The effct of propranolol on plasma renin activity and blood pressure in mild essential hypertension

Three Generations of β-blockers: History, Class Differences and Clinical Applicability

BACKGROUND

Beta-adrenergic receptors are expressed in cardiomyocytes and activated by either noradrenaline released from sympathetic synapses or circulating catecholamines. Their corresponding receptors have three subtypes, namely, β1, β2 and β3, which are members of the G protein-coupled receptors (GPCRs) family. Activation of β1-adrenergic receptors causes various physiological reactions including cardiac contraction and renin secretion from juxtaglomerular cells of the kidney. Antagonists of β-adrenergic receptors, known as β-blockers, have been used effectively for over four decades and have beneficial effects in the treatment of cardiovascular diseases. There are three generations of β-blockers according to their pharmacological properties. Firstgeneration β-blockers are non-selective, blocking both β1- and β2-receptors; second-generation β- blockers are more cardioselective in that they are more selective for β1-receptors; and thirdgeneration β-blockers are highly selective drugs for β1-receptors. The latter also display vasodilator actions by blocking α1-adrenoreceptors and activating β3-adrenergic receptors. In addition, thirdgeneration β-blockers exhibit angiogenic, antioxidant, anti-proliferative, anti-hypertrophic and antiapoptotic activities among other effects that are still under investigation.

CONCLUSION

The objective of this review is to describe the evolution observed during the development of the three distinctive generations, thereby highlighting the advantages of third-generation β- blockers over the other two drug classes.

Pindolol and systolic time intervals in patients with hypertension

Two studies of systolic time intervals (STIs) in patients with mild to moderate hypertension (HBP) revealed that no mean change in systolic intervals occurred with pindolol therapy, although some patients had significant alterations in their STIs. Pindolol responders with normal pretreatment preejection period to left ventricular ejection time (PEP/LVET) ratios had a significant increase in this ratio following pindolol therapy, whereas those with abnormal pretreatment PEP/LVET ratios had improvement in this ratio on administration of the drug. Patients on propranolol showed no change in PEP/LVET ratio. Propranolol administration slowed heart rate and lengthened Q-S2, S1-S2, and LVET, however, without altering the Q-S2 and LVET index, indicating that the changes were caused by the effect of propranolol on the heart rate alone. Chlorthalidone in high doses significantly reduced the Q-S2 index and the S1-S2 index, indicating that these changes were not caused by alteration of the heart rate. The second study suggests that STIs may provide a predictive clue for clinical response to pindolol. Patients with normal cardiac function (group I) are more likely to respond to pindolol than are those with abnormal cardiac function (group II). Directionally opposite changes in STIs in the two subgroups suggest different mechanisms for changing cardiac function. Pindolol's dual role as a beta-blocking agent with intrinsic sympathomimetic activity is proposed as a possible explanation, beta-blocking effects predominating in group I and sympathomimetic activity balancing the beta effect in group II.

Time course of blood pressure, pulse rate, plasma renin and metoprolol during treatment of hypertensive patients

Eleven patients were treated for essential hypertension with metoprolol (Selokén) for more than three months. The time course of changes in blood pressure, pulse rate and plasma renin activity was studied during treatment with an oral maintenance dose of 100 mg twice daily. Significant decreases in pulse rate, diastolic blood pressure and plasma renin activity were observed even after the first dose. The plasma concentration of metoprolol reached equilibrium after the second dose. After the third dose there was no further significant change in blood pressure. There was a significant correlation (p < 0.001) between the initial (after three doses) and final (after > 90 days) effect of metoprolol on blood pressure (r = 0.86 and 0.91 for systolic and diastolic blood pressure change, respectively).

Sympathetic responsiveness and antihypertensive effect of beta-receptor blockade in essential hypertension

The relationship between sympathetic responsiveness and the blood pressure reduction induced by long-term beta-blockade was assessed in patients with essential hypertension. The increase in plasma noradrenaline concentration during physical exercise was used as an index of sympathetic responsiveness. The cardioselective beta-blocker, atenolol, was given to 16 patients with sustained benign essential hypertension for five weeks at a dose of 200 mg/day. Atenolol induced a marked decrease in blood pressure and pulse rate during recumbency, orthostasis and exercise concomitant with a marked increase in plasma noradrenaline concentration (p less than 0.0125) and a pronounced decrease in plasma renin concentration (p less than 0.01). The ratio of plasma noradrenaline during exercise to the base line concentration correlated significantly with the subsequent decrease in mean arterial blood pressure induced by beta-blockade (r = 0.840; p less than 0.001). A less significant correlation was observed between the plasma renin concentration and the subsequent decrease in mean arterial pressure (r = 0.542; p less than 0.05). The results obtained indicate that sympathetic responsiveness is an important determinant of blood pressure response to beta-blockade induced by atenolol.

The effects of antihypertensive medication on the control of the cardiovascular system during halothane anaesthesia in rats

The effects of hydralazine, clonidine, propranolol and methyldopa medication on the control of the circulatory system during halothane anaesthesia were studied in spontaneously hypertensive (SH) rats. Special attention was directed to the problem of circulatory emergencies. Under 1 and 3% halothane anaesthesia, the mean arterial pressure was lowest in methyldopa-treated rats. During 3% anaesthesia, plasma renin activity was markedly increased in the methyldopa group and decreased in the propranolol group. Hydralazine medication suppressed the pressor responses to dopamine and metaraminol, whereas clonidine, propranolol and methylopa increased the response to dopamine. These sympathetic agents induced more cardiac arrhythmias in SH controls than in normotensive ones. These arrhythmias were antagonized by hydralazine. The SH controls also tolerated haemorrhagic shock more poorly than did normotensive control rats. Among the pretreated animals, tolerance to this shock was highest in hydralazine-and clonidine-treated animals and lowest in the methyldopa group. The results suggest that during halothane anaesthesia SH rats are more prone to disturbances in the control of circulation than are normotensive controls. Hydralazine and, to a lesser extent, clonidine have a protective action against these disturbances, but the effect of methyldopa seems to be disadvantageous.

Blood pressure, renin activity and heart rate changes during propranolol therapy of hypertension

To investigate the importance of plasma renin activity in determining the antihypertensive action of propranolol and of heart rate in guiding propranolol therapy, the effect of a graded oral dose of propranolol on blood pressure, plasma renin and heart rate was studied in 24 men with essential hypertension and differing initial levels of renin activity. Although plasma renin decreased substantially in patients with normal and high renin levels tasking a small dose of propranolol (40 mg/day), blood pressure was unchanged in the normal renin group, and a small decrease in systolic pressure alone was noted in those with high plasma levels of renin. Patients with low renin levels had no reduction in blood pressure with small doses of propranolol, but as a group they responded well to a dose of 320 mg/day. Standing heart rate decreased maximally at plasma propranolol concentrations greater than 25 ng/ml, and did not exceed 72 beats/min with plasma concentrations above that level. Overall, the observed responses for plasma renin and heart were more pronounced at lower plasma propranolol concentrations than those for blood pressure.

[Behaviour of plasma renin activity during long-term treatment with propranolol (author's transl)]

17 patients (40 +/- 9 years) with essential hypertension were included in the study. A monotherapy of 120, 160 or 240 mg propranolol per day was administered orally according to the antihypertensive effect. Four weeks after treatment, blood pressure and heart rate showed a statistically significant decrease and remained unchanged over a period of six months. Plasma renin activity decreased significantly from 5.7 +/- 6.3 ng/ml/h at the beginning of the study to 1.8 +/- 1.3 ng/ml/h after 4 weeks. 5 months later however plasma renin activity increased again to 5.0 +/- 0.9 ng/ml/h. The difference was statistically significant. From 17 patients 7 (41%) had lower levels of plasma renin activity after 6 months treatment when compared with pretreatment values. In only 3 patients (18%) was plasma renin activity lower after 6 months than after four weeks. We conclude that the increase in plasma renin activity is a reactive mechanism to the reduced blood pressure under long-term conditions. The decrease of plasma renin activity in short-term treatment of essential hypertension is not a mechanism responsible for the antihypertensive effect of propranolol during long-term treatment.

The effect of pindolol on plasma renin activity and blood pressure in hypertensive patients

1 The effect of pindolol administered to twenty-six patients with hypertension of unknown origin was compared with respect to blood pressure and plasma renin activity change after increase of the dose over a period of 6 weeks. 2 There was no clear correlation between the fall of plasma renin activity, which in some patients was very marked, and the fall in blood pressure. Some patients with a fall in plasma renin activity did not drop their pressure. Conversely, some with a fall of pressure did not drop their plasma renin activity. 3 The addition of hydrochlorothiazide to the pindolol finally caused further lowering of the blood pressure in all but one patient and the plasma renin activity rose in all but two patinets. There was no clear correlation between change in plasma renin activity and the effect on blood pressure.

Beta-adrenoreceptor blockade in hypertension

There is good evidence from many sources that beta-adrenoreceptor blockade is an effective form of therapy in mild, moderate and severe hypertension either alone or in combination with other antihypertensive agents. Although a number os such beta blocking compounds are now available, they appear to have a hypotensive effect of approximately equal magnitude. This hypotensive effect is obtained in both the supine and standing positions thus avoiding postural hypotension. The maximum hypotensive effect may take some time to become apparent. Despite considerable work the mode of action remains uncertain, reduction in cardiac output, resetting of baroreceptors, reduction in plasma renin and a central nervous system effect have been suggested but remain unproved. There is evidence to suggest that these compounds can control, to some degree, the surges in blood pressure resulting from either mental or physical stress. A low incidence of serious side effects has been reported by many workers. Only the long-term use of these compounds in comparison with other antihypertensive agents will determine their place in the management of hypertension.

Controlled treatment of primary hypertension with propranolol and spironolactone. A crossover study with special reference to initial plasma renin activity

Twenty-seven patients with hypertension were randomly allocated to a 10 month crossover study. Treatment consisted of spironolactone (200 mg/day for 2 months), propranolol (320 mg/day for 2 months) and combined administration of both drugs at half the dosage. Between treatment periods placebo was given for 2 months. Fourteen patients were previously untreated. The average pretreatment blood pressure for the entire group was 188/114 +/- 16/7(mean +/- standard deviation) mm Hg supine and 188/118 +/- 20/9 mm Hg standing. Both spironolactone and propranolol reduced blood pressure significantly in both the supine and standing positions. Upright plasma renin activity was determined by radioimmunoassay of angiotensin I. The average initial level was 1.9 +/- 1.2 (range 0.4 to 5.0) ng/ml/hr. There was a close correlation between plasma renin activity and the effects of the drugs: With increasing renin level the response to propranolol was better whereas the opposite was true for spironolactone. The combination of spironolactone and propranolol decreased the blood pressure still further in the supine and standing positions, irrespective of initial plasma renin activity. All patients achieved a normal supine pressure. Blood pressure and plasma renin activity returned toward pretreatment values during placebo administration. It is concluded that pretreatment levels of plasma renin activity can predict the antihypertensive response to propranolol and spironolactone. The combination of the two drugs, which have different modes of action, will effectively reduce blood pressure in hypertension. The results support the concept that the renin-angiotensin-aldo-sterone system may be involved in primary hypertension.

Renin concentrations and effects of propranolol and spironolactone in patients with hypertension

In a crossover study 32 patients with hypertension were randomly allocated to treatment with spironolactone 200 mg/day for two months, propranolol 320 mg/day for two months, and a combination of both drugs at half the dose. Between the treatments placebo was given for two months. Both spironolactone and propranolol lowered the blood pressure significantly in both positions. The initial plasma renin activity (PRA) levels ranged from 0-4 to 5-0 mug angiotensin I l-1 h-1, and there was a close correlation between these levels and the effects of the drugs: with increasing PRA the response to propranolol was better while the opposite was true for spironolactone. Spironolactone reduced the blood pressure more at eight than at four weeks, while no such difference could be shown for propranolol. Spironolactone and propranolol together decreased the blood pressure still further irrespective of the initial PRA. All patients achieved a normal supine blood pressure.

Pharmacokinetics and effects of propranolol in terminal uraemic patients and in patients undergoing regular dialysis treatment

Propranolol blood and plasma levels were measured after a single oral dose of 40 mg in patients with chronic renal failure, in patients undergoing regular dialysis treatment, and in healthy volunteers. Peak levels were observed in all cases within 1.5 to 3 hours. However, peak blood and plasma concentrations of propranolol in the chronic renal failure group were 2- to 3-fold higher (161 +/- 41 ng/ml) than those observed in the dialysis patients (47 +/- 9 ng/ml) and in the healthy volunteers (26 +/- 1 ng/ml). The apparent plasma clearance was also significantly reduced in the patients with chronic renal failure. The data suggest a reduced hepatic extraction in chronic renal failure patients. A significant increase in the fraction of the dose available to the systemic circulation was also found, together with a modification of apparent plasma half-life and volume of distribution in regular dialysis patients during the dialysis day as compared with the after-dialysis day. No extraction of propranolol by the dialyzer was noticed. Marked fluctuations in propranolol blood concentrations were also observed in patients on regular dialysis following continuous propranolol treatment. The suppressive effect of propranolol on plasma renin activity did not fully correlate with the hypotensive effect of the drug. On the basis of the reported data, propranolol should be used with great caution and at low doses in chronic renal failure.