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

Comparison of the effects of intravenous alfentanil and esmolol on the cardiovascular response to double-lumen endobronchial intubation

We compared the effect of alfentanil 10 microg.kg-1 and esmolol 1.5 mg.kg-1 on the cardiovascular responses to laryngoscopy and double-lumen endobronchial intubation in two groups of 20 ASA 2-3 patients undergoing pulmonary surgery, in a randomised double-blind study. Arterial pressure and heart rate decreased after induction of anaesthesia and increased after intubation in both groups (p < 0.05) but remained at or below baseline values, and changes were comparable in both groups. Plasma catecholamine concentrations decreased after induction of anaesthesia in both groups (p < 0.05). Epinephrine concentrations increased in the esmolol group after intubation (p < 0.05) but remained below baseline in the alfentanil group (p < 0.05). Norepinephrine concentrations increased significantly in both groups after intubation but were higher in the esmolol group (p < 0.05). Although both esmolol 1.5 mg.kg-1 and alfentanil 10 microg.kg-1 similarly attenuated the arterial pressure and heart rate response to endobronchial intubation, plasma catecholamine concentrations increased in the esmolol group to values greater than previously reported after tracheal intubation.

A review of why and how we may use beta-blockers in congestive heart failure

The history of the use of beta-blockers for congestive heart failure, beginning with the innovative seminal study by the Swedish group in 1975 to studies in 1995, is reviewed and shows that almost all trials favored the use of beta-blockers. They tended to demonstrate an increase in ejection fraction, a decrease in left ventricular mass, and in some studies, even a decrease in mortality. Even after the introduction of angiotensin-converting enzyme inhibitors, additional improvement in function and mortality was observed. Patients with nonischemic dilated cardiomyopathy derived more benefit from beta-blockers than did patients with ischemic cardiomyopathy. Least likely to benefit were patients treated for <2 months, patients with alcoholic cardiomyopathy, and those with marked intercellular fibrosis. Although the starting dose of metoprolol, the most common beta-blocker used, may have to be as low as 2.5 mg/d, mortality analysis failed to show a decrease in sudden death unless the dose was raised to about 300 mg/d, a dose at which beta-selectivity is generally not expected to be present. The non-beta-specific bucindolol or carvedilol may ultimately be preferred to metoprolol because they are better tolerated initially due to a slight vasodilatation effect. Initial studies with carvedilol showed remarkable promise in reducing mortality. However, these agents cannot yet be said to have been studied adequately.

Exercise hemodynamics and myocardial metabolism during long-term beta-adrenergic blockade in severe heart failure

Hemodynamics and myocardial metabolism at rest and during exercise were investigated in 21 patients with heart failure. The patients were evaluated before and after long-term treatment (14 +/- 7 months) with the beta-adrenergic blocking agent metoprolol. Clinical improvement with increased functional capacity occurred during treatment. Maximal work load increased by 25% (104 to 130 W; p less than 0.001). Hemodynamic data showed an increased cardiac index (3.8 to 4.6 liters/min per m2; p less than 0.02) during exercise. Pulmonary capillary wedge pressure decreased at rest (20 to 13 mm Hg; p less than 0.01) and during exercise (32 to 28 mm Hg; p = NS). Stroke volume index (30 to 39 g.m/m2; p less than 0.006) and stroke work index (28 to 46 g.m/m2; p less than 0.006) increased during exercise and long-term metoprolol treatment. The arterial norepinephrine concentration decreased at rest (3.72 to 2.19 nmol/liter; p less than 0.02) but not during exercise (13.2 to 11.1 nmol/liter; p = NS). The arterial-coronary sinus norepinephrine difference suggested a decrease in myocardial spillover during metoprolol treatment (-0.28 to -0.13 nmol/liter; p = NS at rest and -1.13 to -0.27 nmol/liter; p less than 0.05 during exercise). Coronary sinus blood flow was unchanged during treatment. Four patients produced myocardial lactate before the study, but none produced lactate after beta-blockade (p less than 0.05). There was no obvious improvement in a subgroup of patients with ischemic cardiomyopathy. In summary, there were signs of increased myocardial work load without higher metabolic costs after treatment with metoprolol.

Cardiovascular reactivity to psychosocial stressors. A review of the effects of beta-blockade

Fifty-nine studies examining the effects of beta-blockers on cardiovascular reactivity to psychosocial stressors are reviewed. Across all classifications of beta-blockers, heart rate reactivity was reduced (p less than 0.0001), while there were no significant changes in either systolic or diastolic blood pressure reactivity. Nonselective beta-blockers were more often associated with a reduction in heart rate reactivity than selective blockers (p less than 0.05). There was no evidence that drug lipophilicity or intrinsic sympathomimetic activity differentially affected blood pressure or heart rate reactivity; nor was there evidence that the reactivity of hypertensive subjects was differentially affected by blockade compared to the reactivity of normotensive subjects. While beta-blockers are effective in reducing resting blood pressure, they are not effective agents in reducing blood pressure reactivity to mild psychosocial stressors.

Beta-adrenergic blockade decreases norepinephrine release in humans

Beta-Adrenergic blockade with propranolol (PRP) has been reported to cause an increase in plasma norepinephrine (NE) levels in humans, which suggests that a reflex increase in sympathetic nervous system (SNS) vasoconstrictor tone compensates for the hypotensive effect of beta-adrenergic blockade. However, plasma NE levels are an indirect measure of SNS activity. We have developed a two-compartment model of NE kinetics to estimate NE release into an extravascular compartment as a more comprehensive measure of systemic SNS activity. To determine whether beta-adrenergic blockade alters extravascular NE release, we studied nine healthy subjects during sequential infusions of saline and PRP. During PRP infusion, there was an increase in plasma NE levels [1.03 +/- 0.13 to 1.27 +/- 0.21 (SE) nM; P = 0.05], but the extravascular NE release rate decreased significantly (15.5 +/- 1.6 to 9.2 +/- 1.2 nmol.min-1.m-2, P = 0.0002). The plasma NE concentration increased despite the fall in extravascular NE release rate primarily because the clearance of NE from plasma declined (1.55 +/- 0.08 to 1.18 +/- 0.07 l.min-1.m-2, P = 0.0001); the NE spillover rate into plasma did not change (1.73 +/- 0.18 to 1.75 +/- 0.23 nmol.min-1.m-2, P = 0.89). We conclude that PRP decreases extravascular NE release in humans. Suppression of SNS activity may be an additional mechanism of action of nonselective beta-adrenergic antagonists in humans.

Extraction of epinephrine and norepinephrine by the dog pancreas in vivo

This study determined the fractional extraction of epinephrine and norepinephrine by the in situ dog pancreas. Plasma samples for epinephrine measurements were taken simultaneously from the femoral artery and the superior pancreaticoduodenal vein. Pancreatic extraction of epinephrine was 73 +/- 5% when basal arterial epinephrine levels were 380 +/- 93 pg/mL, 76 +/- 4% when arterial levels were 896 +/- 123 pg/mL (epinephrine infused intravenously at 20 ng/kg/min), and 84 +/- 1% when arterial levels were 2,956 +/- 414 pg/mL (epinephrine infused intravenously at 80 ng/kg/min) suggesting that the process of epinephrine extraction by the pancreas is not saturable over this range. During a similar sampling protocol, norepinephrine was infused intravenously at 4 micrograms/kg/min; pancreatic extraction of norepinephrine was then 65 +/- 7% when arterial norepinephrine levels were 107,000 +/- 28,000 pg/mL. In separate experiments, lower rates of norepinephrine (12 to 1,200 ng/min) were infused directly into the pancreatic artery and pancreatic norepinephrine extraction was calculated; it ranged between 66% and 75%. Because the pancreas produces as well as extracts norepinephrine, a third technique was required to determine pancreatic norepinephrine extraction at the lower endogenous levels of norepinephrine; 3H-norepinephrine was infused intravenously and the arteriovenous difference of 3H-norepinephrine was measured. Fractional extraction of 3H-norepinephrine was 74 +/- 4% both in the basal state (arterial norepinephrine level = 202 +/- 44 pg/mL) and during systemic, glucopenic, stress induced by 2-deoxy-glucose (arterial norepinephrine level = 636 +/- 70 pg/mL). These data suggest that also the norepinephrine extraction process by the pancreas is not saturable.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of bisoprolol on blood pressure, serum lipids and HDL-cholesterol in essential hypertension

Fifty patients with essential hypertension WHO Grades I-II have been treated for 3 months with bisoprolol, a new selective beta blocker, in doses up to 40 mg once daily. Forty-three patients reached the preset target diastolic blood pressure of less than or equal to 90 mmHg on a mean daily dose of 16.8 mg bisoprolol. There was no effect on serum lipids and HDL-cholesterol during the study. The side-effects were mild and were those usually associated with beta-blocking therapy.

Blood pressure and catecholamines following exercise during selective beta-blockade in hypertension

This study examines and compares the hemodynamic and sympathoadrenal response to bicycle exercise in hypertensive subjects during two weeks' treatment with a cardio-selective (metoprolol) and nonselective (propranolol) beta-blocker. The increase in plasma norepinephrine and epinephrine concentration following exercise was augmented to a similar degree with each beta-blocker. Pre-exercise blood pressure and heart rate were similar for the two drugs. However immediately after exercise and particularly after resting for 20 min post exercise, diastolic blood pressure was lower during metoprolol treatment. Systolic blood pressure was also lower 20 min post exercise during metoprolol treatment. These observations indicate that cardio-selective beta-blockers offer advantages in blood pressure control during exercise through intact vascular beta 2-adrenoceptors opposing sympathetically mediated vasoconstriction.

Cardioselective and nonselective beta-adrenoceptor blocking drugs in hypertension: a comparison of their effect on blood pressure during mental and physical activity

The ability of cardioselective and nonselective beta-adrenoceptor blocking drugs, with and without partial agonist activity, to control increases in blood pressure associated with mental and physical activity was compared in 35 subjects with hypertension. Direct measurements of blood pressure and radioenzymatic determinations of plasma norepinephrine were obtained before, during and after four activities, and were repeated after random allocation to treatment with atenolol, metoprolol, pindolol or propranolol. Cardioselective and nonselective drugs modestly reduced the pressor response to reaction time testing, but not to mental arithmetic or isometric exercise. The increase in systolic blood pressure during bicycling was attenuated significantly by the cardioselective drugs atenolol (by 23 mm Hg, or 38%) and metoprolol (21 mm Hg, or 41%), but not by the nonselective agents pindolol (with partial agonist activity) (13 mm Hg, or 20%) and propranolol (10 mm Hg, or 17%) (p less than 0.02 cardioselective versus nonselective; p = NS pindolol versus propranolol). Only bicycle exercise increased plasma norepinephrine concentrations (by 80%). These results suggest that beta-adrenoceptor blocking drugs will not attenuate increases in blood pressure during mental or physical activities unless intense sympathoadrenal activation also occurs. Marked elevations in circulating epinephrine, with or without norepinephrine, and peripheral beta 2-blockade appear necessary for alpha-mediated vasoconstriction to predominate and for the contrasting effects of cardioselective and nonselective drugs to be appreciated.

Clinical implications of the hemodynamic effects of beta blockade

Beta-adrenoceptor blockade produces a well-established constellation of hemodynamic effects at rest and during exercise. The beneficial clinical response in patients with hypertension and angina pectoris relates directly to these hemodynamic effects. A number of molecular modifications have accomplished more selective or additional circulatory effects designed to improve the efficacy and reduce the adverse effects of these drugs. In considering further clinical application of these agents, new data relating to the role of the sympathetic nervous system in the response to exercise, in the control of heart rate and in the progression of the syndrome of heart failure must be considered.

Role of cardiac factors in the initial hypotensive action by beta-adrenoreceptor blocking agents

The blood pressure decrease after beta-blockade is delayed and there are little data on the hemodynamic events associated with the initial decrease in blood pressure. The present study measured the hemodynamics of the initial hypotensive action of timolol maleate, a nonselective beta-adrenoreceptor blocking agent, in 10 patients with essential hypertension. Frequent measurements were made for the first 30 hours of treatment, and follow-up measurements made at 3 and 6 weeks. Before treatment, mean arterial blood pressure, cardiac output, and arteriovenous oxygen difference were 115.9 +/- 9.1 mm Hg, 4.65 +/- 1.05 liter/min, and 55.0 +/- 9.6 ml/liter, respectively. At 3 hours after the first dose of timolol, blood pressure had fallen 13.5 +/- 8.2 mm Hg (p less than 0.05). This was preceded by an initial decrease in cardiac output, which was not associated with a simultaneous decrease in blood pressure, and by an increase of arteriovenous oxygen difference. The early, statistically significant, decrease in cardiac output was followed by a return to normal output, which coincided with the onset of blood pressure reduction. The magnitude of the initial decrease of cardiac output and of the initial increase in arteriovenous oxygen difference was significantly correlated to the later decrease in blood pressure (7 hours after first dose). These hemodynamic observations are consistent with the notion that early underperfusions of tissue play a role in the initial hypotensive action of beta-blockers. After 6 weeks, the blood pressure remained lower but the cardiac output was again decreased at that point. As with many antihypertensive agents, there was a difference between the early and late hemodynamic pattern.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta blockers in hypertension

Beta-adrenoceptor antagonists are effective in the management of patients with mild-to-moderate hypertension. Noncardioselective agents, cardioselective agents and beta blockers with intrinsic sympathomimetic activity (ISA) are equally effective, provided they are used in equipotent doses. Beta blockers can be used as first-line therapy in the management of hypertension and can be safely combined with diuretics, vasodilators, or both, for a better control of blood pressure. The exact mechanism by which beta blockers decrease blood pressure remains speculative, but they all reduce cardiac output during long-term therapy; drugs with ISA lower cardiac output and heart rate less than do drugs without ISA. Pharmacokinetic properties of beta blockers differ widely; drugs metabolized by the liver have shorter plasma half-lives than drugs primarily excreted by the kidneys. Although many of the side effects of various beta blockers are similar, differences in water and lipid solubility account for a higher incidence of central nervous system side effects with lipid-soluble drugs (such as propranolol and metoprolol) than with hydrophilic drugs (such as atenolol and timolol). The incidence of cold extremities has been reported to be less with drugs with ISA, and the incidence of bronchospasm less with cardioselective drugs. In the management of uncomplicated mild-to-moderate hypertension, all beta blockers are equally effective and produce less troublesome side effects than alternative antihypertensive agents. For effective therapy beta blockers can be used in 2 divided daily doses or even once daily.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of atenolol and hydrochlorothiazide on blood pressure and plasma catecholamines in essential hypertension

The antihypertensive effects of atenolol and hydrochlorothiazide were compared with placebo in a randomized, double-blind crossover study, with the blood pressure responses related to sympathetic nervous system activity. Twelve patients with essential hypertension were given atenolol (100 mg), hydrochlorothiazide (50 mg), and placebo as single daily doses, each for 6 weeks. Mean supine, standing, and post-exercise blood pressures (mm Hg) on atenolol (155/94, 152/95, 177/93, respectively) and hydrochlorothiazide (154/99, 150/103, 172/96) were lower (p less than 0.01) than corresponding placebo values (172/109, 166/113, 204/111) at 6 weeks. The role of the sympathetic nervous system in the antihypertensive actions of atenolol and hydrochlorothiazide was examined. The supine plasma norepinephrine on placebo was used as an index of sympathetic activity to categorize each patient's "adrenergic status." The six "hyperadrenergic" patients with high resting norepinephrine values (mean, 302 pg/ml) exhibited a greater (p = 0.05) decrease in BP (-30/-20 mm Hg) on atenolol compared with the BP fall of -9/-11 mm Hg observed in the lower norepinephrine group (mean, 211 pg/ml). Resting plasma norepinephrine values did not predict the BP fall on hydrochlorothiazide. The "adrenergic status" of each patient as measured by the plasma norepinephrine concentration tended to be relatively constant regardless of therapy or the state of activity. In this study, atenolol was an effective antihypertensive agent comparable to hydrochlorothiazide in potency. Adrenergic status tended to predict the BP response to atenolol and was a relatively constant feature of the patients in all treatment phases.

A dose ranging study of atenolol in hypertension: fall in blood pressure and plasma renin activity, beta-blockade and steady-state pharmacokinetics

The relationship between the oral dosage and plasma concentration of the long-acting cardioselective beta-adrenoceptor blocker atenolol and the antihypertensive response to the the degree of beta-adrenoceptor blockade and change in plasma renin activity (PRA) was evaluated in patients with mild-to-moderate essential hypertension in a double-blind, randomized, between-patient, dose-ranging (25, 50 or 100 mg once daily for 4 weeks) study. The optimum, or minimum, daily dose of atenolol to treat patients with mild-to-moderate hypertension was not clearly identified in this study. A between-treatment comparison did not demonstrate that all blood pressure falls were always less in the 25 mg group than in the other two groups. Calculation of beta-error or the power for the negative results between doses suggested that a large sample size is required to draw a conclusion that no dose-antihypertensive relationship of atenolol exists in the treatment of mild-to-moderate hypertension. A relatively flat plasma concentration-antihypertensive response relationship was observed. Steady-state plasma concentrations of atenolol were dose-related and renal drug clearance was well correlated with individual creatinine clearance. beta-adrenoceptor blockade was better correlated with plasma atenolol concentration. Correlations which were less strong were between plasma drug concentration and change in various blood pressures and between blood pressure falls and beta-adrenoceptor blockade. There was no relationship between the fall in blood pressure and change in PRA. Atenolol appeared to suppress PRA in an all-or-none fashion.

Systolic time intervals as possible predictors of pressure response to sustained beta-adrenergic blockade in arterial hypertension. A within-patient, placebo-controlled study

Systolic time intervals (STI) were recorded at rest and during isometric exercise (IHG) in 20 hypertensive outpatients, WHO Stage 1 or 2. In a double-blind crossover study, slow-release metoprolol 200 mg once daily and matched placebo were given for 4 weeks each, at the end of a 2-week placebo washout. Blood pressure and STI were taken in the last day of washout and of either crossover period. Treatment decreased blood pressure and heart rate values at rest and on peak IHG; it didn't modify preejection period index (PEPI), left ventricular ejection time index (LVETI), and their ratio at rest, but decreased the ratio between diastolic blood pressure and PEPI (DBP/PEPI ratio) at rest and on peak IHG and lengthened the PEPI at peak IHG. Resting PEPI values on placebo treatment showed a negative correlation with systolic (r = -0.72) as well as diastolic (r = -0.80) pressure reduction on slow-release metoprolol as compared with placebo treatment. The PEP/LVET ratio at rest on placebo treatment showed a negative correlation with systolic (r = -0.78) as well as diastolic (r = -0.82) pressure reduction at rest on metoprolol compared with placebo treatment. Patients with a resting PEP/LVET ratio less than 0.43 showed a reduction in both systolic and diastolic pressure approximating or exceeding 20 mm Hg, whereas patients with a PEP/LVET ratio greater than 0.47 showed a decrease in systolic and diastolic blood pressure of less than 10 mm Hg. In patients with a PEP/LVET ratio of 0.43 to 0.47 (50% of the trial population), STI didn't show any correlation with the pressure response to beta-blockade. A positive correlation was found between the DBP/PEPI ratio at rest on placebo treatment and systolic (r = 0.56) as well as diastolic (r = 0.76) pressure reduction at rest on slow-release metoprolol compared with placebo treatment. Thus, STI appeared as promising predictors of the magnitude of blood pressure response to sustained beta-blocking therapy in mild-to-moderate essential hypertension, mostly in patients with a resting PEP/LVET ratio less then 0.43 or greater then 0.47.

Effects of acute and long-term beta-adrenoceptor blockade with propranolol on haemodynamics, plasma catecholamines and renin in essential hypertension

The effect of an acute intravenous and repeated oral doses of propranolol on haemodynamics, plasma and urinary catecholamines and plasma renin activity was studied in patients with essential hypertension. Intravenous injection of propranolol 5 mg produced a fall in cardiac output but had no consistent effect on blood pressure. Treatment with oral propranolol for 24 weeks lowered cardiac output and blood pressure; total peripheral resistance did not differ from the pretreatment values. Neither acute intravenous nor chronic oral administration of the beta-blocker affected the resting plasma levels of noradrenaline and adrenaline. Long-term treatment with propranolol reduced urinary excretion of vanilmandelic acid without affecting urinary catecholamine excretion. Acute intravenous injection of propranolol decreased plasma renin activity less than did chronic oral treatment with the drug. The observed time course of plasma renin activity was compatible with the view that suppression of this enzyme contributed to the antihypertensive effect of propranolol.

Effects of pindolol and methyldopa on blood pressure and plasma norepinephrine

Thirty patients with essential hypertension (supine diastolic blood pressure 100 to 115 mm Hg) were treated in a randomized, double-blind study with either pindolol (mean dose 28 +/- 5 mg twice a day) or methyldopa (673 +/- 158 mg three times a day) for 12 weeks after a 3-week, single-blind placebo period. In 17 pindolol-treated patients mean supine blood pressure was 163 +/- 3/106 +/- 1 during the placebo period and 155 +/- 3/99 +/- 2 mm Hg (p less than 0.01) during the high-dose period. In 13 patients treated with methyldopa mean supine blood pressure fell from 160 +/- 4/104 +/- 1 to 156 +/- 5/97 +/- 2 mm Hg. Mean standing heart rate was reduced during pindolol therapy from 84 +/- 2 to 79 +/- 2 bpm (p less than 0.05) but was unchanged during methyldopa treatment. Mean supine pretreatment plasma norepinephrine fell from 379 +/- 40 to 337 +/- 33 pg/ml in patients on pindolol therapy and from 448 +/- 76 to 223 +/- 39 pg/ml (p less than 0.02) in the methyldopa-treated group. Although norepinephrine generally decreased in pindolol responders and not in nonresponders, changes in supine diastolic blood pressure and supine plasma norepinephrine did not correlate. In contrast, norepinephrine declined consistently in methyldopa-treated patients regardless of the blood pressure response; changes in diastolic blood pressure and norepinephrine correlated (r = 0.59; p less than 0.05). The results suggest that suppression of sympathetic nervous system activity may play a role in the hypotensive effect of both pindolol and methyldopa.

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.

Responses of catecholamines and blood pressure to beta-blockade in diuretic-treated patients with essential hypertension

Twenty patients (mean age 49 +/- 4 (SEM) yr) with mild to moderate essential hypertension were studied during placebo conditions, following 6 weeks of chlorthalidone monotherapy and 6 to 28 weeks of combined beta-blocker-chlorthalidone treatment, or vice versa. Compared to chlorthalidone therapy alone, addition of a beta-blocker to this diuretic caused a further blood pressure reduction in 11 patients (Responders); in 9 patients addition of a beta-blocking agent failed to further reduce blood pressure (Non-responders). Supine and upright plasma renin, aldosterone, norepinephrine and epinephrine levels and catecholamine excretion rates were always comparable between Responders and Non-responders. In both groups plasma and urinary norepinephrine or epinephrine values were not significantly altered following addition of a beta-blocker; heart rate and plasma renin activity were decreased in both groups to a similar extent. It is concluded that the antihypertensive mechanism of beta-blockers given to diuretic treated patients with essential hypertension is independant of renin and not associated with changes in plasma or urinary catecholamines.

Comparison of beta-adrenoceptor blockers under maximal exercise (pindolol v metoprolol v atenolol)

1 The time-related, comparative beta-adrenoceptor blocking effect of metoprolol 150 mg twice daily, atenolol 100 mg once daily and pindolol 7.5 mg twice daily on heart rate, blood pressure, work performance, blood lactate, free fatty acids and plasma catecholamines was studied in ten males aged 19--25 years by means of repeated maximal bicycle exercise tests. 2 At steady state several differences in effects were noted among the drugs. These could be explained by differences in beta 1-selectivity, potency of the chosen drug-doses and intrinsic sympathomimetic activity (ISA). 3 This study emphasizes the importance of including strong sympathetic stimuli in any model used for comparing beta-adrenoceptor blockers with and without ISA in order not to underrate the effects of beta-adrenoceptor blockers with ISA. 4 In the chosen doses pindolol was more effective, and atenolol less effective than metoprolol in suppressing heart rate and blood pressure responses to maximal exercise.

How intrinsic sympathomimetic activity modulates the haemodynamic responses to beta-adrenoceptor antagonists. A clue to the nature of their antihypertensive mechanism

1 A survey has been made of the literature on acute and long-term haemodynamic effects of ten different β-adrenoceptor antagonists.

The β-adrenoceptor blockers are: pindolol, practolol, alprenolol, oxprenolol, acebutolol, penbutolol, metoprolol, atenolol, propranolol and timolol. The total numbers of patients included in this review are 396 patients in 41 acute studies and 410 patients in 36 long-term studies.

2 The effects of β-adrenoceptor blockers on the concentrations of plasma noradrenaline have also been reviewed. Ten studies including 110 patients on non-ISA-β-adrenoceptor blockers and eight studies including 116 patients on pindolol are presented.

3 In the acute studies (i.e. 15-90 min) arterial pressure was lowered by 1-7% and in the long-term studies (i.e. 3 days-5 years) by 6-17%.

4 The degree of cardio-depression induced by the various β-adrenoceptor blockers was inversely correlated with their pharmacologically defined quantity of intrinsic sympathomimetic activity (ISA) both in acute and in long-term studies.

5 In the acute studies the increments in peripheral vascular resistance were directly correlated with the degree of cardio-depression. This suggests that a fall in arterial pressure immediately after administration of a β-adrenoceptor blocker is prevented by increased vasoconstrictor nerve activity mediated through the arterial baroreflex.

6 The compensatory response of vascular resistance to cardio-depression was similar for β₁-selective and non-selective blockers, thereby indicating that extra-junctional vascular β-receptors are relatively unimportant for maintaining basal vascular tone.

7 In the long-term studies the correlation between changes in cardiac output and changes in vascular resistance was shifted to a lower level of vascular resistance. This means that the onset of blood pressure reduction during β-adrenoceptor blockade was associated with a fall in vascular resistance at any level of cardiac output. Thus vascular resistance was higher during treatment with a non-ISA-β-adrenoceptor blocker than during treatment with an ISA-β-adrenoceptor blocker.

8 The level of vascular resistance ultimately attained during treatment with the various β-adrenoceptor blockers appears to be inversely related to their effects on plasma renin activity.

9 The concentration of noradrenaline in plasma rose by approximately 30% during treatment with non-ISA-β-adrenoceptor blockers and fell by more than 30% after pindolol.

10 There is evidence that under propranolol, which reduces cardiac output and hepatic blood flow, the plasma noradrenaline clearance is diminished. Since noradrenaline is mainly cleared from the circulation by the lungs and by the liver, and since pindolol has no effect on cardiac output and hepatic blood flow, one may expect the plasma noradrenaline clearance not to be diminished by pindolol.

11 The reported effects of β-adrenoceptor blockers on plasma noradrenaline may indicate that the release of neurotransmitter is diminished, but in the case of non-ISA-β-adrenoceptor blockers this effect is not reflected by a decreased concentration of noradrenaline in plasma, because its clearance is also reduced.

12 The hypotensive effect of β-adrenoceptor blockers appears to be independent of blockade of postjunctional cardiac-β-receptors, juxtaglomerular-β-receptors and extrajunctional vascular β-receptors. This indicates that blockade of β-receptors at other sites (i.e. centrally and/or prejunctionally) is more important.

Effect of beta-adrenergic receptor blockade with propranolol on the response of plasma catecholamines and renin activity to upright tilting in normal subjects

1 Relationship between plasma catecholamines (measured as noradrenaline and adrenaline) and plasma renin activity (PRA) were examined at rest and during passive head-up tilting for 30 min in nine normal subjects, before and after treatment with propranolol 160 mg daily for 7 days. 2 Noradrenaline (NA) and adrenaline (A) increased substantially after tilting for 15 min. There were no changes in PRA. After 30 min tilting, NA remained elevated, whereas A had returned to resting levels. A significant increase in PRA was apparent at 30 min. Pulse rate and diastolic blood pressure increased progressively during tilting. Systolic pressure did not change. 3 Treatment with propranolol reduced pulse rate and systolic blood pressure at rest and during tilting. Resting catecholamine concentrations and the response of NA to tilting were unaffected. In contrast, treatment prolonged the A response leading to significantly higher levels after 30 min tilting. Propranolol reduced PRA in six of the nine subjects and prevented the increase with tilting observed before treatment.

Effect of propranolol on noradrenaline kinetics in patients with essential hypertension

1 The rates of noradrenaline spillover to, and removal from, plasma were measured in ten patients with essential hypertension treated with propranolol, to ascertain if long-term administration of this drug reduces sympathetic nervous system tone. 2 The plasma clearance of noradrenaline fell with propranolol, leading to a small rise in the mean plasma noradrenaline concentration. Sympathetic nervous activity in treated patients cannot be reliably gauged from plasma noradrenaline values because these are distorted by the reduction in noradrenaline clearance. 3 There was no consistent effect on noradrenaline spillover rates, which fell in six patients, but rose in the remaining four. The magnitude of the antihypertensive response was unrelated to these changes in noradrenaline release. During propranolol treatment, noradrenaline spillover rates were in every case within the normal range, much higher than in patients treated with the known sympathetic nervous systems suppressant, clonidine. 4 The principal mode of antihypertensive action of propranolol is something often than central suppression of sympathetic tone or pre-synaptic inhibition of noradrenaline release.

Short-term systemic hemodynamic adaptation to beta-adrenergic inhibition with atenolol in hypertensive patients

Early systemic hemodynamic adjustments to antihypertensive therapy with the cardioselective beta inhibitor, atenolol, were investigated in 12 hospitalized men, mean age 52 years, with uncomplicated mild-to-moderate essential hypertension. Twice daily measurements of cardiac output (CO) by CO2 rebreathing, blood pressure by cuff, and heart rate were performed in all subjects for 3 days before and 5 days after initiation of oral atenolol therapy (50 or 100 mg daily). Cardiac output by CO2 rebreathing was checked with dye dilution just before, and 4 hours and 4 days after the start of therapy. Plasma volume (radioiodinated albumin) was measured before therapy and on Day 5 of therapy. The CO results obtained with the two methods were not significantly different (r = 0.88, p less than 0.01, n = 12). A reduction in heart rate, 18 +/- 2 beats/min (mean +/- SE), occurred in all patients while taking atenolol. By 4 hours after the first dose of atenolol, CO fell from 5.49 +/- 0./30 to 4.24 +/- 0.21 liters/min (p less than 0.01), while the control mean arterial pressure (MAP) of 108 +/- 4 mm Hg was not significantly changed, 110 +/- 4 mm Hg. At 24 hours, CO returned near baseline (5.10 +/- 0.21 liters/min) but MAP was reduced (95 +/- 3 mm Hg, p less than 0.001) and remained so thereafter. CO remained at baseline at 48 hours (5.50 +/- 0.29 liters/min) but fell again (p less than 0.01) to 4.81 +/- 0.11 on Day 4 and to 4.68 +/- 0.25 liters/min on Day 5 of atenolol therapy. Plasma volume, 3110 +/- 100 ml before therapy, was reduced to 2850 +/- 100 by Day 5 of atenolol therapy (p less than 0.01). The findings indicate a delayed onset of the antihypertensive action of atenolol. The transient return to baseline of CO on Day 2 and 3 of atenolol therapy suggests a reverse autoregulatory adjustment to the initial fall in CO.

Urinary kallikrein in normotensive subjects and in patients with essential hypertension

Basal 24 hour urinary kallikrein excretion of 20 patients with uncomplicated essential hypertension did not differ significantly from that of 18 normotensive age-matched control subjects. 4 of the 20 hypertensive patients, however, had low kallikrein excretion. Furosemide (40 mg i.v.) caused an increase of urinary kallikrein excretion in the normotensive subjects and in most of the patients with essential hypertension. The stimulating effect of furosemide was less pronounced or even absent in 7 hypertensives. No circadian rhythm of urinary kallikrein excretion was observed. There were weak correlations between 24 hour kallikrein excretion and urinary volume (r=0.59; p < 0.05), and potassium excretion (r=0.51; p < 0.05) in the normotensives. In the hypertensives correlations were found between 24 hour kallikrein excretion and potassium excretion (r=0.51; p < 0.05), aldosterone excretion (r=0.57; p < 0.01), and creatinine clearance (r=0.59; p < 0.01). Our findings do not support the concept that the renal kallikrein-kinin system might play a primary role in the pathogenesis of essential hypertension.

Mechanism of propranolol withdrawal phenomena

Nine patients on chronic treatment with propranolol for essential hypertension for 3 months or longer were studied after abrupt discontinuation of the drug. Each patient demonstrated transient supersensitivity to the chronotropic effects of isoproterenol, beginning 2--6 days (median 4 days) after propranolol withdrawal, lasting for 3--13 days (median 6 days), with the maximum sensitivity on day 6. A significantly lower dose of isoproterenol was necessary to increase heart rate 25 beats/min on day 6 (median dose 1.2 microgram, range 0.3--3.4 microgram) compared with after day 14, when sensitivity had stabilized (median dose 2.3 microgram, range 1.4--7.6 microgram). Six patients had transient symptoms (headache, chest pain, palpitations and sweating) after abrupt propranolol withdrawal, coinciding with supersensitivity to isoproterenol in five. Transient increases in plasma catecholamines and blood pressures and sustained increases in heart rate occurred during the period of isoproterenol supersensitivity in most patients, and may have contributed to symptoms noted. The delayed onset and potentially long duration of beta-adrenergic supersensitivity after abrupt propranolol withdrawal have important clinical implications.

Sympathetic nervous system and blood-pressure control in essential hypertension

In normotensive subjects an inverse correlation was observed between an index of sympathetic nervous activity (the plasma-noradrenaline concentration during physical exercise) and reactivity to exogenous noradrenaline. This relationship was invariably disturbed in age-matched patients with essential hypertension. Multiple-regression analysis revealed a highly significant correlation between the combination of both factors and the height of mean arterial blood-pressure (r=0.91). The findings suggest that sympathetic nervous activity and pressor response to noradrenaline together form an important determinant of the arterial blood-pressure level. An inverse relationship could be demonstrated between plasma-renin concentration and pressor response to angiotensin II in normotensives, and this relationship was unchanged in hypertensive patients. Therefore angiotensin II does not appear to contribute directly to high blood-pressure.