A crossover comparison of clonidine and methyldopa in hypertension

Dose-response relationships with antihypertensive drugs

A variety of antihypertensive drugs have been introduced into clinical practice at excessively high dose. Examples include most thiazide diuretics, propranolol, oxprenolol, atenolol, methyldopa, hydralazine and captopril. These very high doses have usually resulted from studies in which doses have been increased at regular intervals until the desired antihypertensive effect has been achieved or until unacceptable adverse effects have resulted. Frequently the starting doses were too high and the intervals between dose adjustment too short. In many cases these large doses resulted in unnecessary adverse effects--the adverse biochemical effects of thiazide diuretics, nephrotic syndrome, taste disturbances and neutropenia with captopril, the lupus syndrome with hydralazine and the central nervous system effects of methyldopa. Parallel group design with single doses and sufficient statistical power to distinguish between the upper and lower ends of the antihypertensive dose-response relationship should replace the dose-escalating design.

Interaction of berberine with human platelet alpha 2 adrenoceptors

Berberine, an alkaloid, has been found to have a myriad of pharmacological effects including hypotensive, antisecretory, sedative, and antimicrobial effects, some of which are similar to those of clonidine, an alpha 2 adrenoceptor partial agonist. The interaction of berberine with human platelet alpha 2 adrenoceptor was investigated in this study. Berberine was found to inhibit competitively the specific binding of [3H]-yohimbine. The displacement curve was parallel to those of clonidine, epinephrine, norepinephrine, with the rank order of potency (IC50) being clonidine (0.4 microM) greater than epinephrine (7.5 microM) greater than norepinephrine (14.5 microM) = berberine (16.6 microM). Increasing concentrations of berberine from 0.1 microM to 10 microM inhibited [3H]-yohimbine binding, shifting the saturation binding curve to the right without decreasing the maximum binding capacity. In platelet cyclic AMP accumulation experiments, berberine at concentrations of 0.1 microM to 0.1 mM inhibited the cAMP accumulation induced by 10 microM prostaglandin E1 in a dose dependent manner, acting as an alpha 2 adrenoceptor agonist. In the presence of L-epinephrine, berberine blocked the inhibitory effect of L-epinephrine behaving as an alpha 2 adrenoceptor antagonist. These properties are similar to those of clonidine on human platelets, suggesting that berberine is a partial agonist of platelet alpha 2 adrenoceptors. These findings may provide potential mechanisms for the hypotensive, antisecretory, and sedative effects of berberine.

Clonidine withdrawal. Mechanism and frequency of rebound hypertension

1. The frequency and pathophysiology of the clonidine withdrawal syndrome was studied in fourteen hypertensive patients on chronic clonidine therapy. 2. After sudden cessation of clonidine (900 microgram daily) almost all of the patients showed an excessive increase of the heart rate and blood pressure. Seven of the fourteen patients had subjective symptoms, in three severe enough to require interruption of observation by therapeutic intervention 12 to 60 h after the last dose of clonidine. After clonidine withdrawal, NAE increased to abnormally high values in correlation with the blood pressure (P less than 0.01) and heart rate (P less than 0.001), whereas PRA even decreased initially, probably secondary to the rise of the blood pressure, and only rose, although not significantly, 48 h after withdrawal. PRA was not correlated with NAE, heart rate, or blood pressure. 3. It is concluded that the clonidine withdrawal phenomenon is a frequently occurring and potentially dangerous syndrome. Overactivity of the sympathetic nervous system is mainly responsible, without the mediation of the renin angiotensin system. This also explains our experience that adrenergic beta-receptor blocking drugs do not prevent the rise in BP, although they alleviate some of the symptoms.

Clonidine in hypertension: a 6-year review

A study of the use of clonidine in the treatment of patients with hypertension, both in hospital and in the community, during a period of 6 years is reported. Seventy-eight patients (42 males and 36 females) with hypertension were studied during this period both in terms of their immediate response to introduction of clonidine and also the effect of maintenance treatment for periods varying between 2 and 6 years. During the period of study, the hypertension was well controlled in 51 (65.4%) patients. In the remaining 27 patients, clonidine treatment was stopped for a variety of reasons but, despite this, the drug provided effective treatment for period of up to 4 years. Nineteen patients died in the course of the study, but in all these cases the blood pressure remained under adequate control. The clinical response to treatment and the incidence of side-effects are described.

The effects of sympatholytic drugs on the cardiovascular response to tilting in anaesthetized cats

1. Using cats anaesthetized with chloralose and urethane, comparison was made of the abilities of several antihypertensive and sympatholytic drugs to lower systemic blood pressure, and to depress the compensatory cardiovascular responses to bilateral carotid occlusion and to 45 degrees head-up tilting. Similar comparisons were also made of the effects of these drugs on the perfusion pressure of the vascularly isolated autoperfused hindquarters, and the response of this to carotid occlusion and tilting. The effects of bilateral vagotomy and haemorrhage on these responses were also studied. 2. It was found that hypotensive doses of both bretylium and guanethidine (3.0 mg/kg, i.v.) markedly depressed the ability of cats to restore their systemic blood pressure and to constrict their hindquarters vasculature during tilting. Both drugs depressed the carotid occlusion reflex in the systemic, but not in the hindquarters, circulation. Neither propranolol, 2.0 mg/kg, i.v., nor bilaterial vagotomy had any effect on these parameters and haemorrhage sufficient to cause marked hypotension was without effect on the systemic responses to carotid occlusion or tilting. 3. Clonidine (1.0, 5.0 and 25 microgram/kg, i.v.), xylazine (62.5, 125 and 250 microgram/kg, i.v.) and reserpine (0.5 and 2.0 mg/kg, i.v.) all caused considerable hypotension but had no effect on the response to tilting of the systemic circulation, apart from somewhat prolonging recovering time. The highest dose of clonidine moderately depressed the hindquarters perfusion pressure, and the response of this to tilting. 4. Clonidine (5.0 and 25 microgram/kg, i.v.) and xylazine (125 and 250 microgram/kg, i.v.) depressed the systemic pressor responses elicited by the ganglion stimulants DMPP and McN-A-343. This may indicate that the ability of clonidine to prolong the pressure recovery during tilt may be due to impaired peripheral sympathetic transmission. 5. It is concluded that drugs which significantly reduce the compensatory pressure reponses to tilting in anaesthetized cats may also cause postural disturbances in man, whilst drugs which merely prolong the period required for pressure compensation seem much less likely to cause serious clinical impairment of orthostatic reflexes. It appears that the cardiovascular response to bilateral carotid occlusion may not provide a good index of the integrity of orthostatic reflexes.

Pharmacokinetics and side-effects of clonidine

A single oral dose of clonidine 300 microgram was administered to 8 healthy, normotensive subjects and the time course of its plasma concentrations was followed for 24 h. The plasma concentration of clonidine rose to a peak of 1.17 +/- 0.12 ng/ml at about 2 h: the absorption half-life was 0.6 +/- 0.2 h. Elimination followed first order kinetics with a half-life of 7.7 +/- 2.0 h. The correlation between the two most common side-effects of clonidine, sedation and dryness of the mouth, with the time course of its plasma concentrations was highly significant, p less than 0.01. All the subjects complained of severe sedation. During continuous administration of clonidine (75 microgram t.i.d.) for one week a steady state serum level of 0.30-0.35 ng/ml was achieved. One 75 microgram tablet of clonidine raised the serum level to about 0.69 +/- 0.13 ng/ml in two hours. After cessation of dosing, the serum level declined with a half-life of 7.5 +/- 1.5 h. The urinary excretion of unchanged clonidine was found to be about one third of the administered dose in 24 h during continuous administration and in the first 24 h after the single oral dose.

Provision of medicines in a developing country

In many developing countries the money spent on drugs could often be used more effectively to prevent disease. A large proportion of the drug budget of one developing country is spent on expensive proprietary preparations for use mainly in the larger hospitals, draining resources from health care in rural areas. A major factor in determining the country's expenditure on drugs is the promotional activities of pharmaceutical companies; the number of drug company representatives in the country is proportionately five times that in Britain. Many drugs are promoted for diseases for which they are not indicated and in which their use may be hazardous, and information on side-effects and contraindications is inadequate. Information supplied by drug firms to health workers in different countries must be standardised and the purchase and use of drugs in Third-World countries made more appropriate to their needs.

Clonidine withdrawal in hypertension. Changes in blood-pressure and plasma and urinary noradrenaline

Treatment was interrupted abruptly in 6 hypertensive patients receiving clonidine 0-45-5-4 mg daily. Blood-pressure rose to pretreatment levels within 24-48 h of withdrawal and was accompanied by insomnia, headache, flushing, sweating, and apprehension. These symptoms began 18-20 h after the last dose of clonidine. Plasma-noradrenaline levels and urinary catecholamine excretion increased 24-72 h after withdrawal of clonidine. The subjective symptoms were most prominent in patients on higher doses (greater than 1 mg/day) and in those who had previously been receiving treatment with other antihypertensive drugs. One patient on a very low daily dose (0-15 mg) of clonidine had no symptoms and no significant changes in blood-pressure or catecholamine production after drug withdrawal.

A comparative trial of clonidine, propranolol and placebo in the treatment of moderate hypertension

1 A double-blind cross over trial between clonidine, propranolol and a placebo in patients with moderate hypertension has been performed. 2 Thirty-two patients completed the study which consisted of three treatment periods in random order of 3 months each. Patients had their blood pressure recorded by an unbiased observer using a random-zero machine. 3 Both clonidine and propranolol produced a significant reduction in blood pressure (P less than 0.01), which was apparent by the second week of therapy. Propranolol gave a greater reduction in pulse rate than clonidine (P less than 0.01) but clonidine also reduced the pulse rate significantly (P less than 0.05). There was no evidence of postural hypotension on either drug. Side-effects were more common with clonidine but these tended to wear off after several weeks of therapy. 4 Clonidine and propranolol were equipotent in reducing blood pressure, but clonidine has more initial side-effects than propranolol.

The use of clonidine by intramuscular injection in the treatment of hypertension

Clonidine (Catapres) administered intramuscularly in a dose of 150 microng produced a satisfactory reduction in blood pressure in 13 of 16 hypertensive patients. Its effect occurred within five minutes, was maximal at 75 minutes and persisted for five hours. In six patients who received two doses (150 microng and 300 microng), the response was shown to be dose-related. No serious side effects were noted. Intramuscular administration of clonidine thus appears to be safe and effective. It has a place in the management of uncontrolled hypertension when a rapid reduction in blood pressure is undesirable and in the maintenance of blood pressure control when oral therapy cannot be tolerated.

Centrally acting alpha-adrenoceptor agonists in hypertension: mechanisms and their role in therapy

Experimental work in animals has shown that both methyldopa and clonidine lower the blood pressure predominantly by an action on the central nervous system. This action is due to alpha-adrenoceptor stimulation by clonidine and by alpha-methyl-noradrenaline. The evidence for a central site of action for these drugs in man is indirect but persuasive. Both drugs lower blood pressure and heart rate without causing much postural or exercise hypetension. Sympathetic responses like the overshoot of pressure following Valsalva's manoeuvre are reduced but not abolished. Failure of ejaculation in the male is much less common than with drugs that are known to blockade sympathetic adrenergic neurones in the periphery.

Efficacy of antihypertensive drugs

The magnitude of the fall in blood-pressure in response to an antihypertensive drug depends on the level of the pretreatment pressure, and there is a direct relationship between the two in that the higher the pretreatment pressure the greater the fall in pressure in response to treatment. This simple relationship is inherent in the practical situation of titrating the diastolic blood-pressures of a group of hypertensive patients to a predetermined level. It is assumed that notionally the dose of an antihypertensive drug can be increased in all patients until the diastolic pressure is reduced to the predetermined level. When the fall in diastolic pressure (deltaD.P.) is plotted against pretreatment diastolic pressure (P.T.D.P.), the points for all patients lie on a straight line of slope unity and negative deltaD.P.-intercept numerically equal to the predetermined diastolic-pressure level. This straight-line relationship is termed the predetermined ideal response line. Analysis of data from clinical trials shows that, despite the variability inherent in the practical situation, the data appear to conform to this straight-line relationship. The method of assessing the efficacy of antihypertensive agents is essentially a comparison of each experimental point with the theoretical predetermined response line. In its simplest form the method consists in constructing a scatter diagram of deltaD.P. against P.T.D.P. for all patients. Patients can then be classified as responders or non-responders according to their position on the diagram relative to the predetermined response line. This method of assessing the efficacy of antihypertensive agents has several advantages, the most important of which is that it provides a simple method for displaying all the relevant information in a readily comparable form.