Role of renin and aldosterone suppression in the antihypertensive mechanism of clonidine

Effects of clonidine on diuretic response in ascitic patients with cirrhosis and activation of sympathetic nervous system

The effects of the addition of clonidine to diuretics on the mobilization of ascites in the short term (diuretic response and requirement of diuretics) and the long term (readmissions for tense ascites and requirement of diuretics) were examined in patients with cirrhosis and with increased sympathetic nervous system (SNS) activity. We also studied neurohormonal, hemodynamic effects and side effects of clonidine and diuretics. Patients were randomized to receive placebo (group 1, n = 32) or clonidine (0.075 mg) twice daily (group 2, n = 32) for 3 months. After 8 days and for 10 days duration, spironolactone (200 mg/day) was added in both groups. After this period, the dosages of diuretics were individually increased until diuretic response. Responding patients were discharged and followed at the outpatient clinic. During the first hospitalization, the time needed for diuretic response was shorter in group 2 than in group 1. The mean requirement for diuretics was significantly higher in group 1 than in group 2, and the diuretic complications (hyperkalemia and renal impairment) were significantly lower in group 2. Clonidine induced a permanent decrease in SNS activity and delayed decrease in renin/aldosterone levels. During the follow-up, the time to the first readmission for tense ascites was shorter in group 1 than in group 2. Readmissions related to tense ascites or diuretic complications were significantly lower in group 2. The mean requirement for diuretics was significantly higher in group 1 than in group 2. In conclusion, the additional administration of clonidine to diuretics induced an earlier diuretic response associated with fewer diuretic requirements and complications.

Comparative pilot study of repeated large volume paracentesis vs the combination on clonidine-spironolactone in the treatment of cirrhosis-associated refractory ascites

OBJECTIVES

To study the usefulness of the combination of clonidine--spironolactone in refractory ascites.

METHODS

Twenty cirrhotic patients with refractory ascites were randomly assigned to receive repeated large volume paracentesis plus intravenous albumin (group 1), or a combination of clonidine (0.075 mg twice daily) and spironolactone (200 to 400 mg daily) (group 2).

RESULTS

During the first hospitalisation,, the mean weight loss in group 1 was higher than in group 2 (12.4 +/- 3.2 versus 4.3 +/- 1.1 kg, P < or = 0.01). Mean stay in hospital was shorter in group 2 (20 +/- 1.5 versus 10 +/- 2.8 days; P < or = 0.01). Paracentesis did not induce changes in neuro-hormonal measurements. Oppositely, clonidine induced a decreased sympathetic activity, an increased glomerular filtration rate and a delayed reduction of the renin-aldosterone levels. During the follow-up in group 1, the number of rehospitalisations for ascites was higher than in group 2 (37 versus 3; P < or = 0.01), and the mean time to the first readmission was shorter (10 +/- 2.7 versus 23.7 +/- 5.6 days; P < or = 0.01). The total duration spent in hospital were similar in both groups.

CONCLUSION

Paracentesis is more effective for short-term treatment of ascites but clonidine-spironolactone association might provide better long-term control.

Influence of an uremia-associated serum factor on CNS alpha 2-adrenoceptors

The action of blood serum from uremic rats and chronically hemodialyzed patients was investigated for effects on alpha 2-adrenoceptors labeled with 3H-clonidine. Compared to blood sera of rats and patients with normal kidney function, uremic serum significantly inhibited specific 3H-clonidine binding. In saturation experiments the density and affinity of alpha 2-adrenoceptors for 3H-clonidine was lowered by uremic serum. Heating, or trypsin or lipase treatment of the serum did not affect this phenomenon. The effect of the patient's serum could likewise be demonstrated after hemodialysis treatment. The presence of an allosteric regulating substance for clonidine binding to adrenoceptors could at least partially explain the altered and attenuated action of this drug in renal insufficiency.

Clinical pharmacokinetics of clonidine

Clonidine is a centrally active antihypertensive agent effective in the treatment of mild, moderate and severe hypertension, alone or in combination with other drugs. Use of oral clonidine has often been limited by side effects which include dry mouth and drowsiness. Transdermal clonidine was therefore developed as an alternative to oral therapy. Ideally, a drug administered at a constant rate into the systemic circulation should attain steady-state concentrations with less peak-to-trough fluctuation than that associated with intermittent oral dosing. In theory, transdermal administration should thus minimise the adverse effects associated with peak plasma drug concentration, while avoiding the potential for decreased efficacy associated with trough levels. Clonidine has been incorporated into a small, pliable adhesive cutaneous delivery device designed to provide therapeutically effective doses of drug at a constant rate for at least 7 days. The transdermal therapeutic system is a laminate consisting of an external film impermeable to moisture and to the drug, a thin layer of active drug dispersed within a highly drug-permeable matrix, a membrane with a controlled intrinsic permeability regulating the rate of delivery of drug to the skin, and an adhesive coating that attaches the system to the skin surface. The permeation of drug through the skin occurs primarily by diffusion. Application of the clonidine transdermal system to both normotensive and hypertensive subjects has consistently reduced systolic and diastolic blood pressures. Maximum reduction in blood pressure occurs 2 to 3 days after initial application, and is maintained for at least 7 days or until the system is removed. The rate at which clonidine is presented to the skin surface is controlled by the microporous membrane: this rate is the same for all strengths of transdermal clonidine, the amount of clonidine released being proportional to its surface area. Thus, the daily dose is regulated by the area of skin covered. Typically, steady-state plasma concentrations are reached on the fourth day after initial transdermal system application. The lack of dose dependency in half-life and renal clearance estimates emphasise that the transdermal absorption of clonidine is linear. The plasma clonidine concentration produced by a particular transdermal dose varies considerably between individuals as a result of interindividual variation in renal clearance. For this reason, it is recommended that dosages be titrated up from the smallest system (3.5 cm2) until the desired pharmacological effect has been obtained.(ABSTRACT TRUNCATED AT 400 WORDS)

Diuretic treatment alters clonidine suppression of plasma norepinephrine

The effect of short-term diuretic treatment on the action of clonidine was evaluated in eight subjects with mild, uncomplicated hypertension. A single oral dose of clonidine (0.3 mg) was given before and after 1 week of therapy with hydrochlorothiazide, 50 mg, and amiloride, 5 mg, taken daily. Changes in mean arterial pressure, heart rate, plasma norepinephrine and epinephrine levels, and plasma renin activity were assessed. Diuretic treatment caused a significant weight loss, increased plasma renin activity, and reduced serum potassium concentration but did not significantly alter the absolute reduction in mean arterial pressure caused by clonidine. Absolute clonidine-induced reduction in plasma renin activity after diuretic treatment was three times greater than before treatment, although percent changes were similar. Before diuretic therapy, clonidine significantly reduced the level of norepinephrine (absolute and percent change). After diuretic treatment, clonidine failed to suppress norepinephrine, and the difference from prediuretic changes was significant. The level of epinephrine was not altered significantly either by diuretic treatment or clonidine. These results indicate that diuretic therapy alters the clonidine-activated mechanism for reduction of arterial pressure through a shift from overall suppression of sympathetic tone to pathways that are more restricted to renal tone. This shift may be due to changes in fluid or electrolyte balance that alter the action of alpha 2-adrenergic receptor-mediated pathways. Use of the clonidine suppression test for the diagnosis of pheochromocytoma may give false-positive results in diuretic-treated patients.

Role of sympathetic nerve inhibition and body sodium-volume state in the antihypertensive action of clonidine in essential hypertension

A simultaneous analysis of the interrelationships between mean blood pressure (MBP), plasma catecholamines, plasma renin activity (PRA) and aldosterone, exchangeable body sodium, and blood volume was carried out before and after acute and chronic administration of clonidine in 15 patients with essential hypertension, in order to further elucidate the mechanism(s) of action of this drug. After a single oral dose of 200 microgram, clonidine produced a significant fall in MBP, heart rate, plasma norepinephrine (NE), PRA, and aldosterone. There were significant correlations (P < 0.01) between NE and MBP, but not between PRA or aldosterone and MBP both before and after the ingestion of clonidine. Furthermore, there was a significant correlation (P < 0.05) between the magnitude of fall of plasma NE and change in MBP. After 6 weeks of treatment, conidine at a dose of 960 +/- 80 microgram/day produced a significant decrement in MBP and NE, but not in PRA and aldosterone; with upright posture, the magnitude of the fall in MBP was significantly greater (P < 0.01), and the rise in plasma NE was significantly smaller (P < 0.01) after chronic clonidine treatment. There was also a significant fall (P < 0.01) in exchangeable sodium and plasma volume. The data provide evidence that inhibition of the sympathetic nervous system by clonidine plays a major role and that the decrease in exchangeable body sodium and blood volume after chronic treatment may contribute to the antihypertensive action of the drug.

Effect of guanfacine on blood pressure and renin activity in hypertensive patients

1. The effect of treatment with guanfacine for 12 weeks on blood pressure and plasma renin activity was investigated in 24 hypertensive patients (WHO I-III). 2. Systolic, diastolic and mean blood pressure values underwent significant reductions with a constant dose of guanfacine 3 mg daily for 1 week: from 197/115/149 (systolic/diastolic/mean) mmHg before therapy to 166/97/126 mmHg. After 4 weeks of treatment at varying doses, the blood pressure decreased to 157/91/118 mmHg, and after 12 weeks of treatment it levelled off at 147/83/109 mmHg. Heart rate remained essentially unchanged throughout the observation period. 3. During exercise, blood pressure and heart rate increased to the same extent during therapy as before treatment. Orthostatic hypotension was not seen in any of the patients. 4. Renin values decreased under therapy from 3.3 to 2.3 ng ml-1 h-1, but at no time during treatment did this decrease correlate with that of blood pressure. 5. Guanfacine treatment was accompanied by relatively few and mild side-effects.