choline-deficient diet; and Group CD + D, fed a cholinedeficient diet and treated with α-methyldopa. The appropriate groups received daily i.p. injections of α-methyldopa (300 mg/kg body wt). The kidneys of all surviving rats were studied grossly and histologically, and the levels of noradrenaline and adrenaline determined. All animals from Control Groups CS and CS + D showed essentially normal kidneys on gross and light microscopic examination. On the other hand, CD rats showed marked renal injury, while the kidneys lesions of CD + D animals were significantly less pronounced than those of rats from Group CD. The total content of noradrenaline and adrenaline in the kidneys of CD + D and CD rats were not statistically different, although the CD + D animals tended to have lower levels of catecholamines. The content of noradrenaline and adrenaline of rats from Group CD was significantly higher than the corresponding values in CS rats. Also, the total content of renal noradrenaline of CD + D animals was found to be unaltered when compared to that of CS rats, while their content of adrenaline was found to be higher than the corresponding value in CS group. The noradrenaline levels of CS and CS + D rats were similar, but the latter group had a higher renal adrenaline content than the former. These findings, besides confirming our previous observations, clearly show that α-methyldopa afforded a protective effect against the renal injury of choline deficiency, thus giving strong additional support to the theory that the kidney haemorrhagic necrosis of choline deficiency in young rats is probably due to an autonomic imbalance.

Studies on the hypotensive action of alpha-methyldopamine

1. Intraventricular alpha-methyldopamine (50-200 mug) produced a dose-related fall in blood pressure in conscious spontaneously hypertensive rats. Pretreatment with intraventricular 6-hydroxydopamine prevented this hypotensive effect of alpha-methyldopamine. 2. The hypotensive effect of alpha-methyldopamine was prevented by intraventricular injection of phentolamine or desmethylimipramine, but not by intraperitoneal injection of haloperidol. 3. Pretreatment with U-14,624, a selective central dopamine-beta-hydroxylase inhibitor, prevented the hypotensive effect of alpha-methyldopamine. 4. Alpha-methyldopamine was considerably less potent than noradrenaline as a pressor agent in the pithed rat, but noradrenaline and alpha-methylnoradrenaline were found to be equipotent. 5. Alpha-methyldopamine (1-5 mg i.c.v.) reduced pressor responses elicited by electrical stimulation of the midbrain reticular formation in cats anaesthetized with chloralose. 6. It is concluded that the hypotensive action of alpha-methyldopamine in conscious animals involves intact central alpha-adrenergic neurones and a central adrenergic uptake mechanism for the formation of alpha-methylnoradrenaline.

Further evidence for a central hypotensive action of alpha-methyldopa in both the rat and cat

1. alpha-Methyldopa (300 mg/kg i.p.) produced a fall in blood pressure in conscious genetic hypertensive rats. Pretreatment with intraventricular 6-hydroxydopamine prevented this hypotensive effect of alpha-methyldopa, whilst intravenous 6-hydroxydopamine reduced but did not prevent the hypotension.2. The hypotensive effect of alpha-methyldopa was prevented or reversed by intraventricular injection of phentolamine (200 mug/rat).3. Pressor responses obtained by stimulation of the entire sympathetic outflow in the Gillespie & Muir preparation, were unaffected by pretreatment with alpha-methyldopa (300 mg/kg i.p.).4. Vasoconstrictor responses to periarterial nerve stimulation of the isolated renal artery preparation of the rat were markedly reduced by pretreatment with alpha-methyldopa. Furthermore, alpha-methylnoradrenaline was found to have one-eighth the vasoconstrictor potency of noradrenaline in this particular artery preparation.5. Pressor responses obtained by stimulation of the posterior hypothalamus or midbrain reticular formation in the rat anaesthetized with urethane were markedly reduced by pretreatment with alpha-methyldopa. FLA-63, a selective dopamine-beta-hydroxylase inhibitor, prevented the reduction of the pressor responses to hypothalamic stimulation produced by alpha-methyldopa.6. Stimulation of the posterior hypothalamus in the anaesthetized cat caused both an increase in sympathetic nerve activity and a rise in blood pressure. These responses were markedly reduced 3-4 h after the injection of alpha-methyldopa (100 mg/kg i.v.).7. These results strongly suggest that the central actions of alpha-methyldopa are important for its hypotensive effect, although a possible peripheral effect cannot be excluded.

Actions of noradrenaline, other sympathomimetic amines and antagonists on neurones in the brain stem of the cat

1. The effects of (-)-noradrenaline ((-)-NA) and related compounds on brain stem neurones in decerebrate unanaesthetized cats have been investigated using the technique of iontophoretic application from micropipettes.2. Four types of response to (-)-NA have been described. These were short lasting inhibition, long lasting inhibition, excitation, and a biphasic response consisting of short lasting inhibition followed by excitation. A variable amount of desensitization of the excitatory response, but not of inhibitory responses, was observed.3. Experiments in which small currents were used to pass (-)-NA from pipettes with smaller tips did not lead to any appreciable change in the proportions of neurones excited or inhibited.4. A variety of sympathomimetic agonists was tested. Short lasting inhibition was less sensitive than excitation to changes in molecular structure. Long lasting inhibition was more sensitive to molecular change and was not mimicked by some of the agonists which mimicked short lasting inhibition.5. Although agonists without one ring hydroxyl had weaker effects than those with both, compounds in which both ring hydroxyl groups were absent (beta-hydroxyphenylethylamine, ephedrine and amphetamine) mimicked excitation strongly. It is possible that the compounds without both ring hydroxyl groups had some effect other than simple agonistic activity.6. A dissociation was observed between responses to dopamine and (-)-NA. p-Tyramine mimicked dopamine, rather than (-)-NA.7. Neither the alpha-agonist, phenylephrine nor the beta-agonist, isoprenaline mimicked neuronal responses to (-)-NA. The alpha-antagonists phentolamine and phenoxybenzamine and the beta-antagonists dichloroisoprenaline, propranolol and D(-)-INPEA and combinations of propranolol with phentolamine or phenoxybenzamine were ineffective in blocking either excitation or inhibition. Thus, the central receptors appear to be different from peripheral alpha- and beta-receptors.8. The most effective antagonist of excitation was (-)-alpha-methylnoradrenaline. Metaraminol and dihydroergotamine also had some antagonistic activity. None of the compounds tested blocked inhibition. The effects of (-)-alpha-methylnoradrenaline have been discussed in relation to the hypotensive action of alpha-methyldopa.

An investigation of alpha-methyl amino-acids and their derivatives on isolated tissue preparations

1. The ability of alpha-methyl amino-acids and their corresponding amines to restore the sympathomimetic actions of tyramine, and the uptake of the amino-acids and the amines, were studied in isolated tissue preparations obtained from reserpine pretreated animals.2. Tyramine relaxed isolated rat ileum preparations from non-reserpinized rats but not from reserpine treated animals. alpha-Methyldopa, alpha-methylnoradrenaline and lower concentrations of metaraminol restored the responses of reserpinized preparations. alpha-Methyldopamine, alpha-methyl-m-tyrosine, alpha-methyl-p-tyrosine and higher concentrations of metaraminol did not do so. The restoring effect of alpha-methyldopa was blocked by disulphiram. alpha-Methyl-p-tyrosine or alpha-methyl-m-tyrosine blocked the restoring action of alpha-methyldopa but not of alpha-methylnoradrenaline. Cocaine blocked the restoration of responses to tyramine by alpha-methylnoradrenaline but not by alpha-methyldopa. alpha-Methyldopa and alpha-methylnoradrenaline failed to restore responses to tyramine in the presence of sodium-free Tyrode solution.3. Tyramine increased the perfusion pressure in isolated rabbit ear preparations obtained from non-reserpinized animals but was very much less active in preparations obtained from reserpine treated animals. alpha-Methyldopa, alpha-methyl-m-tyrosine, alpha-methylnoradrenaline, alpha-methyl-p-tyrosine and metaraminol restored the effects of tyramine. alpha-Methyldopamine did not do so. The restoring effect of alpha-methyldopa and alpha-methyl-m-tyrosine was blocked by disulfiram. alpha-Methyl-p-tyrosine blocked the restoring effect of alpha-methyl-m-tyrosine.4. Tyramine produced positive inotropic effects in isolated rabbit heart preparations. This was either reduced or absent in preparations obtained from reserpine pretreated animals. alpha-Methyldopa, alpha-methylnoradrenaline, alpha-methyl-m-tyrosine and metaraminol restored the responses to tyramine. alpha-Methyldopamine and alpha-methyl-p-tyrosine did not do so.

On the role of a central adrenergic mechanism in morphine analgesic action

The influence of drugs interfering with brain monoamine metabolism on morphine analgesia of rats was estimated by the tail pressure method. Central adrenergic stimulation produced by apomorphine, cocaine, pyrogallol or amphetamine led to stereotyped behaviour and elevation of the pain threshold. Lower doses of these drugs potentiated morphine analgesic action. Reserpine, iproniazid and disulfiram weakened morphine analgesic action. α-Methyldopa increased morphine action and (±)-tryptophan did not influence it significantly.

Central hypotensive effect of alpha-methyldopa

In cats under chloralose anaesthesia l-α-methyldopa (20 mg/kg) was infused for 1 hr into the left vertebral artery. One to 3 hr after the end of the infusion a gradual and significant lowering of mean arterial blood pressure was observed. The dopamine and noradrenaline contents of the brain were significantly reduced while brain 5-hydroxytryptamine and heart noradrenaline concentrations remained normal. The same low dose of l-α-methyldopa infused into a systemic vein did not affect the blood pressure. However, brain dopamine and noradrenaline were depleted to the same extent as observed after infusion into the vertebral artery. Intravenous infusion of a large dose of l-α-methyldopa (200 mg/kg) did not significantly alter mean arterial blood pressure but lowered brain dopamine, noradrenaline and 5-hydroxytryptamine levels. No effect on heart noradrenaline was observed. Infusion of saline or the d-isomer of α-methyldopa (20 mg/kg) into the vertebral artery had no effect on blood pressure or tissue monoamines.