The effects of alpha-methyldopa on renal function in hypertensive patients

How Do Antihypertensive Drugs Work? Insights from Studies of the Renal Regulation of Arterial Blood Pressure

Though antihypertensive drugs have been in use for many decades, the mechanisms by which they act chronically to reduce blood pressure remain unclear. Over long periods, mean arterial blood pressure must match the perfusion pressure necessary for the kidney to achieve its role in eliminating the daily intake of salt and water. It follows that the kidney is the most likely target for the action of most effective antihypertensive agents used chronically in clinical practice today. Here we review the long-term renal actions of antihypertensive agents in human studies and find three different mechanisms of action for the drugs investigated. (i) Selective vasodilatation of the renal afferent arteriole (prazosin, indoramin, clonidine, moxonidine, α-methyldopa, some Ca(++)-channel blockers, angiotensin-receptor blockers, atenolol, metoprolol, bisoprolol, labetolol, hydrochlorothiazide, and furosemide). (ii) Inhibition of tubular solute reabsorption (propranolol, nadolol, oxprenolol, and indapamide). (iii) A combination of these first two mechanisms (amlodipine, nifedipine and ACE-inhibitors). These findings provide insights into the actions of antihypertensive drugs, and challenge misconceptions about the mechanisms underlying the therapeutic efficacy of many of the agents.

Evolution of drugs that preserve renal function

Over the past 50 years, many advances have been made in slowing the progression of renal disease from various causes. These advances have been primarily linked to defining new lower levels for blood pressure goals as well as understanding the importance of proteinuria reduction. To achieve these goals, it is also appreciated that agents that lower blood pressure must also lower proteinuria. This is not true for all antihypertensive drug classes--notably, direct-acting vasodilators, alpha-blockers, and dihydropyridine calcium antagonists. Interestingly, antihypertensive agents that also reduce proteinuria have been associated with cardiovascular risk reduction. Moreover, an understanding of combinations of antihypertensive medications that provide additive reductions in proteinuria may be even more efficacious for slowing renal disease progression. It is hoped that these advances and the projected advances in pharmacogenetics will reduce the current increasing incidence of people going on dialysis.

Current status of putative imidazoline (I1) receptors and renal mechanisms in relation to their antihypertensive therapeutic potential

1. A 'second generation' of centrally acting antihypertensive agents has recently been developed. Unlike the 'first generation' of these agents (e.g. alpha-methyldopa, clonidine, guanabenz), which act predominantly by an agonist action at a alpha 2-adrenoceptors, these agents (e.g. rilmenidine, moxonidine) are believed to exert their antihypertensive effects chiefly by an interaction at putative imidazoline (I) receptors of the I1-type, and so have a reduced profile of alpha 2-adrenoceptor-mediated side effects. There is also evidence from studies in experimental animals that activation of I1-receptors mediates a natriuretic effect. This review evaluates the evidence that they mediate renal effects different from those of alpha 2-adrenoceptors that could contribute to their long-term efficacy. 2. Data from binding studies suggest that I1-binding sites are heterogeneous. There is conflicting evidence concerning whether any of these binding sites are truly receptors. Indeed, the best evidence for the existence of I1-receptors comes from in vivo experiments indicating that imidazoline compounds act at non-adrenoceptor receptive sites in the central nervous system to reduce sympathetic drive and blood pressure. 3. There are a wide range of potential sites and mechanisms through which centrally acting antihypertensive agents can affect renal function, including actions mediated within the central nervous system, heart, systemic circulation and within the kidneys themselves. 'First generation' centrally acting antihypertensive agents cause diuresis and natriuresis in rats, while in dogs and humans a diuresis is often seen with variable effects on sodium excretion. 4. Evidence from studies in anaesthetized rats indicates that rilmenidine and moxonidine can promote sodium excretion by interacting with both central nervous system and renal putative I1-receptors. This does not appear to necessarily be the case in other species. At this time there are few or no published data from clinical studies to suggest that 'second generation' centrally acting antihypertensive agents affect salt and water balance differently from 'first generation' agents.

The evolution of antihypertensive therapy: an overview of four decades of experience

Hypertension is a major public health problem amendable to treatment. Numerous large scale clinical trials have demonstrated that effective, sustained control of elevated arterial pressure to a level below 140/90 mm Hg results in reduced cardiovascular morbidity and mortality. Over the past 4 decades antihypertensive drug therapy has evolved from a stepwise, but physiologically rational, selection of agents to specific programs tailored to individualized therapy for specific clinical situations. This evolution has taken place because of a greater understanding of the pathophysiology of hypertensive diseases, the development of new classes of antihypertensive agents that attack specific pressor mechanisms, and the ability to wed these concepts into a rational and specific therapeutic program. Thus, with the currently available spectrum of antihypertensive therapy, we are now able to select treatment for special patient populations utilizing a single agent and, therefore, we can protect the heart, brain and kidneys and maintain organ function without exacerbating associated diseases. These benefits are clear-cut and have resulted in many millions of patients becoming the beneficiaries of this transfer of careful, painstaking and purposeful investigative experiences into clinical practice.

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.

Clinical pharmacokinetics of methyldopa

Absorption of methyldopa from the gastrointestinal tract is incomplete and variable; bioavailability after oral administration is about 25% (range 8 to 62%). The average time to reach maximum plasma concentration (tmax) [chemically determined] is 2 hours, when the maximum plasma concentration of active drug accounts for 50% of the radioactivity, the remainder representing various metabolites. Physicochemical determination of methyldopa shows that bi-phasic elimination occurs after both intravenous and oral administration, the half-life of the alpha-phase being 0.21 hours (range 0.16 to 0.26 hours) and of the beta-phase 1.28 hours (range 1.02 to 1.69 hours) in normal subjects. Methyldopa is less than 15% protein bound, whereas the primary metabolite, which most probably is the O-sulphate, is about 50% protein bound. The apparent volume of distribution in the central compartment is about 0.23L/kg (range 0.19 to 0.32L/kg), and the total volume of distribution (calculated as Vdarea) is about 0.60L/kg (range 0.41 to 0.72L/kg) in healthy volunteers. Acid-labile conjugates are formed after oral administration. These acid-labile conjugates, in particular the O-sulphate, are probably formed in the intestinal cells, since they are detected in very small amounts after intravenous administration. Additionally, there is a rapid formation of partly unidentified metabolites after both intravenous and oral administration. After intravenous administration the quantitatively most prominent metabolites are methyldopamine and the glucuronide of dihydroxyphenylacetone, but traces of 5 or 6 other metabolites have also been found and identified. These metabolites are probably formed in the liver, but the complete metabolic pattern is still unknown. The renal clearance of methyldopa (95 ml/min/m2) is more than 50% higher than the endogenous creatinine clearance. Renal excretion of some metabolites is slower. Extrarenal elimination accounts for about 50% of the total body clearance of the drug. Renal excretion is very low in patients with renal failure, resulting in accumulation of both active drug and, in particular, its metabolites. There is a marked accumulation of unidentified metabolites in renal failure patients, which possibly explains the strong and prolonged hypotensive action of methyldopa in these patients.

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.

Comparison of the alpha and beta blocking drug, labetalol, and methyl dopa in the treatment of moderate and severe pregnancy-induced hypertension

Twentysix women with pregnancy-induced hypertension have been randomly treated with either labetalol or Aldomet. A more satisfactory control of blood pressure was obtained with labetalol with minimal side-effects. After two weeks of treatment with labetalol renal function had significantly improved with a markedly lower incidence of proteinuria. More patients went into spontaneous labour following labetalol than following Aldomet; the Bishop score was also higher in this group. No adverse effects attributable to labetalol were noted in the baby either ante- or post-natally.