Beta blockers in hypertension

Characterisation of selected active agents regarding pKa values, solubility concentrations and pH profiles by SiriusT3

The aim of this work was to determine pKa values and solubility properties of 34active agents using the SiriusT3 apparatus. The selected drug substances belong to the groups of ACE-inhibitors, β-blockers, antidiabetics and lipid lowering substances. Experimentally obtained pKa and intrinsic solubility values were compared to calculated values (program ACD/ChemSketch) and pKa values to published data as well. Solubility-pH profiles were generated to visualise the substance solubility over the gastrointestinal pH range. The relationship between the solubility characteristic of a substance, its bioavailability and categorisation according to the Biopharmaceutics Classification System (BCS) was examined as well. The results showed a good agreement between experimentally obtained, calculated and published pKa values. The measured and calculated intrinsic solubility values indicated several major deviations. All solubility-pH profiles showed the expected shape and appearance for acids, bases or zwitterionic substances. The obtained results for the pKa and solubility measurements of the examined active agents may help to predict their physicochemical behaviour in vivo, and to understand the bioavailability of the substances according to their BCS categorisation. The easy and reproducible determination of pKa and solubility values makes the SiriusT3 apparatus a useful tool in early stages of drug and formulation development.

Effect of acute atenolol on short-term blood pressure variability and baroreflex sensitivity in rats

Beta-blocker therapy for hypertension or coronary artery disease is common, but there are a lot of controversies about its effects on short-term blood pressure variability and arterial baroreceptor reflexes. The aim of this study was to evaluate the effects of acute atenolol on baroreflex sensitivity (BRS) and on the spontaneous variability of systolic blood pressure (SBP) and RR intervals in conscious rats. Ten Wistar rats equipped with telemetry system were evaluated: 1) under control conditions; 2) after injection of saline; and 3) during beta1-adrenergic blockade by atenolol. Fast Fourier transform analysis was applied to RR intervals and SBP. Atenolol increased RR intervals significantly by 14% and the variation coefficient of the RR intervals by 31%. SBP was reduced significantly by 9%. In frequency domain, beta1-blockade in RR intervals increased very low frequency by 33% and the total power by 22% and decreased low frequency by 25%. The ratio of low to high frequency power decreased by 60%. Frequency domain variables in SBP were not significantly changed after beta1-adrenergic receptor blockade. BRS (gain alpha) was not significantly altered by beta-blockers. Acute atenolol decreased SBP and increased RR intervals with no change in BRS, indicating 'resetting' of baroreflex function.

Relation between tonic sympathetic and vagal control of human sinus node function

BACKGROUND

Clinical conditions, such as heart failure or myocardial infarction are associated with enhanced sympathetic and reduced parasympathetic activity as compared to normal controls. The reciprocal alteration in cardiac autonomic tone likely contributes to the electrical instability of the myocardium. Little information is available on the relation between sympathetic and vagal cardiac control in healthy human subjects.

METHODS AND RESULTS

Heart period changes in response to autonomic blockades were measured in 16 young, healthy human subjects. Adrenergic and cholinergic blockades were induced by i.v. propranolol (0.2 mg/kg) and atropine (0.04 mg/kg) in two opposite orders on two occasions; interindividual correlations were performed between the R-R interval responses to propranolol and to atropine obtained under the various blockade conditions, and the magnitude of the responses were compared by a drug x order two factorial ANOVA design. It was found, that previous adrenergic blockade did not reduce the extent of cardioacceleration produced by subsequent cholinergic blockade and that the R-R interval responses to atropine and to subsequently given propranolol did not share significant variance across subjects (r = 0.22, P = 0.234). Also, no interindividual correlation was found between the R-R interval responses to propranolol and to atropine, with the influence of the other, respective, autonomic division already blocked (r = 0.42, P = 0.114).

CONCLUSIONS

Under resting conditions, activity levels of cardiac vagal and sympathetic outflows are not related across young, healthy human subjects and peripheral interaction is not manifest between the autonomic divisions.

Around the beta-blockers, one more time

We review the pharmacology, pharmacokinetics, and relative costs of beta-blockers, as well as indications for and therapeutic controversies surrounding their use. It is hoped that this discussion will assist clinicians in making informed decisions when choosing a drug for a hospital formulary or a particular patient. Beta-blockers are indicated for a variety of noncardiovascular and cardiovascular conditions, including hypertension, ischemic heart disease, arrhythmias, and prophylaxis of myocardial infarction (MI). These agents compete with catecholamines at beta-adrenoreceptors. They have different ancillary properties, including intrinsic sympathomimetic activity (ISA), cardioselectivity, and membrane stabilizing-activity, and vary in their duration of action, route of elimination, and lipophilicity. Beta-blocking agents decrease oxygen demand by exerting a negative inotropic and chronotropic effect. They also reduce blood pressure and possess antiarrhythmic effects. Beta-blockers penetrate the central nervous system (CNS) to different degrees and can cause a wide variety of CNS adverse effects. Nonselective beta-blockers have been noted to slightly reduce renal blood flow. Nadolol is an exception in that either no change, or even a small increase in renal blood flow, is observed upon initiation of therapy. Beta-blockers also act on the pulmonary bed by preventing beta 2-mediated bronchodilation, thereby exacerbating bronchospastic disease in some patients. Beta-adrenergic blocking agents can potentiate both hypoglycemia and hyperglycemia in diabetic patients. Their effects on total peripheral resistance (TPR) are controversial. Initially it appears that beta-blockade increases TPR. After chronic therapy, however, TPR decreases to or below baseline values. These agents appear to be equally efficacious in the treatment of hypertension, arrhythmias, and ischemic heart disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypertension

An estimated 58 million Americans are at increased risk of morbidity and premature death due to high blood pressure (BP) and require some type of therapy or systematic monitoring. This article focuses on recent advances in our understanding of the pathogenesis of hypertension, new approaches to the diagnosis and treatment of secondary hypertension, and current views of the most appropriate nonpharmacologic and pharmacologic therapy for essential hypertension. In view of the extremely high prevalence of the disorder, emphasis is placed on efficient and cost-effective strategies for diagnosing and managing the hypertensive patient. Recent evidence indicates that nonpharmacologic therapy, including dietary potassium and calcium supplements, reduction of salt intake, weight loss for the obese patient, regular exercise, a diet high in fiber and low in cholesterol and saturated fats, smoking cessation, and moderation of alcohol consumption produces significant sustained reductions in BP while reducing overall cardiovascular risk. Accordingly, nonpharmacologic antihypertensive therapy should be included in the treatment of all hypertensive patients. In persons with mild hypertension, nonpharmacologic approaches may adequately reduce BP, thereby avoiding the expense and potential side effects of drug therapy. In patients with more severe hypertension, nonpharmacologic therapy, used in conjunction with pharmacologic therapy, can reduce the dosage of antihypertensive medications necessary for BP control. Patients treated with nonpharmacologic therapy only should be followed closely, and if BP control is not satisfactory, drug therapy should be added. The large number of drugs available for use in hypertension treatment, coupled with our rapidly expanding knowledge of the pathophysiology of hypertension and of the adverse effects of these drugs in individual patient groups, make it possible to individualize antihypertensive treatment. When used as monotherapy, most agents effectively lower BP in the majority of patients with mild or moderate essential hypertension. Thus, a single agent from one of four classes: diuretics, angiotensin-converting enzyme inhibitors, calcium channel blockers, and beta-adrenergic blockers, usually provides effective BP control with minimal side effects in most patients. Therapy should be initiated with the agent most likely to be effective in BP lowering and best tolerated. If the initial agent is ineffective at maximal recommended therapeutic doses or has undue side effects, an alternative agent from another class should be tried. When monotherapy is unsuccessful, a second agent, usually of a different mechanism of action, should be

Hemodynamic effects of celiprolol at rest and during exercise; a comparison with enalapril

The antihypertensive effects of once-daily administration of celiprolol 400 mg were compared with those of once-daily enalapril 20 mg in 20 mild to moderate essential hypertensives in a single-blind study in which the subjects were randomized to 2 weeks' treatment with either drug, preceded by 2 weeks of placebo administration. Supine and standing systolic blood pressure (SBP), diastolic pressure (DBP), and heart rate (HR) were measured at rest and during exercise (exercise bicycle with increasing work loads up to 100 watts) at the end of either period, 20 to 24 hr after drug or placebo administration. Compared to placebo, both celiprolol and enalapril reduced resting supine and standing SBP and DBP to a marked and similar extent; resting HR was unaffected by enalapril, while celiprolol produced modest reductions. During exercise, however, celiprolol significantly attenuated rises in SBP and HR, while enalapril did not. Indexes of cardiac function at rest obtained by echocardiography and systolic time intervals were unaffected by either drug. Thus, despite similar reductions by both drugs in resting blood pressures, celiprolol produced better antihypertensive effects during exercise than enalapril.

Effect of acute administration of propranolol and atenolol on baroreflex function in normal man

The acute administration of the beta-adrenoceptor antagonists propranolol (80 mg) and atenolol (50 mg) on baroreflex function were investigated in healthy volunteers. Two h after administration both propranolol and atenolol significantly prolonged the supine R-R interval (1126, 1128 ms respectively) compared to placebo (1012 ms); systolic arterial pressure also fell (102.9, 102.0 mm Hg respectively) compared to placebo (112.6 mm Hg). Baroreflex function, assessed using glyceryl trinitrate to deactivate the baroreceptors was unchanged by these drugs compared to placebo. Baroreflex sensitivity (slope of the linear regression line relating R-R interval to systolic blood pressure) using phenylephrine to activate the baroreceptors, was also unchanged (17.2, 17.9 ms/mm Hg respectively) compared to placebo (19.9 ms/mm Hg). However both regression lines were shifted (p less than 0.05) to the left compared to placebo.

Evaluation of two oxprenolol oral osmotic (OROS) delivery systems in patients with essential hypertension

The effects of two oxprenolol oral osmotic (OROS) delivery systems on heart rate and blood pressure before and during recovery from exercise at a predetermined load were examined in twelve patients with hypertension previously responding to beta-blocker monotherapy. Haemodynamic responses were attenuated during the 24 h after single and repeated (15 days') once daily administrations of 10/170 and 16/260 oxprenolol OROS. At 24 h after repeated doses, compared to placebo there were significant reductions in resting blood pressure and in heart rate immediately following exercise. Attenuation of heart rate after exercise was dose related but differences between the systems with respect to resting heart rate and blood pressure were inconsistent. Antihypertensive responses after repeated doses were greater than those after single doses. However, reductions in resting and exercise heart rates were consistently less on chronic therapy. This may reflect enhanced expression of the partial agonist activity of oxprenolol due to altered receptor sensitivity after prolonged beta-blockade. The plasma oxprenolol profiles after both systems indicated slow absorption and substantial concentrations were apparent 24 h after drug administration. These observations suggest that both oxprenolol OROS systems display sustained drug release and on once daily dosing provide 24 h beta-blockade and control of blood pressure at rest and following exercise.

Lipophilicity, hydrophilicity, and the central nervous system side effects of beta blockers

One of the attributes of beta-adrenergic blocking agents that has distinguished these drugs from each other is degree of lipophilicity. While this feature may play a role in facilitating passage across the blood-brain barrier, it is essential to realize that crossing the barrier is not necessarily synonymous with the ability to cause central nervous system (CNS) effects. Several studies have found some degree of CNS side effects, particularly tiredness and fatigue, with atenolol, a hydrophilic beta blocker. Pindolol, a moderately lipophilic beta blocker, has been reported to cause greater disturbances on electroencephalogram (EEG) than propranolol, the most highly lipophilic beta blocker. The investigational agent bevantolol exhibits a moderate degree of lipophilicity and a low frequency of CNS side effects. Drug-induced increases in plasma catecholamine levels, the possible saturation of CNS receptor sites at relatively low drug levels, and the specific structural details of beta-blocker molecules have been suggested as possible contributory factors in determining the degree of CNS effects.

Effect of beta blockers on vascular resistance in systemic hypertension

Over 20 years ago, it was established that beta blockers could reduce high blood pressure. Currently, several beta blockers with different ancillary properties are available. They all have the property of blocking beta 1 receptors but differ from each other in a number of other respects: they may or may not block beta 2 receptors in low doses (beta 1 = receptor selectivity); they may or may not possess varying degrees of partial agonist activity, also known as intrinsic sympathomimetic activity (ISA); they vary in the extent to which they are soluble in fat (lipophilicity). A review of relevant published findings indicates that the effects of beta blockers on cardiac output are not essential for their antihypertensive effect, nor is penetration of these drugs into the brain and cerebrospinal fluid. Reduction in blood pressure during long-term beta blocker therapy is always associated with reduction of total peripheral resistance. Beta blockers with sufficient ISA to prevent cardio-depression, by exerting less negative inotropic and chronotropic effects on the heart, do not cause initial reflex vasoconstriction in response to cardiac beta blockade. Unlike beta blockers devoid of ISA, these agents ultimately reduce blood pressure by lowering the increased vascular resistance in hypertension to below pretreatment values. Recent beta blocker research has revealed a number of ways to manipulate the characteristically elevated vascular resistance in hypertension. Examples of these efforts are the combination of ISA, alpha 1 or alpha 2 receptor blockade and direct vasodilating properties in the enantiomers of a single beta blocker molecule. The practical significance of these developments remains to be established.

Effect of beta1 selective adrenoceptor blockade on physiological response to exercise

The effect of the beta1 selective adrenoceptor blocker, atenolol, on the physiological response to exercise was studied in 12 healthy young men. Oral atenolol (100 mg) and placebo were administered in a randomised double blind crossover fashion an hour and a half before an intermittent multistage cycle ergometer exercise test. At maximal effort oxygen consumption, pulmonary ventilation, carbon dioxide output, and respiratory exchange ratio were not modified by atenolol. In contrast, maximal heart rate and performance time were significantly reduced after atenolol. Nevertheless, because the relation of percentage of maximal oxygen consumption to percentage of maximal heart rate was not changed by atenolol both the absolute and relative oxygen consumption corresponding to 70% and 85% of the maximal heart rate remained unaltered. These data suggest that recommendations of exercise intensity may be determined on the basis of a calculated percentage of the predetermined maximal heart rate in persons without symptomatic coronary heart disease receiving beta1 selective adrenoceptor blockers.

Effect of selective and nonselective beta-adrenoceptor blockade on thermoregulation during prolonged exercise in heat

The effect of selective and nonselective beta-adrenoceptor blockade on the thermoregulatory responses of 11 physically active, healthy, young adult men was studied during 2-hour block-stepping in heat. The trial consisted of 3 periods of 6 days each during which propranolol (160 mg/day), atenolol (100 mg) or matching placebo was administered in a randomized, double-blind crossover fashion. Propranolol and atenolol induced similar, significant (p less than 0.001) increases in subjective ratings of perceived exertion. The mechanism of this increased fatigue was not evident from the documented alterations in serum electrolyte, blood glucose and blood lactate levels or ventilatory parameters. Propranolol did, however, induce a postexercise delayed serum-potassium reversion. Although rectal and mean skin temperature responses were essentially unaltered by beta-adrenoceptor blockade during block-stepping, an increased total sweat loss was observed with propranolol (p less than 0.01 versus placebo) and to a lesser degree with atenolol (p = not significant versus placebo). This indicates that persons receiving beta-adrenoceptor blockers have an increased need to adhere to a strict fluid-replacement regimen during exercise. This potentially adverse response was minimal with atenolol in contrast to propranolol, and this in turn suggests the use of beta1-selective adrenoceptor blockers during prolonged exercise when adequate fluid replacement is not possible.

Hypertension and beta-blockers. Are they all the same?

The circadian rhythm of blood pressure derived from continuous recordings of blood pressure in hypertensive subjects, provides an accurate and repeatable method of describing blood pressure on a temporal scale which can be used to evaluate the effects of anti-hypertensive drugs. It has been postulated that the rapid rise of pressure between 0600 and 1000 hr is due to intrinsic sympathetic activity mediated at the peripheral arteriole by alpha-adrenoreceptors. Drugs which block beta-adrenoreceptors do not appear to influence blood pressure at this time, no matter how frequently they are administered or how long-acting they may be. Meteroprolol and sotolol are beta-adrenoreceptor blocking drugs which appear to be different in that they produce a considerable fall in nocturnal blood pressure, which is similar to the effects of alpha-adrenoreceptor blocking agents. The physiological and clinical significance of these findings remains to be defined, but they emphasise the need for full and complete information on blood pressure changes before the most effective method of blood pressure reduction can be defined.