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A COMPARATIVE STUDY OF THE EFFECTS OF OXPRENOL, ATENOL AND PROPRANOLOL ON SOME PHYSIOLOGICAL PARAMETERS IN HEALTH MAN
1.1 INTRODUCTION AND LITERATURE REVIEW
Norepinephrine, epinephrine and other catecholamines can cause either excitation or inhibition of smooth muscle, depending on the site, the dose and the catecholamine chosen. Norepinephine is the most potent excitatory catecholamine and has correspondingly low activity as an inhibitor. Epinephrine is relatively potent as an excitor and as an inhibitor of smooth muscle. On the basis of such observations, Ahlquist (1948) propounded a hypothesis that two types of adrenoceptors exist: the alpha (α) and the beta (β) adrenoceptors. This classification of receptors has been corroborated by the findings that certain drugs selectively antagonize the alpha – adrenoceptors whereas some other agents selectively antagonize the beta-adrenoceptors. The alpha-adrenoceptors are divided into two types: alpha-1 and alpha-2 adrenoceptors. (Starke, 1977). There are also two types of beta-adrenoceptors: beta-1 and beta-2 adrenoceptors (Lands et al 1967). There are drugs that antagonize the activities of these adrenoceptors either selectively or non-selectively. Hence we have drugs that antagonize both alpha-1 and alpha-2 adrenoceptors (e.g. Phenoxybenzamine) and those that act only on alpha-1 adrenoceptors (e.g. Prazosin). Starke et al (1975; Langer 1977). The beta-adrenoceptor antagonists (beta-blockers) may be divided into beta-1 selective antagonists (e.g. atenolol) and non-selective antagonists with affinity for both beta-1 and beta-2 adrenoceptors (e.g. Propranolol) (Levy and Wilkenfeld, 1969). There are also drugs that antagonize both the alpha and beta-adrenoceptors (e.g. Labetalol). (Brittain and Levy, 1976; Richards and Prichards, 1978).
The first drug shown to produce a selective blockade of beta-adrenoceptors was dichloroisoproternol (DCI) (Powell and Slater, 1958). Studies with DCI made a substantial contribution to the understanding of effects mediated by beta-adrenoceptors, but it was not used in man, largely because it has a prominent beta-receptor stimulant action; that is, it is a partial agonist. The initial report of the hypotensive effect of beta-adrenoceptors blocking drugs appeared in 1964 (Prichard). Pronethalol was administrated to some hypertensive patients and then to some normotensive patients who were being treated for angina pectoris. A significant fall in blood pressure was recorded after three months therapy. However, pronethalol was withdrawn because of its carcinogenic effects in mice. The use of Propranolol in hypertension was described later the same year.
The cardioselective or beta-1-adrenoceptor antagonists include atenolol, and metoprolol. It is important to remember that the selectivity of the beta-1-blockers is not absolute; larger doses of these compounds will inhibit all beta-adrenoceptors (Prichard, 1978). The non-selective beta-adrenoceptor antagonists include propranolol, nadolol, oxprenolol, alprenolol, penbutolol, sotalol and timolol. The beta-blockers in each group may exert two types of effects namely: membrane stabilizing activity and intrinsic sympathomimetic activity.
It has been considered that large doses of some beta-blockers have direct ‘depressing action’ (‘quinidine-like effect’, ‘membrane action’ local anaesthetic effect), on the hear in man. A membrane action is seen with large doses in animals (Coltart, Gibsonand Shand, 1971). It however has no relevance to treatment of hypertension and is certainly not responsible for the haemodynamic effects of beta-adrenoceptor antagonists since no significant membrane stabilizing activity occurs with clinically effective doses of beta-blockers. The quinidine-like effects of beta blockers appear to contribute little to the treatment of cardiac arrhythmias. Effective concentrations of propranolol in blood are below those that cause membrane stabilization. In addition beta-blockers without this property are also effective anti-arrhythmic agents (Black and Prichard, 1973; Shand, 1975).
The clinical significance of Intrinsic Sympathomimetic Activity (ISA) is controversial. From a theoretical point of view it has been argued that the beta-blockers with ISA for example oxprenolol and pindolol, may protect against development of cardiac failure (Imhof, 1976). Others have regarded ISA as a side effect limiting the use of the drugs (Waal – Manning, Simpson, 1975). It has been shown, however, that in the treatment of hypertension and angina pectoris, drugs with ISA are as effective as beta-blockers without this property, for example atenolol and propranolol (Harry, 1975; Prichard, 1970; Thadani et al, 1980). In healthy subjects ISA modified the haemodynamic response to beta-blockers, in that heart rate and cardiac output were less with atendlol and propranolol than when beta-blockers with ISA were used (Svendsen et al, 1980). Beta-blockers with ISA do not change the total peripheral resistance index. Beta-blockers without ISA, on the other hand, increase the total peripheral resistance, reflecting peripheral vaso-constriction (Svendsen et al, 1981). The receptors involved in this action are the peripheral beta-2-adrenoceptors.
Occasionally today one talks of the various generations of beta-blockers. The ratio of beta-1- to beta-2- blocking potency has been calculated as well as the significance of intrinsic sympathomimetic activity or of membrane stabilizing effect. Some of these attempts have demonstrated, to an extent, the superiority of one or the other representative of the whole group in the treatment of high blood pressure.
1.2 MECHANISM OF ACTION OF BETA-ADRENOCEPTOR BLOCKERS
The beta-adrenoceptor blocking drugs appear to lower the blood pressure as a result of a variety of mechanisms involving beta-adrenoceptor blockade. Blood pressure falls regardless of the presence or absence of the associated properties. There have been a number of hypothesis to explain the hypotensive effect of beta-adrenoceptor blocking agents, including a direct action on the Central Nervous System (CNS), adrenergic neurone blockade, anti-renin activity, an increase of vasodilator prostaglandins, effects secondary to reduced cardiac output and resetting of baroreceptors secondary to reduced pressor peaks from the reduction in cardiac activity.
Some observation in animals suggest a possible central mode of action. Intra-arterial injection of propranolol into either the carotid or vertebral artery in the anaesthetized dog produces a fall in blood pressure of more rapid onset than if administered via the femoral artery (Offferhaus and Van Zwieten, 1974). Several beta-adrenoceptor antagonists (propranolol, atenolol, oxprenolol, pindolol, practolol and sotalol) when injected into the cerebral ventricle of conscious normotensive cats have been shown to produce a transient rise followed by a fall in blood pressure (Day and Roach, 1974).
Some beta-adrenoceptor blocking drugs when administered systemically penetrate the CNS and concentrations rapidly achieve a steady state. However, in cats, for example, there was no evidence of accumulation in the cerebrospinal fluid following two weeks administration of oxprenolol compared to acute administration (Offerhaus and Van Zwieten, 1974). Other experiments in cats using equivalent hypotensive doses of metoprolol and atenolol have revealed up to a nine-fold higher concentration of metoprolol compared to lipid insoluble atenolol (Offerhaus et al, 1975).
While beta-adrenoceptor drugs can influence central sympathetic activity it is unlikely that this is the only explanation for their anti-hypertensive action in man. Beta-adrenoceptor blocking drugs, such as sotalol, that apparently do not cross the CNS still have an effective antihypertensive effect in man.
The adrenergic neurone blocking effect that has been observed in rats and rabbits has been found not only from racemic propranolol but also with the d (+) isomer which is devoid of antihypertensive effects in man. The fact that propranolol and other beta-adrenoceptor antagonists reduced blood pressure in man without producing the postural hypotension characteristic of the adrenergic neurone blocking drugs (Prichard and Gillam, 1969) also indicates that an adrenergic neurone inhibition is not important.
iii. Renin Blocking Activity
Beta-adrenoceptor inhibition lowers plasma renin in normotensives and hypertensives (Buhler et al, 1972) although the beta-adrenoceptor stimulation is not the only factor involved in rennin release (Bravo et al, 1974).
Buhler et al (1972) subdivided their hypertensive patients prior to treatment into high, normal and low rennin groups according to their ambulant peripheral rennin levels and daily urinary sodium excretion. 540mg Propranolol was given daily to all the patients. After a week’s therapy it was found that the 12 high renin patients showed the greatest falls in blood pressure, the 14 low renin level patients failed to show a significant fall in pressure while the 23 patients with normal renin levels had intermediate falls of blood pressure. There was a good overall correlation between the fall in blood pressure and plasma renin activity. Propranolol, oxprenolol and cardio selective agents atenolol and metoprolol and cardio selective agents atenolol and metoprolol were studied in a further series of 137 patients (Buhler et al, (1975) and the results of the previous investigators (Buhler et al, 1972) were confirmed.
On the other hand, there is considerable evidence against the effects on renin being the predominant mechanism of the antihypertensive action of beta-blockers. Several investigators have failed to find a relationship between the fall in blood pressure and pretreatment levels of plasma renin (Leonetti et al, 1975; Morgan et al, 1975). Morgan et al (1975) observed similar falls in blood pressure with low, normal and high renin patients. Additionally, relatively small doses of propranolol whichc suppress plasma renin levels have little effect on blood pressure. It has been suggested that renin suppression may be important in lowering blood pressure when beta-adrenoceptor antagonists are given to patients taking vasodilators as they do reverse the rise in renin which results from the ingestion of vasodilators (Pettinger and Mitchell, 1976).
In conclusion, while the fall in blood pressure in some patients with high renin levels may be due, at least in part, to the antirenin activity of beta-blockers, overall, the present evidence cannot be regarded as more than indicating the possibility that the antirenin activity of beta-blockers is the main mechanism of their anti-hypertensive effect.
Tarazi et al (1971) observed that propranolol reduced plasma volume in a series of 14 hypertensive patients although this failed to correlate with the fall in blood pressure. Blood pressure fall in some patients without a fall in plasma volume. Julius et al (1972) noted a fall in plasma volume after acute intravenous propranolol but as others had shown before, blood pressure did not fall. Other investigators have found an increase in plasma volume after the administration of beta-adrenoceptor blocking drugs. Propranolol 30mg a day increased plasma volume after one month despite a fall in blood pressure (Gordon, 1976). Larger doses of propranolol (40mg 3 times daily) produced inconsistent effects with a fall in blood pressure in seven (7) and no change or an increase in the remaining six (6) patients. (Brave et al, 1975). The evidence therefore suggests that it is unlikely that changes in plasma volume are of major importance in the hypotensive action of beta-adrenoceptor antagonists.
Other mechanisms which has been proposed for the action of beta-blockers include increase of vasodilator prostaglandins, effects secondary to reduced cardiac output and resetting of baro-receptors secondary to reduced pressor peaks from the reduction in cardiac activity.
The beta-blockers after a hesitant start have now become first line drugs in the treatment of Caucasian hypertensive patients and their use is continuing to increase. How they lower the blood pressure is yet to be ascertained. Drugs with such varied actions, as beta-blockers, may well lower blood pressure by more than one mechanism. In some cases one of the many effects may be of overriding importance e.g. reducing plasma renin in patients with high plasma renin level.
Oxprenolol is a non selective beta-adrenoceptor antagonist. It exhibits Intrinsic Sympathomimetic Activity (ISA) and also has a considerable membrane stabilizing effect which in potency is about half of lignocaine.
Schlesinger and Barzilay (1980) working on the effect of oxprenolol on patients with essential hypertension found that there was a significant reduction in heart rate and systolic and diastolic blood pressure in both supine and erect positions after treatment for 4 weeks. Also a significant reduction in forced expiratory volume in second (FEV1) was observed and this was due to the inhibition of brnchodilation, (beta-2 effect) by oxprenolol.
ABSORPTION, METABOLISM AND EXCRETION
Oxprenolol is appreciably absorbed when taken orally. Like propranolol, it is subject to first pass effect, hence its short elimination half life which is about 2 hours.
Oxprenolol is considerably bound to plasma proteins. It is virtually completely metabolized in the liver before excretion in the urine as metabolites of less potency compared to the parent drug.
TOXICITY AND SIDE EFFECTS
The minor side effects of exprenolol include nausea, vomiting, dizziness, and mild diarrhea. Owing to its effect on the bronchioles, oxprenolol may precipitate severe bronchoconstriction in predisposed patients. Hence it is contra indicated in asthma patients.
Oxprenolol is used in the treatment of hypertension. Beta-blockers cause unwanted effects due to alterations in peripheral blood flow. These have included an awareness of cold limbs, an increase in intermittent claudication and peripheral gangrene (Rodger et al 1975; Frohlich et al 1969; Simpson 1974; Vale and Jeffreys 1978). There is evidence that this problem may be reduced by using oxprenolol instead of propranolol (Roberts et al 1977). In a controlled trial designed to compare the effect of oxprenolol and propranolol on Resting Forearm Blood Flow (RFBF) when administrated acutely and chronically in patients with essential hypertension, it was shown that a reduction in RFBF after acute administration and after 2 weeks’ therapy occurs with both drugs, the reduction being greater and more prolonbged with propranolol than with oxprenolol. It was also shown that total peripheral resistance was not changed by oxprenolol whereas propranolol increased the total peripheral resistance. It is likely that exprenolol is exerting a different effect because of its ISA either by direct stimulation of the peripheral beta-adrenergic receptors or, reflexly in consequence of it causing a lesser reduction of the cardiac output than prepranolol (Malcolm, 1982). Hence it could be concluded that in hypertensive patients with peripheral vascular disease, oxprenolol is safer milligram for milligram than propranolol. Also in bradycardia and in situations in which a decrease in cardiac output is inappropriate or if the beta-blocking effect is required during physical and psychic stress, a beta-blocker with ISA might be advantageous.
Atenolol is a selective beta-1-adrenoceptor antagonist which is mainly used in the treatment of hypertension. Atenolol lacks both Intrinsic Sympathomimetic Activity (ISA) and membrane stabilizing activity. Its chemical structure is shown in fig. 1.
Fuller and Vallance (1982) found that Atenolol reduces FEV1 and blood pressure in normotensive subjects after acute administration. Leoneti et al (1982) were able to show that blood pressure decrease was very significant during the first day of hypertensive therapy with atenolol and was unchanged after seven and fourteen days of chronic treatment. The degree of blood pressure reduction was very similar in both supine and erect positions. The bradycardiac effect was already at its maximum during the first day of therapy with atenolol and no further changes were observed during the study. Atenolol also reduced the pressor increament during exercise. Atenolol could therefore, be said to have a prompt and long lasting action.
ABSORPTION, METABOLISM AND EXCRETION
Atenolol is appreciably absorbed when administered orally. It is about 20 – 30% bound to plasma proteins and eliminated largely unchanged by the kidney (Rubin et al, 1982). It’s elimination half-life is about 7 hours.
Atenolol is beta-1 selective and has a long half-life. Among the beta-adrenoceptor antagonists these two properties are the ones likely to have the greatest clinical relevance. Indeed long duration of action offers the possibility of reducing the number of daily administration of a drug and it has been shown that the compliance of hypertensive patients to treatment is inversely proportional to the frequency of drug administration (Finnerty et al, 1973).
TOXICITY, SIDE EFFECTS AND PRECAUTIONS
No appreciable inhibition of bronchodilatation has been noticed with atenolol and there is no convincing evidence that atenolol augments hypoglycaemia. However, atenolol has to be used with caution in asthmatics and diabetics receiving insulin or hypoglycaemic drugs.
As with all beta-blockers atenolol should be used if there is a risk of congestive heart failure unless the patient is monitored closely.
The relative selectivity of atenolol is the basis for its therapeutic advantage over less selective agents (Ablad et al 1973). Atenolol is mainly used in the treatment of hypertension and angina pectoris. It’s selective beta-1 adrenoceptor blocking activity makes it preferable to propranolol for use in patients with mild obstructive airways disease.
Propranolol was the first beta-adrenoceptor antagonist to come into wide clinical use. It is a highly potent non-selective beta-adrenoceptor antagonist (blocks both beta-1 and beta-2 adrenoceptors competitively). It does not exhibit any ISA but has a quinidine – like action. The chemical structure is shown in fig. 1.
Propranolol reduces sinus rate, decreases the spontaneous rate of depolarization of ectopic pacemakers and slows conduction in the atria and in the A.V node hence its use in the treatment of some cardiac arrhythmias.
ABSORPTION, METABOLISM AND EXCRETION
Propranolol is almost completely aborbed following oral administration. However much of the administered drug is metabolized by the liver during the first passage through the portal circulation and only up to about 30% reaches the systemic circulation. The degree of hepatic extraction of propranolol is less as the dose is increased. Also somewhat less of the drug is removed during the first circulation through the liver after repeated administration than after the initial dose, which accounts for a gradual increase in the half-life of the drug on chronic oral administration (about 4 hours) compared with the half-life of the initial oral dose (about 3 hours) (Shand, 1975).
Propranolol is about 90 – 95% bound to plasma proteins metabolized before excretion in the urine (Hayes and Cooper, 1971). One of the products of hepatic metabolism is 4-hydroxypropranolol. This metabolite is an active compound and is produced predominantly when the drug is administered orally. Other metabolic products that have been identified in the urine include naphthoxylactic acid, isopropylamine and propanol glycol. A considerable fraction of propranolol is apparently glucuronide conjugates (Shand, 1975).
Abrupt withdrawal of propranolol therapy may give rise to a withdrawal syndrome. This may be due to the supersensitivity of beta-adrenoceptors. Some patients may experience severe exacerbation of angina attacks and patients being treated for hypertension may have a life-threatening rebound of high blood pressure to levels that can exceed pretreatment levels. Hence withdrawal of propranolol therapy should be gradual.
Propranolol causes an increase in airways resistance which can be life-threatening in asthmatics. Asthma is therefore an absolute contra indication to the use of propranolol.
Propranolol augments the hypoglycaemic action of insulin by reducing the compensatory effect of sympathoadrenal activation and masks the tachycardia that is an important sign of developing hypoglycaemia. Therefore uncontrolled diabetes is an absolute contra indication to propranolol therapy.
Other minor side effects that have been reported include nausea, vomiting, diarrhea, and constipation. CNS side effects includes vivid dreams, night mares and depression.
1.6 SCOPE OF THE PROJECT
Beta-adrenoceptor antagonists are widely used in the treatment of ischaemic heart disease and hypertension. It has been widely reported in the literature that there are profound racial differences in the responsiveness to both selective and non-selective beta-adrenoceptor antagonists. (Venter and Joubert, 1984; Salako et al, 1979; Abson et al, 1981). However, the factors underlying these differences have not been clearly defined. Beta-blockade can be assessed from effects on:
Because there is a scarcity of published studies in healthy blacks this study was undertaken to assess the physiological and biochemical effects of standard doses of oxprenolol, atenolol and propranolol. The results were later compared to published results among age and sex matched caucasians with a view to determining any differences in responsiveness. As drug levels were not measured, their pharmacokinetics were not compared.
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