Urinary excretion of prostacyclin and thromboxane A2 metabolites after angiotensin converting enzyme inhibition in hypertensive patients

Systemic prostacyclin and thromboxane production in obstructive sleep apnea

PURPOSE

Obstructive sleep apnea increases the risk of cardiovascular diseases. Alternations in prostacyclin and thromboxane concentrations and balance could constitute one of mechanisms linking sleep apnea and cardiovascular events. Thus we aimed to assess the concentrations of 6-keto-prostaglandin F1α (6-keto-PGF1α) (metabolite of prostacyclin) and thromboxane B2 (TXB2) (metabolite of thromboxane A2) in urine and blood of obstructive sleep apnea patients and controls (snoring subjects with otherwise normal polysomnogram).

MATERIAL AND METHODS

Overnight urine and morning blood samples were taken from subjects and controls at baseline and in sleep apnea group during continuous positive airway pressure (CPAP) treatment. Samples were analyzed using mass chromatography/gas spectrometry.

RESULTS

We analyzed data from 26 obstructive sleep apnea subjects (mean apnea-hypopnea index 45.4±17.3) and 22 well-matched controls. At baseline sleep apnea patients, when compared to controls, have higher 6-keto-PGF1α in urine (0.89±0.15 vs 0.34±0.06, p=0.01) and blood (24.49±1.54 vs 19.70±1.77, p=0.04). TXB2 levels in urine and blood were not different across groups. CPAP treatment significantly decreased 6-keto-PGF1α in urine (0.92±0.17 vs 0.22±0.10, p=0.04), but not in blood. TXB2 levels during CPAP treatment did not change significantly.

CONCLUSIONS

These results suggest augmented systemic prostacyclin production in obstructive sleep apnea patients, which potentially could constitute a protective mechanism against detrimental effects of sleep apnea.

Angiotensin-converting enzyme inhibitors

ACE inhibitors have achieved widespread usage in the treatment of cardiovascular and renal disease. ACE inhibitors alter the balance between the vasoconstrictive, salt-retentive, and hypertrophic properties of angiotensin II (Ang II) and the vasodilatory and natriuretic properties of bradykinin and alter the metabolism of a number of other vasoactive substances. ACE inhibitors differ in the chemical structure of their active moieties, in potency, in bioavailability, in plasma half-life, in route of elimination, in their distribution and affinity for tissue-bound ACE, and in whether they are administered as prodrugs. Thus, the side effects of ACE inhibitors can be divided into those that are class specific and those that relate to specific agents. ACE inhibitors decrease systemic vascular resistance without increasing heart rate and promote natriuresis. They have proved effective in the treatment of hypertension, they decrease mortality in congestive heart failure and left ventricular dysfunction after myocardial infarction, and they delay the progression of diabetic nephropathy. Ongoing studies will elucidate the effect of ACE inhibitors on cardiovascular mortality in essential hypertension, the role of ACE inhibitors in patients without ventricular dysfunction after myocardial infarction, and the role of ACE inhibitors compared with newly available angiotensin AT1 receptor antagonists.

ACE inhibitor effects on platelet function in stages I-II hypertension

Angiotensin II enhances platelet aggregation through activation of the G protein-linked pathway present in platelets. Studies of several angiotensin-converting enzyme (ACE) inhibitors have demonstrated marked differences on platelets. Therefore this prospective, randomized, double-blind, crossover study compared the ex vivo effects of equivalent antihypertensive doses of captopril, enalapril, and fosinopril on platelet aggregation and thromboxane B2 (TxB2) formation in subjects with stage I-II essential hypertension. Nineteen male subjects with a baseline mean seated blood pressure of 141 +/- 3/100 +/- 1 mm Hg were enrolled. The decline in mean arterial pressure after 4 weeks of stable dosing was 10 +/- 1, 12 +/- 1, and 11 +/- 1 mm Hg for captopril, enalapril, and fosinopril, respectively (p = NS). There was no significant change in adenosine diphosphate (ADP)-, epinephrine-, or thrombin-stimulated platelet aggregation from baseline or between ACE inhibitors. Compared with baseline, fosinopril decreased TxB2 concentrations 27.5-67.6% with all stimuli after 1 and 5 min. Captopril also decreased TxB2 formation, but this effect was stimulus and time dependent. Enalapril consistently increased TxB2 concentrations, independent of stimuli or time. We conclude that different ACE inhibitors have distinct effects on platelet TxB2 formation without significant effects on platelet aggregation. Fosinopril may be a direct antagonist ofTxA2 synthase, suggesting benefit in syndromes of platelet activation or vascular occlusion.

Angiotensin converting enzyme inhibitors improve contractile function of stunned myocardium by different mechanisms of action

Angiotensin converting enzyme (ACE) inhibitors enhance contractile function of myocardium "stunned" by a brief episode of coronary artery occlusion, yet their mechanism(s) of action remain unresolved. In addition to possible hemodynamic effects, ACE inhibitors may stimulate the synthesis of cardioprotective prostaglandins. Furthermore, the beneficial effects of ACE inhibitors that contain a sulfhydryl group may be due in part to the ability of thiol compounds to act as nonspecific antioxidants or direct scavengers of cytotoxic oxygen-derived free radicals. To investigate this question we compared the effects of (1) the sulfhydryl-containing ACE inhibitor zofenopril, (2) the sulfhydryl-containing stereoisomer of captopril (SQ 14,534) with essentially no ACE inhibitor properties, (3) the nonsulfhydryl-containing ACE inhibitor enalaprilat, and (4) solvent alone, given at the time of reperfusion, on recovery of contractile function after 15 minutes of coronary occlusion in the anesthetized open-chest dog. Segment shortening in control animals remained depressed or "stunned" after reperfusion, recovering to only -5 +/- 12% of baseline preocclusion values at 3 hours after reperfusion. In contrast, all three treatment agents attenuated postischemic dysfunction: segment shortening was restored to 33 +/- 12%, 54 +/- 6%, and 83 +/- 5% of baseline values at 3 hours after reflow in dogs treated with SQ 14,534 (p less than 0.05), zofenopril (p less than 0.01), and enalaprilat (p less than 0.01), respectively (all vs control value). These improvements in segment shortening did not appear to be the result of altered oxygen supply or demand after reperfusion, inasmuch as no significant differences in systemic hemodynamic parameters or myocardial blood flow were observed among the groups. In the second phase of the study, we found that the improved contractile function associated with enalaprilat treatment could largely be reversed by infusion of the potent cyclooxygenase inhibitor indomethacin: segment shortening was reduced from 69 +/- 12% at 2 hours after treatment/reperfusion to 38 +/- 12% at 2 hours after indomethacin infusion (p less than 0.01 vs 2 hours after reperfusion). Infusion of indomethacin had no effect, however, on the improved contractile function associated with zofenopril treatment. We therefore conclude that sulfhydryl- versus nonsulfhydryl-containing agents enhance contractile function of stunned myocardium by different mechanisms of action: enalaprilat attenuates postischemic dysfunction at least in part by a prostaglandin-mediated mechanism, whereas the salutary effects of zofenopril and SQ 14,534 may be due in part to the antioxidant properties of the sulfhydryl moiety.

Angiotensin-converting enzyme inhibitors and renal function

The effects of angiotensin-converting enzyme (ACE) inhibitors on renal hemodynamics vary widely depending on the preexisting physiologic and pathologic state of the kidneys. Although some studies of ACE inhibitors in primary essential hypertension have demonstrated increases in glomerular filtration rate (GFR) and effective renal plasma flow in patients with renal impairment, other studies have not shown these same beneficial results. The difference may involve the choice of ACE inhibitor used in the investigations, but controlled comparison trials are needed to determine whether this is the case. The use of ACE inhibitors in renovascular hypertension remains controversial. ACE inhibition can interfere with the autoregulation of GFR mediated by angiotensin II and may lead to deterioration of renal function, especially in patients with bilateral renal artery stenosis or stenosis of a solitary kidney. Additionally, ACE inhibitors have been shown to cause a decline in GFR in the kidney affected by the stenosis, whether or not clinically apparent renal insufficiency occurs. Although the functional impairment associated with ACE inhibitors in renal artery stenosis has generally been reversible following removal of the drug, the consequences of a long-term reduction in GFR are unknown. Treatment of stable congestive heart failure (CHF) with ACE inhibitors can result in enhancement of GFR and reduction of sodium and fluid retention, thus improving the clinical state. However, in patients with decompensated cardiac failure, renal perfusion pressures may already be at or near the autoregulatory breakpoint and ACE inhibition may cause deterioration of renal function. In general, ACE inhibitors can be used safely in CHF if they are initiated cautiously, with adjustment of ACE inhibitor and diuretic dosages to avoid systemic hypotension and sodium and fluid depletion. In studies comparing the agents, enalapril and lisinopril have both been shown to cause higher incidences of renal function deterioration than has captopril. These findings suggest that the more complete or sustained ACE inhibition seen with the longer-acting agents may be detrimental to renal function in patients with CHF. The use of ACE inhibitors in the treatment of proteinuria is the newest area of research with these agents. At present it appears that ACE inhibitors reduce urinary protein excretion the most effectively in diabetic patients with mild proteinuria and in hypertensive patients with renal insufficiency and proteinuria due to glomerular disorders. More study is needed to determine whether these agents can reduce the rate of renal failure progression and to define the patient populations expected to benefit most.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Effect of captopril or enalapril on renal prostaglandin E2

Since one of the hypotensive mechanisms of angiotensin-converting enzyme inhibitor (ACEI) has been suggested to be mediated through the renal kinin-prostaglandin (PG) axis, the present study was designed to investigate the effect of captopril (C) or enalapril (E) on renal PGE2 excretion or synthesis. Wistar male rats (BW 200-250 g) were given orally captopril at 30 mg/kg/day or enalapril at 10 or 30 mg/kg for one week. Before and after ACEI, blood pressure (tail cuff method) as well as PRA and urinary PGE2 excretion was determined. Renopapillary slices were obtained from some of the rats including controls and incubated to determine PGE2 synthesis. C or E administration resulted in a blood pressure decrease of 21 to 36 mm Hg with an increase in PRA. Urine volume and sodium excretion increased after daily treatment with C or E at 30 mg/kg. Urinary PGE2 excretion increased 1.4-fold in response to C, but not to E. Papillary PGE2 synthesis demonstrated a marked decrease 2 h after in vivo administration of either ACEI compared to controls. However, when C or enalaprilat was added in vitro to renal slices obtained from controls, only C at 10(-5) M showed a significant 2-fold increase in renal PGE2 synthesis. These results suggest that (1) renal PGE2 synthesis may be dependent on circulating angiotensin II. (2) C, but not enalaprilat, has a direct stimulatory effect on renal PGE2 synthesis and (3) renal PGE2 may not be involved very much in the hypotensive effect of ACEI.

The interaction between indomethacin and captopril or enalapril in healthy volunteers

In this study eight healthy volunteers were involved in a randomized, cross-over trial in which they were treated with either 25 mg of captopril b.i.d. or 20 mg of enalapril o.i.d. alone or in combination with 50 mg of indomethacin b.i.d. in order to detect a difference between both converting enzyme inhibitors when interacting with indomethacin. Before and after each 4-d treatment period, blood pressure (determined by random zero sphygmomanometer), body weight, plasma renin activity, angiotensin converting enzyme, plasma potassium, serum creatinine and the 24-h urinary excretion of 6-keto-prostaglandin F1 alpha were measured. Indomethacin attenuated the decrease of supine diastolic blood pressure during treatment with captopril, but not with enalapril. However, the initial decrease of blood pressure on captopril was greater than on enalapril. Both converting enzyme inhibitors had no effect on the urinary excretion of 6-keto-prostaglandin F1 alpha, while indomethacin reduced it. The results suggest a difference between captopril and enalapril in interaction with indomethacin.

Angiotensin converting enzyme inhibitors (captopril, CGS14831 and CGS14824A) antagonise in vitro smooth muscle prostanoid synthesis: evidence for calcium channel blockade

The present study investigated the effect of the angiotensin-converting enzyme (ACE) inhibitors, captopril, CGS14824A and CGS14831 on in vitro rat aortic and urinary bladder prostacyclin (PGI2; measured as 6-oxo-PGF1 alpha by radioimmunoassay) synthesis. PGI2 synthesis was stimulated with adrenaline (aorta), carbachol (bladder), calcium ionophore A23187, arachidonate and trauma. The ACE inhibitors antagonised adrenaline-, carbachol- and A23187-stimulated PGI2 synthesis in the aorta and bladder (CGS14824A greater than captopril greater than CGS14831) but were without effect on trauma- or arachidonate-stimulated PGI2 synthesis. The patterns of inhibition of these ACE inhibitors, using the same stimulatory regimes, was very similar to those previously observed by us with known calcium channel blockers (nifedipine, verapamil). These data suggest that: (i) ACE inhibitors possess calcium channel blocking properties, which may be of relevance to the antihypertensive action of these drugs; and (ii) ACE inhibitors did not stimulate vascular PGI2 synthesis as has been previously suggested.

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.

Enalapril: a long-acting angiotensin-converting enzyme inhibitor

The development of angiotensin-converting enzyme (ACE) inhibitors is of landmark importance in the understanding and treatment of cardiovascular disorders, particularly hypertension and congestive heart failure. Enalapril has recently joined captopril as an approved, orally active ACE inhibitor. Like captopril, it has been effective in the treatment of hypertension and congestive heart failure, with minimal adverse reactions noted. Differences in pharmacology exist between enalapril and captopril which may prove to be of clinical significance.

Effects of indomethacin on hormonal and blood pressure responses to captopril in spontaneously hypertensive rats

The possible role of vasodilatory prostanoids in the antihypertensive action of captopril was investigated in spontaneously hypertensive rats (SHR). Captopril (100 mg/kg/day for 5 days) decreased systolic blood pressure and increased water consumption, urine excretion and plasma renin activity (PRA). It also enhanced the urinary excretion of the prostacyclin metabolite 6-keto-PGF1 alpha, but did not change the excretion of PGE2. Indomethacin (3 mg/kg/day), given both alone and in combination with captopril, reduced markedly the urinary excretions of 6-keto-PGF1 alpha and PGE2 but did not alter PRA, compared with corresponding groups without indomethacin. The suppression of prostanoid synthesis caused by indomethacin did not affect the antihypertensive effect of captopril or the basal blood pressure in SHR. Neither did indomethacin influence drinking or urine excretion in SHR not receiving captopril, but it reduced the dipsogenic and diuretic effects of captopril. The results suggest that captopril augments the production of vasodilatory prostacyclin. Yet prostanoids have no significant role in the antihypertensive mechanism of captopril in SHR.

Plasma angiotensins and blood pressure during converting enzyme inhibition

The relationship between plasma angiotensin and the reduction of blood pressure with the angiotensin converting enzyme inhibitor enalapril was studied in rats. Blood pressure was measured in conscious rats with indwelling arterial catheters. To measure angiotensin II, plasma was analyzed by physical separation of angiotensins using high performance liquid chromatography followed by radioimmunoassay. The effects of both single (acute) and long-term (chronic) dosages of enalapril were measured. After a single oral dose of enalapril (10 mg/kg), mean arterial pressure fell from 111 +/- 3 to 86 +/- 3 mm Hg (p less than 0.005). Despite the blood pressure reduction, plasma angiotensin II was unaffected (control, 9.9 +/- 1.8 vs 9.7 +/- 1.1 pg/ml). After a higher single oral dose of enalapril (30 mg/kg), there was a reduction in both mean arterial pressure (81 +/- 5 mm Hg, p less than 0.005) and plasma angiotensin II concentration (2.3 +/- 0.6 pg/ml, p less than 0.01). The chronic effects of converting enzyme inhibition were evaluated in rats given enalapril in their drinking water (30 mg/kg 24 hr) for 1 week or 2 months. Mean arterial pressure remained equally low after chronic administration (for 1 week or 2 months), but plasma angiotensin II increased above normal (after 1 week, 28.9 +/- 8.7, p less than 0.01 vs control; after 2 months, 43.1 +/- 16.2 pg/ml, p less than 0.05 vs control). Although plasma angiotensin converting enzyme activity was undetectable at any time after enalapril administration, there was a partial return of the angiotensin I pressor response with chronic administration. The data are most compatible with actions of converting enzyme inhibitors independent of the blockade of plasma angiotensin II formation.

Enalapril. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use in hypertension and congestive heart failure

Enalapril maleate is an orally active angiotensin-converting enzyme inhibitor. It lowers peripheral vascular resistance without causing an increase in heart rate. Enalapril 10 to 40 mg/day administered either once or twice daily is effective in lowering blood pressure in all grades of essential and renovascular hypertension, and shows similar efficacy to usual therapeutic dosages of hydrochlorothiazide, beta-blockers (propranolol, atenolol and metoprolol) and captopril. Most patients achieve adequate blood pressure control on enalapril alone or with hydrochlorothiazide. In patients with severe congestive heart failure resistant to conventional therapy, enalapril improves cardiac performance by a reduction in both preload and afterload, and improves clinical status long term. Enalapril appears to be well tolerated, with few serious adverse effects being reported. It does not induce the bradycardia associated with beta-blockers or the adverse effects of diuretics on some laboratory values. In fact, the hypokalaemic effect of hydrochlorothiazide is attenuated by the addition of enalapril. The incidence of the main (but rare) side effects of hypotension in hypovolaemic patients and reduced renal function in certain patients with renovascular hypertension, which are also seen with captopril, might be reduced by careful dosage titration, discontinuation of diuretics, and monitoring of at-risk patients. Thus, enalapril is a particularly worthwhile addition to the antihypertensive armamentarium, as an alternative for treatment of all grades of essential and renovascular hypertension. It also shows promise in the treatment of congestive heart failure.

Urinary excretion of TXB2 after angiotensin converting enzyme inhibition in hypertensive patients

Urinary excretion of thromboxane B2 (TXB2), a stable metabolite of thromboxane A2 (TXA2), was measured by radioimmunoassay in 7 essential hypertensive patients before and after a converting enzyme inhibitor, SQ 14225, administration. When a single oral dose of SQ 14225 (50mg) was given to 7 patients with essential hypertension, urinary excretion of TXB2 was increased significantly (from 58.9 +/- 18.1 to 116.1 +/- 20.7 pg/min, mean +/- SE, P less than 0.02) with simultaneous increase in plasma renin activity, urine volume, urinary sodium, urinary potassium and urinary excretion of PGE (from 58.8 +/- 12.8 to 135.1 +/- 30.0 pg/min, mean +/- SE, P less than 0.05). These results indicate that SQ 14225 stimulates vasoconstricting TXA2 production as well as vasodilating PGE production.

The role of prostaglandins in human hypertension

A large body of evidence supports the concept that prostaglandins (PG) are importantly involved in arterial pressure regulation. Various PGs, especially PGE2 and prostacyclin (PGI2) may influence blood pressure through control of vascular tone, sodium excretion, and renin release. Inhibition of PG synthesis by nonsteroidal antiinflammatory drugs (NSAID) augments the vasoconstrictor response to exogenous pressors such as angiotensin II, arginine vasopressin (AVP), and fludrocortisone. The acute administration of NSAID to either normotensive or untreated hypertensive subjects results in an increase in arterial pressure and peripheral resistance; long-term administration, however, is associated with little or no change in blood pressure, possibly because of a reduction in cardiac output. Although NSAID have little influence on blood pressure in normotensive subjects or untreated hypertensives, inhibition of PG synthesis blunts or abolishes the antihypertensive effect of most antihypertensive agents. NSAID antagonize the vasodepressor action of diuretics, beta-adrenoreceptor antagonists, vasodilators, and converting enzyme inhibitors. Consequently, potent NSAID should be used with caution, if at all, during treatment of hypertensive patients. Numerous studies have examined renal PG production in essential hypertension (EH). The majority have demonstrated reduced basal and stimulated urinary PGE2 excretion in EH compared to normotensive subjects, but there is substantial overlap. Nevertheless, renal PGE2 synthesis is significantly decreased in approximately one-third of patients with EH. A recent innovative approach to arterial pressure regulation has focused on dietary supplementation with polyunsaturated fatty acids (PUFA), especially linoleic acid and eicosapentaenoic acid. Several groups have demonstrated that long-term dietary supplementation with PUFA reduces blood pressure in both normotensive individuals and in patients with EH.(ABSTRACT TRUNCATED AT 250 WORDS)