Actions of the decarboxylase inhibitor, alpha-methyl-3,4-dihydroxyphenylalanine, in the dog

SOME OBSERVATIONS ON THE PHARMACOLOGY OF ALPHA-METHYLDOPA

alpha-Methyldopa in high concentrations impaired the responses of rabbit isolated ileum and guinea-pig isolated vas deferens to stimulation of the sympathetic nerves and to noradrenaline, but these preparations taken from animals previously treated with alpha-methyldopa showed no sign of impairment. Contractions of the cat nictitating membrane were reduced but not abolished by alpha-methyldopa. In cats, dogs and rats, pressor responses to noradrenaline were usually slightly increased by alpha-methyldopa. Pressor responses to tyramine were not affected consistently. alpha-Methyldopa, alpha-methyldopamine and alpha-methylnoradrenaline behaved like dopa, dopamine and noradrenaline respectively in restoring the responses of tissues from reserpine-treated animals to stimulation of the sympathetic nerves to the rabbit ileum, the guinea-pig vas deferens and the cat nictitating membrane and in restoring responses to tyramine of the cat blood pressure and nictitating membrane, and the rat blood pressure. The potency of alpha-methylnoradrenaline relative to noradrenaline ranged from one-half to one-ninth on various preparations. The results are discussed in relation to the antihypertensive action of alpha-methyldopa.

Alpha-methyl-DOPA induced mydriasis in the cat. Relationship between pupillary response and the oculomotor perfusate concentration of methyldopa and its metabolites

In pentobarbital anaesthetized cats, intravenous administration of 30 mg/kg, alpha-methyl-DOPA produced mydriasis that reached a maximum plateau in 2--2.5 h. The oculomotor nucleus was perfused with saline using a push-pull cannula system chronically implanted over the nucleus. Perfusate samples were collected and subsequently analyzed by liquid chromatography with electrochemical detection (LC-EC). Alpha-methyl-DOPA administration resulted in a gradual build up of alpha-methyl-noradrenaline to approximately 32 mumoles over the 3 h sampling period. In contrast, the concentration of alpha-methyl-dopamine was below the detection level for the first 90 min with peak levels of less than 6 mumoles after 3 h. A linear regression analysis demonstrated a negative correlation (R = 0.90) between the pupil size and the perfusate concentration of alpha-methyl-DOPA and a positive correlation for both alpha-methyl-dopamine and alpha-methyl-noradrenaline (R = 0.88 and 0.94 respectively). Pretreatment with the DOPA-decarboxylate inhibitor, 3-hydroxy-benzyl-hydrazine (NSD-1015; 25 mg/kg, i.p.) completely blocked the mydriatic response to alpha-methyl-DOPA, with neither alpha-methyl-dopamine nor alpha-methyl-noradrenaline reaching detectable levels in the oculomotor perfusate. After treatment with the dopamine-beta-hydroxylase inhibitor, bis (4-methyl-homopiperazinyl thiocarbonyl) disulfide (FLA-63; 2.5 mg/kg, i.p.) there was a significant accumulation of alpha-methyl-dopamine when compared to that obtained in the alpha-methyl-DOPA controls but with no apparent alpha-methyl-dopamine related pupillary dilation. However, the correlation between alpha-methyl-noradrenaline concentration and the increase in the diameter of the pupil was maintained.(ABSTRACT TRUNCATED AT 250 WORDS)

Idiopathic oedema: a study of the effects of bromocriptine

We have studied the effect of bromocriptine in seven patients with idiopathic oedema. Two patients were unable to tolerate the drug; the remaining five were maintained on doses of between 5 and 7.5 mg a day. Symptomatic improvement was noted in two patients and in both of these and one other there was a significant improvement in mean diurnal weight gain. The fall in free water and sodium clearance on standing was reduced by bromocriptine, but there was no significant change in the excessive fall in plasma volume.

Effects of dopamine on haemodynamics and myocardial energetics in man: comparison with effects of isoprenaline and L-noradrenaline

The effects of dopamine (DOPA) on haemodynamics, coronary blood flow and myocardial oxygen requirements were compared with those of L-noradrenaline and isoprenaline (isoproterenol) in patients after open-heart surgery, performed because of mitral valvular and congenital heart disease. The patients were in low cardiac output state but not in shock. DOPA increased heart rate less than isoprenaline, averaging 22 and 31 beats/min, and was less arrhythmogenic. DOPA increased mean arterial pressure by an average of 7 mmHg, whereas isoprenaline had little effect; noradrenaline uniformly increased all measurements of arterial pressure. DOPA and isoprenaline increased cardiac index by an average of 1.01 and 0.94 1 min--1 m--2; noradrenaline did not significantly improve peripheral perfusion. DOPA and isoprenaline decreased systemic vascular resistance by an average of 465 and 549 dynes s cm--5; noradrenaline increased resistance in all patients. For similar cardiac outputs average urine flow increased more with DOPA (75 ml/h) than with isoprenaline (28 ml/h). DOPA increased coronary blood flow and myocardial O2 consumption by an average of 28 and 3.60 ml min--1 100 g--1, noradrenaline by 16 and 1.93 and isoprenaline by 62 and 4.25 ml min--1 100 g--1 respectively. Arterial--coronary sinus O2 differences remained unchanged (normal) with DOPA and noradrenaline and decreased with isoprenaline on average by 1.10 ml/100 ml. Myocardial lactate utilization was normal before and during catecholamine administration. It is concluded that, in its haemodynamic effects, DOPA is intermediate between noradrenaline and isoprenaline. The effects of DOPA on coronary blood flow and myocardial O2 consumption are closer to those of noradrenaline than of isoprenaline. DOPA increase coronary blood flow according to myocardial metabolic demand; it is not a potent primary coronary vasodilator. DOPA, although increasing myocardial O2 consumption more than noradrenaline, is by far less O2-demanding than isoprenaline. DOPA appears to be the superior vasoactive agent among the three catecholamines for the treatment of low cardiac output state in patients with preserved coronary reserve.

Effects of alpha-methyldopa on blood pressure in the anaesthetized dog

Intravenous infusion over 1 h of 20 mg kg(-1) of alpha-methyldopa produced hypotension in the anaesthetized dog. The magnitude of this effect was, however, inversely correlated with bodyweight. Either rapid intravenous injection of alpha-methyldopa or slow infusion of alpha-methyldopate, was less effective in lowering blood pressure than infusion of free alpha methyldopa in dogs of equivalent bodyweight. Infusion of alpha-methyldopa into a vertebral or internal carotid artery produced hypotensive responses but these were no greater and generally less than those obtained to intravenous infusion. alpha-Methyldopa was therefore capable of producing hypotension in the dog but no evidence was obtained for this being the result of an action within the brain.

Studies on the mechanism of the cardiovascualr effects of methyldopa

Oral administration of methyldopa (100 mg/kg, twice daily for 3 days) to mongrel dogs produced a significant decrease in blood pressure and heart rate. The drug treatment affected neither the resting venous tone nor the cardiac output. Thus, the hypotensive effect of the drug was predominantly due to a reduction in total peripheral resistance. Vasoconstrictor responses of the renal vasculature to sympathetic nerve stimulation were significantly impaired after methyldopa at all the frequencies, while mesenteric vasoconstrictor responses to sympathetic nerve stimulation were impaired only at the lower stimulation frequencies. In addition, methylnorepinephrine was a significantly less potent vasoconstrictor than norepinephrine in the renal vasculature, but was equipotent to norepinephrine in the mesentery. The finding of a reduction in the renal vascular resistance of methyldopa-treated dogs, with no such alteration in the mesenteric vascular resistance, is consistent with the nerve stimulation studies. Therefore, the results of the present investigation indicate that in addition to the existing evidence favoring a central site of action for methyldopa, the impairment of peripheral sympathetic neuronal function is also of importance in accounting for the hemodynamic alterations observed following treatment with methyldopa.

Attenuation of postganglionic sympathetic nerve activity by L-dopa

In anesthetized dogs and cats, intravenous L-dopa reduced the increase in mean blood pressure in response to bilateral carotid occlusion by 77.5% ( P <0.01). In the presence of a dopa decarboxylase inhibitor (Ro 4·4602), this effect was absent. Transmission across the superior cervical sympathetic ganglion in the cat was unaltered by the administration of L-dopa. Carotid artery infusion in the dog of 20% of the effective intravenous dose of L-dopa or dopamine failed to inhibit the response to bilateral carotid occlusion. The pressor response to intravenous tyramine in the dog was potentiated ( P <0.05) by L-dopa. In the dog, the increase in femoral vascular resistance to lumbar sympathetic nerve stimulation was attenuated ( P <0.05) by L-dopa while the response to intra-arterial norepinephrine was unchanged. Thus, inhibition of postganglionic sympathetic nerve activity is a possible mechanism by which L-dopa impaired the carotid sinus reflex.

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.

Studies on the Metabolism and Mechanism of Action of Methyldopa

The metabolism and particularly the decarboxylation of methyldopa were studied in hospitalized patients with uncomplicated hypertension. Both unlabeled and radioactive drugs were employed in these studies. Absorption from the intestine, as indicated by plasma levels of the drug and recovery of radioactivity in urine, was shown to be incomplete. The drug disappeared from the plasma with a half-time of less than 2 hours. On the average, about half of the radioactivity administered orally as 2-C 14 -methyldopa could be recovered in the urine. That portion not excreted in the urine appeared in the feces. Urinary excretion of radioactivity was rapid and essentially complete within 24 hours.

Confirming previous studies with dl -α-methyl-dopa, the major urinary excretory products of l -α-methyl-dopa (methyldopa, Aldomet) were found to be the drug itself and conjugates thereof. Small fractions appear as other metabolites, the most significant of which is the decarboxylated derivative, αmethyl-dopamine. The pharmacologically inert d -isomer of α-methyl-dopa was found not to be decarboxylated. Alpha-methyl-dopa hydrazine was shown to reduce significantly the excretion of α-methyl-dopamine during treatment with methyldopa but the effects of the latter drug on blood pressure were not altered. On the basis of unchanged urinary excretions of vanilmandelic acid, decarboxylase inhibition during treatment with methyl-dopa was judged to be inadequate to inhibit endogenous synthesis of norepinephrine.

The relationships between metabolism of the drug and clinical response are discussed. The hypothesis is presented and defended that the hypotensive effects of methyldopa are mediated by amine metabolites of the compound, which are formed following its decarboxylation. Probably norepinephrine depletion is the final denominator but other possibilities exist.

Relationships Between Alterations in Amine Metabolism and Blood Pressure

The principal processes of synthesis and metabolism for aromatic amines have been discussed in terms of their status in essential hypertension and in relation to alterations induced by enzyme inhibitors with hypotensive properties. From a practical standpoint, the clinical use of monoamine oxidase inhibitors and a decarboxylase inhibitor (α-methyldopa) approach the achievement of medical sympathectomy without prohibitive side effects. From a basic standpoint, use of these inhibitors as biochemical tools affords a new perspective into connections between aromatic amine metabolism and blood-pressure regulation. A combined pharmacological-biochemical approach in man will be an important phase of future investigations.