Studies on the mechanism of the cardiovascualr effects of methyldopa

Chronic ethanol attenuates centrally-mediated hypotension elicited via alpha(2)-adrenergic, but not I(1)-imidazoline, receptor activation in female rats

AIMS

This study dealt with the effect of chronic ethanol administration on hemodynamic responses elicited by alpha(2)-adrenergic (alpha-methyldopa) or I(1)-imidazoline (rilmenidine) receptor activation in telemetered female rats.

MAIN METHODS

The effects of alpha-methyldopa or rilmenidine on blood pressure (BP), heart rate (HR) and their variability were investigated in rats that received liquid diet without or with ethanol (5% w/v) for 12 weeks. To evaluate the effect of each drug on cardiovascular autonomic control (BP and HR variability) in the absence or presence of ethanol, three time-domain indices of hemodynamic variability were measured: (i) standard deviation of mean arterial pressure (SDMAP), (ii) standard deviation of beat-to-beat intervals, and (iii) root mean square of successive differences in R-R intervals.

KEY FINDINGS

In liquid diet-fed control rats, i.p. rilmenidine (600 microg/kg) or alpha-methyldopa (100 mg/kg) reduced BP along with decreases and increases, respectively, in HR. Both drugs had no effect on HR variability but reduced BP variability (SDMAP), suggesting a reduced vasomotor sympathetic tone. Ethanol feeding attenuated reductions in BP and SDMAP evoked by alpha-methyldopa but not by rilmenidine.

SIGNIFICANCE

We conclude that chronic ethanol preferentially compromises alpha(2)- but not I(1)-receptor-mediated hypotension in female rats probably via modulation of vasomotor sympathetic activity. These findings highlight the adequacy of rilmenidine use to lower BP in hypertensive alcoholic females.

Effects of acute and short-term treatment with antihypertensive drugs in sinoaortic denervated rats

Sinoaortic denervation (SAD) produced a marked increase of the systolic blood pressure (SBP). Clonidine (50 micrograms kg-1, i.p.) reduced SBP in SAD but not in sham rats. L-alpha-methyldopa (alpha-MD) (50 mg kg-1, i.p.) also induced a more effective hypotensive action in SAD than sham rats. The withdrawal of alpha-MD in SAD rats after the first treatment was not abrupt and the hypotension persisted for several days, but after the second treatment the withdrawal induced a rapid rebound hypertension. Our results suggest that SAD increases the response to the hypotensive agents. An alteration in the availability of alpha-MD to accumulate or synthesize the active metabolites was also observed after second treatment.

Recent developments in noradrenergic neurotransmission and its relevance to the mechanism of action of certain antihypertensive agents

This report reviews a number of significant developments in the fields of noradrenergic transmission and adrenergic receptors which suggest that, in addition to the classical postsynaptic adrenoceptors, there are also presynaptic adrenoceptors that help modulate the release of norepinephrine (NE) from peripheral as well as central noradrenergic nerve endings during nerve stimulation. In particular, stimulation of presynaptic alpha-adrenoceptors reduces this release of transmitter and the reverse is observed after blockade of these receptors. Clearcut pharmacological differences exist between the postsynaptic alpha 1-adrenoceptors that mediate the responses of certain organs and the presynaptic alpha 2-adrenoceptors that modulate the NE release during nerve stimulation. Therefore, subclassification of alpha-adrenoceptors into alpha 1 and alpha 2 subtypes is warranted but must be considered to be independent of the anatomical location of these receptors. Some noradrenergic nerve endings have also been shown to possess beta-adrenergic receptors, the stimulation of which increases the quantity of transmitter released by nerve impulses. Physiologically, these receptors could be activated by circulating epinephrine (E) and be involved in essential hypertension. A third type of catecholamine receptor found at the noradrenergic nerve ending is the inhibitory dopamine (DA) receptor, which might be of significance in the development of new antihypertensive agents. Application of these new concepts of noradrenergic neurotransmission and the subclassification of alpha-adrenoceptors to the treatment of hypertension is presented. Clonidine, for example, appears to be a potent alpha 2-adrenoceptor agonist; the central receptor involved in its antihypertensive action is pharmacologically an alpha 2-type but located postsynaptically. Clonidine also induces activation of peripheral presynaptic alpha 2-adrenoceptors, which might contribute to its cardiovascular action. The antihypertensive effects of alpha-methyldopa are related to the formation of alpha-methylnorepinephrine, a preferential alpha 2-adrenoceptor agonist, which can stimulate peripheral presynaptic alpha 2-adrenoceptors leading to a decrease of NE release and a reduction in sympathetic tone. Prazosin is a new antihypertensive agent the mechanism of action of which involves a selective blockade of postsynaptic alpha 1-adrenoceptors. This drug does not antagonize several effects of clonidine that are mediated via alpha 2-adrenoceptors. The mechanisms presently considered to account for the antihypertensive activity of beta-adrenoceptor blocking agents are numerous. It is proposed that blockade of peripheral presynaptic facilitatory beta-adrenoceptors could be of significance in the antihypertensive action of these drugs.

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.

Effects of several catecholamines on sympathetic transmission to the myocardium: role of presynaptic alpha-adrenoceptors

Effects of several alpha-adrenoceptor agonists on presynaptic alpha-adrenoceptors were evaluated by studying chronotropic responses to cardioaccelerator nerve stimulation in anesthetized dogs. I.v. infusions of norepinephrine (NE), methylnorepinephrine (MNE), epinephrine (E) or phenylephrine (PHE) in desipramine-treated dogs caused significant attenuation of nerve stimulation responses. The inhibitory influence of all these agents could be prevented by prior treatment with phentolamine, but not with haloperidol. Comparisons of relative pressor and presynaptic inhibitory actions of these compounds revealed that in equipressor doses, MNE caused a significantly greater attenuation of nerve stimmulation responses than NE, while PHE had similar pressor and presynaptic inhibitory activity to that of NE. The inhibition of chronotropic responses to nerve stimulation observed following E was significantly greater compared to its relative pressor effect. These results demonstrate the existence of a presynaptic alpha-adrenoceptor mechanism modulating cardiac rate in intact dogs and indicate that the false transmitter MNE may be more potent than NE in imparing neuronal transmission by an action on presynaptic alpha-adrenoceptors.