The effects of L-dopa and alpha-methyldopa on reflexes and sympathetic nerve function

Potentiation of the carotid artery occlusion reflex by a cholinergic system in the posterior hypothalamic nucleus

Bilateral occlusion of the common carotid arteries of urethane-anesthetized rats evoked a pressor response of 14 +/- 1 mm Hg. Injection into the lateral cerebral ventricle of neostigmine (0.2-1.0 microgram) or physostigmine (10-15 microgram) caused a dose-dependent increase in basal blood pressure and in the magnitude of the carotid artery occlusion (CAO) pressor reflex. Neostigmine (1 microgram) and physostigmine (15 microgram) caused nearly maximal and approximately equal degrees of cholinesterase inhibition in several brain regions. The recovery of the cardiovascular parameters and of brain cholinesterase activity was significantly faster following physostigmine compared to neostigmine. Prior intracerebroventricular injection of atropine (0.3 microgram) or hemicholinium-3 (20 microgram) prevented the increases in basal pressure and the CAO pressor response. Potentiation of the CAO reflex also followed injection of physostigmine or neostigmine into the posterior hypothalamic nucleus and of injection of physostigmine intravenously. Injection of atropine bilaterally into ther posterior hypothalamic nucleus prior to intravenous injection of physostigmine prevented the potentiation of the CAO reflex but not the increase in basal blood pressure. These results indicate that acetylcholine in the posterior hypothalamic nucleus serves as a neurotransmitter in a pathway which can potentiate the pressor response to carotid artery occlusion and thus modulate baroreceptor reflexes.

Antihypertensive and orthostatic responses to drugs in conscious dogs

In view of the frequent occurrence of orthostatic hypotension in antihypertensive therapy, the orthostatic potential of various classes of drugs was evaluated in conscious normotensive dogs in order to assess the utility of this model. Antihypertensive effectiveness was ascertained initially in hypertensive dogs. Inhibitors of peripheral sympathetic vasoconstrictor mechanism (phentolamine, prazosin, guanethidine) produced marked orthostatic hypotension at antihypertensive doses. In contrast, clonidine did not. The vasodilator hydralazine and, to some extent, minoxidil also caused postural effects. It is concluded that conscious dogs are useful in assessing the orthostatic potential of antihypertensive drugs but that their predictability to humans is not universal.

Comparison of hypotensive, orthostatic and sympathetic inhibitory actions of antihypertensive drugs in rats

Hypotensive and orthostatic activities of a variety of orally administered antihypertensive drugs were concurrently evaluated in conscious spontaneously hypertensive rats and attempts were made to relate these responses to effects on peripheral sympathetic function. The alpha-adrenergic blockers phentolamine and prazosin and the adrenergic neuron blocker guanethidine inhibited compensatory responses to upright tilt at antihypertensive doses. The ganglionic blocker mecamylamine produced milder inhibition. The centrally active agents methyldopa and clonidine and the vasodilator minoxidil did not alter tilt responses appreciably. In contrast, the vasodilator hydralazine exerted marked postural effects. Autonomic interactions were evaluated in pithed rats. alpha-Adrenergic receptor, adrenergic neuron and ganglionic blockers inhibited pressor responses to i.v. phenylephrine and/or to sympathetic nerve activation. Vasodilators did not produce specific effects. The tilt and pithed rat models, when used in conjunction, are very useful in predicting orthostatic potential of drugs in humans although absolute correlation cannot be expected.

Evidence for an inhibitory action of methyldopa on spinal sympathetic reflexes

The antihypertensive activity of methyldopa is mediated primarily by effects on the central nervous system. A number of centrally acting hypotensive substances has recently been shown to depress spinal pathways and reflexes as well as inhibiting supraspinal areas. The present series of experiments evaluated the potential influence of methyldopa on spinal mechanisms. Stimulation of an intercostal nerve evoked a two-component response (early and late) in the splanchnic nerve of anesthetized cats. The "late" phase involves supraspinal neurons, whereas, the "early" component involves only spinal pathways. The i.v. administration of 100 mg/kg of methyldopa reduced systemic blood pressure and both components of the evoked potential progressively. These experiments indicate that methyldopa exerts an effect of spinal sympathetic mechanisms which may contribute to the antihypertensive response to the compound.

Some paradoxical aspects of the cardiovascular pharmacology of alpha-methyldopa

In an effort to help explain why it is often difficult to demonstrate hypotension with alpha-methyldopa on an acute basis in normotensive anesthetized animals, the drug was administered intravenously in single doses of either 50, 100, or 200 mg/kg to chloralose-anesthetized cats while monitoring mean arterial pressure; the systolic pressor response in arterial pressure to bilateral occlusion of the common carotid arteries; hind limb vascular resistance in a vascularly isolated, extracorporeally perfused but neurally intact hind limb; and the response of hind limb vascular resistance to CCO. Alpha-methyldopa failed to cause any significant hypotension and also failed to affect vasomotor tone to the hind limb vasculature, but the drug augmented the pressor response to CCO on MAP and hind limb vascular resistance. Alpha-methyldopa also had no effect on hind limb vascular resistance when added directly to the extracorporeally perfused hind limb vascular resistance when added directly to the extracorporeally perfused hind limb circuit, indicating a lack of direct vascular smooth muscle dilating properties in these preparations. It is postulated that the augmented baroreceptor demonstrated may help th explain why it is difficult to record hypotension on an acute experimental basis with this drug. Such actions of alphamethyldopa might also provide a basis for understanding the development of tolerance to the antihypertensive effects of the drug and instances of paradoxical hypertension with it when these occur in the hypertensive human.

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.

Centrally mediated hypotension and bradycardia by methysergide in anesthetized dogs

In anesthetized dogs, methysergide (1 and 3 mg/kg i.v.) caused reductions in systolic and diastolic blood pressure, heart rate, left ventricular pressure and peripheral resistance. Caardiac output was unchanged because of an increase in stroke volume. Methysergide exhibited no alpha-receptor, ganglion, or adrenergic neuron-blocking properties nor did it have marked direct vasocilator action. The BCO, but not the orthostatic, reflex was severely inhibited by the drug, evidence for a central inhibitory action. Atropine, vagotomy or carotid sinus debuffering had little or no effect on the hypotension and bradycardia produced by methysergide, whereas guanethidine pretreatment essentially abolished these effects. Direct intracerebronventricular administration of small doses of methysergide (0.2 mg/kg) caused significant hypotension and bradycardia. It is concluded that methysergide causes centrally mediated hypotension and bradycardia, the mechanism of which is not clearly understood.

Effect of methyldopa treatment on peripheral sympathetic nerve function in the dog

Administration of methyldopa (100 mg/kg, p.o. twice daily for 3 days) to mongrel dogs produced a significant decrease in the mean blood pressure (MBP) and heart rate (HR). Chronotropic responses to cardioaccelerator nerve stimulation and vasoconstrictor responses to lumbar sympathetic nerve stimulation were significantly attenuated in the treated dogs. Pressor and chronotropic responses to bilateral carotid occlusion (BCO) and tyramine were also markedly reduced following treatment with methyldopa, which is consistent with the clinical findings that chronic methyldopa treatment in hypertensive patients impairs cardiovascular reflexes. Methylnorepinephrine (MNE) was significantly less potent than norepinephrine (NE) in producing an increase in MBP or hindleg perfusion pressure, while the chronotropic and inotropic potency of MNE was about equal to NE. These results indicate that treatment with methyldopa in the dog does indeed produce an impairment of peripheral sympathetic neuronal function to certain organs and this peripheral effect of the drug may be of importance in accounting for its antihypertensive action in the dog.

Central inhibitory effect of alpha-methyldopa on blood pressure, heart rate and body temperature of renal hypertensive rats

The central inhibitory effect of alpha-methyldopa on blood pressure, heart rate and body temperature was studied in conscious renal hypertensive rats. Systemic administration of alpha-methyldopa decreased mean arterial blood pressure and body temperature and caused a short lasting increase in heart rate followed by a long lasting decrease. Inhibition of central decarboxylase activity prevented the decrease in blood pressure, heart rate and body temperature but not the initial increase in heart rate. Inhibition of peripheral decarboxylase activity blocked the increase in heart rate and partially reduced the decrease in heart rate and body temperature but did not affect the decrease in blood pressure. Alpha-Methyldopa also decreased blood pressure at an ambient temperature of 30 degrees C, but the decrease of body temperature was absent and the heart rate remained elevated for 7 hr. Similar results were obtained in normotensive rats. The decrease in heart rate was correlated with the decrease in body temperature in normotensive and renal hypertensive rats. These findings suggest that in the renal hypertensive rat the decrease in blood pressure and in body temperature depends on a central action of alpha-methyldopa metabolites.

An analysis of the peripheral effects of l-DOPA on autonomic nerve function

1 The ability of intravenous L-DOPA to block sympathetic and parsympathetic nerves has been studied in cats and dogs pretreated with a monoamine oxidase inhibitor.2 L-DOPA inhibited positive chronotropic and pressor responses to dimethylphenylpiperazinium (DMPP) and McN-A-343 in dogs, and contractions of the nictitating membrane produced by these ganglion stimulants in cats.3 Responses of the cat nictitating membrane to preganglionic stimulation were inhibited by L-DOPA to a greater extent than those to postganglionic stimulation of the cervical sympathetic chain.4 In dogs, L-DOPA had no vagolytic action, but depressed vasoconstrictor responses elicited in the perfused hind-limb by electrical stimulation of the lumbar sympathetic chain.5 The degree of lumbar sympathetic chain inhibition correlated with the pressor response following L-DOPA, and both effects were prevented by prior decarboxylase inhibition.6 These results suggest that the decarboxylation products of L-DOPA do not impair parasympathetic nerve activity but depress sympathetic nerve function predominantly by inhibiting both muscarinic and nicotinic sites of sympathetic ganglia.