The possible importance of aldosterone as well as renin in the long-term antihypertensive action of propranolol

Mechanisms of target organ damage caused by hypertension: therapeutic potential

Hypertension is a major risk factor for cardiovascular mortality and morbidity through its effects on target organs like the brain, heart, and kidney. Structural alterations in the microcirculation form a major link between hypertension and target organ damage. In this review, we describe damages related to hypertension in these target organs and the mechanisms involved in the pathogenesis of hypertension-induced cardiovascular diseases such as dementia, cardiac ischemia and remodeling, or nephropathy. We also focus on the therapeutical potential on the basis of such mechanisms. Several antihypertensive agents like diuretics, angiotensin converting enzyme (ACE) inhibitors, angiotensin II (Ang II) receptor antagonists, beta-blockers, or calcium channel blockers (CCBs) have been shown to reduce effectively hypertension associated cardiovascular events and to improve end organ damage. More recently, aldosterone antagonism has also shown beneficial effects. Part of the favorable effects of these agents is due to blood pressure lowering as such. Other mechanisms such as oxidative stress, inflammation, or endothelial dysfunction have appeared to play a key role in the pathogenesis of target organ damage and therefore represent another important pathway for therapy. In this review, we discuss the different therapeutic approaches aiming at reducing cardiovascular events and damages induced by hypertension.

Circulatory and extracirculatory effects of angiotensin-converting enzyme inhibition

The antihypertensive effects of angiotensin-converting enzyme (ACE) inhibitors cannot be fully explained by their actions on the circulating renin-angiotensin system (RAS). Agents such as captopril or enalapril maintain efficacy during long-term therapy even when plasma concentrations of converting enzyme or angiotensin II are not fully suppressed. Components of the entire RAS exist at several sites, thereby making it possible for drugs to produce effects at extracirculatory locations. An ACE inhibitor such as quinapril that has a comparatively short plasma concentration half-life binds strongly to plasma ACE as well as to ACE in key tissues including artery wall, heart, and kidney. The effects of ACE inhibition on the tissue RAS are of potential importance in fully explaining the blood pressure-lowering effects of these drugs. ACE inhibitors might also reduce blood pressure by blocking nonhemodynamic actions of angiotensin II. They affect vascular properties by increasing compliance of arteries and they act on baroreceptors and central regulatory mechanisms. Furthermore, ACE inhibitors affect other neuroendocrine systems, including aldosterone, kinins, and prostaglandins; attenuation of sympathetic activity can contribute further to their antihypertensive properties. Actions independent of circulating renin effects do not necessarily require plasma ACE inhibition throughout a 24-hour period. Sustained antihypertensive effects by drugs with short durations of plasma ACE inhibition give credibility to therapeutic targets beyond the circulating RAS.

Calcium antagonists and the second drug for hypertensive therapy

Calcium antagonist monotherapy is more effective in older patients and in those with low plasma renin activity, whereas beta blockers control blood pressure better in younger patients and in those with normal or high renin activity. Monotherapy with a calcium antagonist has been shown to result in the reduction of diastolic blood pressure to equal to or less than 95 mm Hg in more than 80 percent of patients with essential hypertension. We investigated the antihypertensive efficacy of verapamil plus an angiotensin converting enzyme inhibitor and nifedipine plus a beta blocker in 24 patients (aged 41 to 68) with moderate to severe hypertension in whom monotherapy with a calcium antagonist had been ineffective. Blood pressure recorded in patients during the placebo period was 175 +/- 3/111 +/- 2 mm Hg (mean +/- SEM). Twelve patients received monotherapy with nifedipine (50.0 +/- 5.2 mg per day) and 12 others received verapamil (460 +/- 20 mg per day); neither treatment resulted in the reduction of diastolic blood pressure to less than 90 mm Hg. However, this goal was achieved when atenolol (89.5 +/- 25.7 mg per day) was added to the regimen of patients receiving nifedipine and enalapril (29.5 +/- 5.0 mg per day) was added to the regimen of those receiving verapamil; resultant blood pressures were 127 +/- 3/83 +/- 2 mm Hg and 137 +/- 5/85 +/- 1 mm Hg, respectively. It is suggested that in patients in whom hypertension is inadequately controlled by calcium antagonist monotherapy, counter-regulatory mechanisms can be blocked by the addition of a beta blocker or an angiotensin converting enzyme inhibitor to the calcium antagonist regimen, resulting in greatly improved, simple, well-tolerated, and safe control of blood pressure.

Relative potency of a beta-blocking and a calcium entry blocking agent as antihypertensive drugs in black patients

The short-term efficacy of nitrendipine (N) as a first stage antihypertensive drug in black patients has been assessed and compared with acebutolol (A) in a double-blind study. Forty patients were randomized and after a 4 week run-in period on placebo, the active treatment was administered for 6 weeks starting with 20 mg N or 200 mg A once daily. The dose was increased up to 60 mg N or 600 mg A as needed. Nitrendipine appeared to be more efficient than acebutolol in reducing blood pressure and the N-induced fall in blood pressure was achieved after 2 weeks. After 2 and 6 weeks on N, the recumbent blood pressure was decreased by 13% and 12% for the systolic and by 14% and 11% for the diastolic pressure. The concurrent decreases in the A group averaged 4% and 5% for the systolic and 5% and 10% for the diastolic pressure after 2 and 6 weeks. Pulse rate and plasma renin activity in the N group were slightly increased and body weight was decreased at the end of the active treatment period.

Indenolol: a new antihypertensive agent: efficacy, toxicity, and hemodynamic effects in a crossover double-blind study with metoprolol

The antihypertensive efficacy of a new agent, indenolol, was compared with that of the well-established antihypertensive drug, metoprolol, and its hemodynamic effects were investigated using echocardiography. Eighteen hypertensives completed a double-blind, crossover, randomized study using indenolol and metoprolol. Two four-week courses with indenolol or metoprolol were preceded and followed by a two-week placebo period; the total duration of the study was 14 weeks. Indenolol proved to be significantly more effective than metoprolol in decreasing blood pressure values at rest (P less than .05). Furthermore, three patients that failed with metoprolol were successfully treated with indenolol. Both drugs induced a significant decrease in cardiac output that was mediated mainly through a reduction in heart rate, because stroke volume, left ventricle circumferential fiber shortening velocity, and ejection fraction were not significantly reduced by either drug. However, after indenolol, a significant direct relationship was found between the basal values of both cardiac output (r = .809) and total peripheral resistance (r = .800), and the reduction of these parameters. On the contrary, after metoprolol only, the correlation between the basal value of cardiac output and its reduction was significant (r = .790).

Age related antihypertensive effect of nitrendipine, a new calcium entry blocking agent

The effect of nitrendipine 20 mg o.d., a new calcium entry blocker similar in structure to nifedipine, on blood pressure has been evaluated in 14 patients (aged 24-62 years) with uncomplicated mild or moderate arterial hypertension. A significant decrease both in systolic (160 +/- 12 at baseline vs 141 +/- 8 mm Hg, p less than 0.001) and diastolic (106 +/- 8 vs 93 +/- 3 mm Hg, p less than 0.001) blood pressure was observed at the end of 8 weeks of nitrendipine treatment. An inverse correlation was found between age and the reduction in diastolic blood pressure (r = 0.772, p less than 0.001 as absolute reduction; r = 0.791, p less than 0.001 as percentage reduction versus baseline). This peculiar characteristic differentiates the effect of nitrendipine from that of other calcium entry blockers, which appear to be more effective in older patients.

Age and cardiovascular response adaptation. Determinants of an antihypertensive treatment concept primarily based on beta-blockers and calcium entry blockers

The patient's age has great impact on the development of hypertension, its duration, and severity. In patients with essential hypertension, sympathetic cardiovascular control changes from an early phase with increased beta-adrenoceptor-mediated responses, e.g., cardiac output and renin, into a later phase where these responses are blunted and alpha-adrenoceptor-mediated vasoconstriction prevails, associated with higher intracellular free sodium and calcium concentration. This pathophysiological view of essential hypertension has its corollary in the pharmacotherapeutic approach. Younger patients, who often have high renin levels, respond better to monotherapy with a beta-blocker or with a converting-enzyme inhibitor. Older patients, who often have low renin levels, respond less well to beta-blockers but particularly well to calcium entry blockers as an alternative to diuretics. Therefore, beta-blockers and calcium entry blockers form new cornerstones for antihypertensive treatment and strategy, with the potential of cardioprotection.

Vasoconstrictor effects of aldosterone in isolated vascular tissue

Isolated rings of rabbit ear artery or thoracic aorta were prepared for the measurement of isometric contraction. Treatment of ear artery with 1 X 10(-4)M aldosterone had no effect. However, in ear artery pretreated with 1 X 10(-7)M desipramine to block the catecholamine neuronal uptake process, 1 X 10(-6), 1 X 10(-5) and 1 X 10(-4)M aldosterone elicited contractions of 0.16, 0.48 and 1.31 g, respectively. Phentolamine, 1 X 10(-7)M, an alpha adrenoceptor antagonist, both prevented and reversed the aldosterone-induced contractions. Desipramine-pretreated thoracic aortas were contracted to a steady-state of 1-2 g with norepinephrine and immersed in mineral oil to prevent diffusion of drugs from the tissue. Relaxation, which reflects the rate of tissue inactivation of norepinephrine, was markedly slowed by exposure of these tissues to either aldosterone, deoxycorticosterone acetate (an inhibitor of the catecholamine-extraneuronal uptake process) or the combination of these steroids prior to oil immersion. It is concluded that in the ear artery, endogenous norepinephrine was spontaneously released from intramural nerves and that aldosterone blocked the catecholamine extraneuronal uptake process within the muscle coat allowing the concentration of norepinephrine to increase well above the threshold for contraction.

The role of aldosterone in the response to treatment of primary hypertension

The role of the renin-aldosterone axis in mediating responses to antihypertensive treatment was examined in patients with essential hypertension treated with propranolol (n=50) or chlorthalidone (n=50). Control plasma renin activity tended to be predictive of the response to treatment: it correlated with the diastolic blood pressure changes produced by both the renin-lowering agent, propranolol (r=-.43, P less than 0.001), and the diuretic, chlorthalidone (r= .48, P less than 0.001). However, when patients were divided into responders (fall in diastolic blood pressure of greater than 10%) and non-responders, the treatment-induced changes in aldosterone excretion rate appeared to be more important than the changes in renin in determining outcome. During propranolol there was no difference between the renin changes in the responders (n=21) and the non-responders (n=29), but aldosterone fell significantly more (P less than 0.025) in the responders than in the non-responders. Similarly, renin changes during chlorthalidone were not markedly different between the two groups, but the increase in aldosterone excretion in the non-responders was significantly greater (P less than 0.001) than in the responders (n=25). Thus, regardless of the treatment used, responders were characterized by having lower levels of aldosterone during therapy than non-responders. This possible role for aldosterone in influencing blood pressure changes was not related to clinically measurable effects on fluid and electrolyte balance; it has been conjectured that this pressor effect of aldosterone could reflect a direct action in the peripheral circulation.

[High dose betablocker treatment in experimental renal hypertension (author's transl)]

Rats with experimental renovascular hypertension were treated with high doses of beta-blocking agents. Pindolol 10 mg/kg per day increased Goldblatt-type hypertension, whereas Propranolol 100 mg/kg per day showed an antihypertensive effect. A linear correlation between the weight of the left-heart ventricle and the systolic blood pressure was found in animals treated with Pindolol as well as in untreated Goldblatt rats. On the contrary, the weights of the left-heart ventricle were significantly higher in those animals treated with Propranolol than in the other groups. The results show that Pindolol in high doses, possibly on account of its sympathomimetic activity, leads to an increase in Goldblatt hypertension, but does not influence the pressure-dependent progression of left-heart hypertrophy. On the other hand, Propranolol, possibly on account of a cardiodepressory effect, leads to a decrease in blood pressure.

The substitutive role of ACTH in supporting aldosterone response to head-up tilt during acute renin suppression in patients with essential hypertension

The relative contribution of the renin-angiotensin system, adenocorticotrophic hormone (ACTH) and plasma electrolytes in the response of plasma aldosterone to 30 minutes of 65 degrees head-up tilt was assessed in 10 essential hypertensive patients. Studies were carried out before and during acute blockade of renin release by propranolol, ACTH suppression by dexamethasone and combined renin and ACTH blockade. In control studies orthostasis induced significant increases only in plasma renin activity and aldosterone. In contrast, when the renin response to tilt was acutely suppressed by propranolol administration, the aldosterone response was nonetheless maintained but now appeared to be under ACTH control, since concurrent increases in cortisol were observed. During ACTH suppression aldosterone increased during tilt and so did renin. However, during combined ACTH and renin blockade aldosterone failed to increase during tilt. These studies suggest that the aldosterone secretory response to head-up tilt is normally mediated by the renin-angiotensin system but, when the renin response is suppressed, an ACTH response is elicited which assumes a backup role. However, when these two systems are blocked other factors appear unable to respond during tilt to support a normal aldosterone response.