Effect of captopril and hydrochlorothiazide on the response to pressor agents in hypertensives

Abated cardiovascular responses to chronic oral lisinopril treatment in conscious elderly rats

To determine whether the cardiovascular effects of chronic treatment with lisinopril are age related, we compared baroreflex sensitivity and pressor responsiveness in 4-mo- and 21-mo-old male rats that had been given oral lisinopril daily for 4 wk. Reflex bradycardia elicited by elevating blood pressure with phenylephrine was stronger in 4-mo-old rats than it was in 21-mo-old rats and also stronger in lisinopril-treated rats than it was in untreated rats of the same age. Pressor responses to angiotensin or norepinephrine were recorded after combined cholinergic and beta-adrenergic blockade and then analyzed not only as absolute but also as percent increases in mean pressure. Although pressor responses seemed to be slightly reduced by lisinopril when expressed as absolute increases in mean pressure, corresponding percent increases were always larger in 4-mo-old rats than they were in 21-mo-old rats and were clearly enhanced by lisinopril more in younger rats. The stronger overall enhancement of pressor responsiveness and reflex bradycardia in younger rats suggests that the cardiovascular effects of lisinopril diminish with advancing age.

ACE inhibition and pressor responsiveness to norepinephrine in hypertensive patients

The pressor response to norepinephrine (NE) was assessed in normal renin essential hypertensive patients before and after they were randomly assigned to receive in parallel groups of treatment a single dose of an angiotensin converting enzyme (ACE) inhibitor (captopril or lisinopril) or a prolonged therapy with lisinopril (30-45 days) or with hydrochlorothiazide (9 days). Blood pressure was significantly reduced by all treatments. The pressor response to NE was unchanged after the single administration of the ACE inhibitors, while it was blunted after chronic administration of lisinopril and after the diuretic. On the basis of these results, it is suggested that the attenuation of the sympathetically mediated vasoconstriction may represent an additional mechanism contributing to the antihypertensive effect of ACE inhibitors administered chronically.

A comparison of the haemodynamic and hormonal effects of low and conventional dose cyclopenthiazide in normal volunteers

In this study we compared low (125 micrograms) and conventional (500 micrograms) doses of cyclopenthiazide on the renin angiotensin system, plasma and extracellular fluid volumes and the pressor responsiveness to angiotensin II since we have previously shown that the two doses have the same antihypertensive effect but different effects on plasma renin activity. Following a two week placebo run-in period, 8 healthy male volunteers received 125 micrograms or 500 micrograms of cyclopenthiazide for 2 treatment periods of 4 weeks as part of a double blind, 2-part crossover study with treatment periods separated by a 4-week placebo washout phase. Measurements were made on two study days at the beginning and end of the active treatment periods. On the first day serum potassium, plasma renin activity and plasma angiotensin II levels were measured after a 1 h period of supine rest. Plasma and extracellular fluid volumes were also measured after appropriate equilibration times. The blood pressure responses to angiotensin II were assessed on day 2. The 500 micrograms dose of cyclopenthiazide had a greater effect than the 125 micrograms dose on plasma renin activity, serum potassium, angiotensin II levels and extracellular fluid volumes. Neither drug had any effect on plasma volume or the responsiveness to infused angiotensin II. Low dose cyclopenthiazide failed to increase angiotensin II levels, contract body fluid volumes or attenuate vascular reactivity in normotensive volunteers.

Angiotensin-converting enzyme inhibitors: mechanistic controversies

Many studies have investigated the mechanisms responsible for the therapeutic effects of the angiotensin converting enzyme inhibitors. Initially, the hemodynamic changes that occur with these agents were attributed solely to the inhibition of the renin-angiotensin-aldosterone system in plasma. Further research suggested other mechanisms were operable as a relationship was not always evident between hemodynamic changes and inhibition of the plasma renin-angiotensin-aldosterone system. A relationship between the pharmacodynamics of these agents and the inhibition of vascular and tissue renin-angiotensin systems, however, has been observed. Mechanisms less likely to contribute to the actions of the angiotensin converting enzyme inhibitors are increases in bradykinin and prostaglandin concentrations, or inhibition in the renin-angiotensin system within the central nervous system. Ancillary cardiovascular effects of angiotensin converting enzyme inhibitors offer possible new therapeutic gains. An understanding of these mechanistic controversies and newly-defined cardiovascular actions of angiotensin converting enzyme inhibitors are important to clinicians using these agents.

Pathogenetic and therapeutic relevance of cardiovascular pressor reactivity to norepinephrine in human hypertension

In normotensive humans with a positive family history of essential hypertension, blood pressure (BP) is often dysregulated. Body sodium, blood volume, plasma angiotensin II (AngII), epinephrine and norepinephrine (NE), their responses to changes in sodium intake or posture, as well as baroreflex function, beta-receptor-mediated cardiac responsiveness, and the responsiveness of BP to AngII appear to be largely unaltered. However, the responsiveness of BP to NE is commonly exaggerated. An increase in potassium intake may improve the NE hyperreactivity and concomitantly lower BP within the "normotensive" range. Therefore, a selective vascular NE hyperreactivity relative to existing sympathetic activity seems to be a common familial abnormality predisposing for the development of essential hypertension. In borderline or established essential hypertension, an inappropriate vascular reactivity relative to sympathetic activity probably complements other mechanisms contributing to the maintenance of hypertension. Various antihypertensive treatments may lower BP at least in part by improving cardiovascular NE (hyper)reactivity without provoking an equivalent rise in sympathetic activity. These include dietary potassium supplementation, thiazide-type agents, indapamide, calcium channel blockers, postsynaptic alpha 1-blockers, and AngII converting enzyme inhibitors.

Angiotensin-converting enzyme inhibitors

There is convincing evidence that ACE inhibitors, alone or in combination with a diuretic, effectively lower blood pressure in patients with all grades of essential or renovascular hypertension and that they are of particular benefit as adjunctive therapy in patients with congestive heart failure. The hemodynamic, hormonal and clinical effects of the presently available ACE inhibitors, captopril and enalapril, are comparable and their side effect profiles are extremely favorable. One important difference between the two oral ACE inhibitors, however, is their pharmacokinetics; enalapril's action is slower to begin and is of longer duration. Compared with other agents, ACE inhibitors offer important advantages, among them an improved feeling of well being. It is, therefore, expected that ACE inhibitors will gain greater acceptance by patients and physicians in the future.

The effect of acute and chronic captopril therapy on baroreflex function in man

1. The effect of captopril 12.5 mg three times daily for 14 days on baroreflex sensitivity was investigated in six normotensive salt-replete male subjects. 2. Baroreflex sensitivity [delta R-R interval ms/mmHg systolic blood pressure (SBP)] was assessed following decreases and increases in SBP with glyceryl trinitrate and phenylephrine respectively and during Valsalva's manoeuvre. 3. Captopril had no effect on supine intra-arterial SBP or R-R interval on days 1 and 15 compared with placebo. However decreases (P less than 0.05) which occurred in plasma angiotensin II on day 1 (12.3 +/- 0.8 to 7.6 +/- 1.5 pg ml-1) and on day 15 (14.5 +/- 1.5 to 6.3 +/- 0.5 pg ml-1) and increases (P less than 0.02) in plasma renin activity on day 1 (1.2 +/- 0.7 to 4.8 +/- 0.8 ng A I ml-1 h-1) and on day 15 (1.1 +/- 0.2 to 5.4 +/- 0.9 ng A I ml-1 h-1) compared with placebo, indicated angiotensin-converting enzyme inhibition. 4. Baroreflex sensitivity was unchanged by captopril on day 1 and day 15 compared with placebo following the use of glyceryl trinitrate and phenylephrine to deactivate and activate respectively the baroreceptors, and during the strain and release phases of Valsalva's manoeuvre.

Double-blind comparison of captopril and enalapril in mild to moderate hypertension

To compare the antihypertensive and humoral effects of the angiotensin-converting enzyme inhibitors captopril and enalapril, 20 patients with essential hypertension, not receiving treatment for 2 weeks and consuming a prescribed sodium ion intake, were randomly assigned to two parallel, double-blind treatment groups with stratification based on race and untreated seated diastolic blood pressure. These groups received a placebo (day -1) followed by either captopril, 200 mg every 12 hours (n = 9), or enalapril maleate, 20 mg every 12 hours (n = 11), alone (days 1 to 14) and then with hydrochlorothiazide, 25 mg every 12 hours (days 16 to 28). Captopril and enalapril were coadministered alone (day 15) and with hydrochlorothiazide (day 29) to assess whether further decreases in blood pressure would occur. Captopril and enalapril alone caused comparable decreases (p less than 0.05) in the mean 12 hour time-averaged seated diastolic blood pressure from values on day -1 (placebo), on day 1 (11 and 9 mm Hg, respectively) and day 14 (8 and 7 mm Hg, respectively). The addition of hydrochlorothiazide further decreased (p less than 0.05) blood pressure in each group (7 and 8 mm Hg, respectively) from values on day 14. Combined use of captopril and enalapril did not result in further reduction. Coupled with the comparable changes observed in each treatment group in serum angiotensin-converting enzyme activity, plasma renin activity and plasma aldosterone concentration, these data support the view that captopril and enalapril have similar antihypertensive effects and mechanisms.