Alpha-Methyldopa in Arterial Hypertension

Comparison of long-term renal hemodynamic effects of methyldopa and propranolol in patients with hypertension and renal insufficiency

Studies were carried out in 15 patients with renal insufficiency and hypertension to compare the long-term effects of methyldopa and propranolol on renal hemodynamics. Inulin and PAH clearance measurements were made under baseline conditions and four to six months of antihypertensive therapy with each of the two drugs. Eight of the 15 patients (group I) were started on methyldopa and then switched to propranolol; and in the other seven (group II), the sequence was reversed. There were no statistical differences in blood pressure or inulin or PAH clearances under baseline conditions between the two groups of patients. Blood pressure was controlled equally with the two drugs in combination with furosemide. In group I, there was no significant effect of either antihypertensive drug on inulin clearance, but PAH clearance was significantly higher during methyldopa than propranolol therapy. In group II, the same higher PAH clearance was found with methyldopa, even though the sequence of drug administration was opposite to that of group I. Challenge with iv furosemide resulted in a greater 3-hour natriuresis during methyldopa than propranolol treatment. The observations indicate that glomerular filtration rate (GFR) is not significantly affected by long-term treatment with methyldopa or propranolol but that renal plasma flow (RPF) is higher during treatment with methyldopa in patients with renal insufficiency and hypertension. The higher RPF apparently enhances the acute natriuretic effect of iv furosemide.

Antihypertensives and their impact on renal function

Antihypertensive agents possess many properties that could cause alterations in renal function. These are: alterations in systemic hemodynamics, changes in the renin-angiotensin aldosterone system, direct intrarenal effects, and alterations in salt and water metabolism. This article reviews the antihypertensive agents in general usage and major points are made concerning potential deleterious effects of methyldopa and nonselective beta-adrenergic blocking drugs on renal function. In particular, recent data are shown concerning the effects of labetalol on renal function indicating the absence of decrements in glomerular filtration rate and renal plasma flow in patients with normal renal function and in patients with mild to moderate renal insufficiency. A possible decrease in these parameters in patients with severe renal insufficiency is presented and discussed.

Review of therapeutic modalities acting directly via central pathways

The brain influences arterial pressure through central mediation of a variety of neurotransmitters, including norepinephrine, and translates this action into changes in peripheral autonomic tone. Two opposed adrenergic systems have been described in brain: a hypothalamic pathway in which adrenergic receptor stimulation raises arterial pressure and a brainstem pathway related to the baroreflex arc in which adrenergic receptor stimulation lowers arterial pressure. Antihypertensive drugs with primarily central effects, including clonidine and alpha methyldopa, act as alpha 2 adrenoceptor agonists. The central receptor involved in their antihypertensive action is of the alpha 2 type but is located postsynaptically. Activation of this receptor by either clonidine or alpha methylnorepinephrine, a metabolite of alpha methyldopa, engages the depressor pathway in the brainstem and leads to a decrease in norepinephrine release and a reduction in peripheral sympathetic tone. Clonidine and alpha methyldopa share a similar pattern of peripheral effects, including reductions in preganglionic sympathetic nerve traffic, bradycardia, decreases in plasma renin activity, reductions in blood pressure in the supine position and adverse effects such as depression, sedation and bad dreams. Because of the frequency and severity of these side effects, there is an ongoing search for new centrally acting antihypertensive agents which might be better tolerated.

Antiadrenergic therapy: special aspects in hypertension in the elderly

The effect of antiadrenergic treatment with methyldopa was studied in 17 patients with established essential hypertension who were subdivided with respect to age in a group younger (n = 10; mean age, 47 +/- 2.4 (SEM) years; and a group older than 60 years of age (n = 7, mean age, 67 +/- 2.8 SEM). The fall in arterial pressure was associated with a significant (p less than 0.05) decrease in cardiac output and heart rate in patients over 60 years of age and no change in total peripheral resistance, whereas a (nonsignificant) fall in resistance occurred in younger patients. In both age groups, a significant (p less than 0.05 and less than 0.01, respectively) decrease in plasma norepinephrine levels was observed, whereas epinephrine and dopamine showed no changes. Pre- and posttreatment values of mean arterial pressure correlated directly with plasma norepinephrine values (r = 0.35 p less than 0.05). Regardless of whether cardiac output was reduced or remained unchanged, renal blood flow, plasma and total blood volume did not change in either group with antiadrenergic treatment. Further, reflexive cardiac changes (responses to isometric exercise and upright tilt) remained qualitatively unchanged. It is concluded that antiadrenergic treatment with methyldopa lowers arterial pressure additionally by decreasing circulating norepinephrine levels. The antihypertensive effect is associated with a fall in peripheral resistance in the younger and a decrease in cardiac output in the older patients, and does not compromise renal blood flow or cardiac reflexive responses.

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.

Effects of reserpine, guanethidine and methyldopa on cardiac output and its distribution

In anaesthetized rats with a renal model of hypertension, effects of reserpine, guanethidine and methyldopa on cardiac output (the dye-dilution method) and its distribution among the heart, the lung, the kidney, the large and small intestine, the stomach, the liver, the spleen, skeletal muscle and skin (the rubidium method) were studied. Several hours administration of all three drugs under study, a definite decrease in cardiac output was observed. Cardiac output then escaped from the influence of the drugs despite a reduction in heart rate. Fall of arterial pressure 24 hr after administration of the drugs was only due to the decrease in total peripheral resistance. Effects of the drugs on distribution of cardiac output were studied at the time when cardiac output was not changed to any significant extent. Under this condition the features common to all the drugs were an increase of the cardiac output fractions to the gastro-intestinal tract and a decrease to the heart and the spleen. This fact indicates that decrease in resistance of gastro-intestinal vascular region plays a basic role in the hypotensive effect of reserpine, guanethidine and methyldopa.

Systolic time intervals in chronic hypertension: Alterations and response to treatment

Systolic time interval studies were performed to evaluate left ventricular performance in 28 patients with untreated systemic hypertension but without clinical heart failure. The pre-ejection period (PEP) was significantly prolonged (p smaller than 0.001) and left ventricular ejectime time (LVET) was shortened (p smaller than 0.02) when compared to rate-corrected predicted values. The PEP/LVET ratio was abnormally high in 18 of the patients and the average ratio was 0.45 ( smaller than 0.001). Eleven patients with abnormal time intervals were restudied during treatment with antilypertensive drugs. The PEP/LVET raio decreased in ten and became normal in nine. The average ratio decreased from 0.49 to 0.41 (p smaller than 0.001), due to both shortening of PEP (p smaller than 0.02) and lengthening of LVET (p smaller than 0.001). These findings indicate that alterations in left ventricular function may occur commonly in chronic hypertension in the absence of clinical heart failure, and can be reversed with appropriate therapy. This technique may be useful in evaluating hypertensive patients and in determining the efficacy of treatment.