Serum uric acid, ACE inhibitors, and natriuresis

The release of the substrate for xanthine oxidase in hypertensive patients was suppressed by angiotensin converting enzyme inhibitors and alpha1-blockers

OBJECTIVE

Hyperuricemia is associated with the vascular injury of hypertension, and purine oxidation may play a pivotal role in this association, but the pathophysiology is not fully understood. We tested the hypothesis that in hypertensive patients, the excess amount of the purine metabolite, hypoxanthine, derived from skeletal muscles, would be oxidized by xanthine oxidase, leading to myogenic hyperuricemia as well as to impaired vascular resistance caused by oxygen radicals.

METHODS

We investigated the production of hypoxanthione, the precursor of uric acid and substrate for xanthine oxidase, in hypertensive patients and found that skeletal muscles produced hypoxanthine in excess. We used the semi-ischemic forearm test to examine the release of hypoxanthine (deltaHX), ammonium (deltaAmm) and lactate (deltaLAC) from skeletal muscles in essential hypertensive patients before (UHT: n = 88) and after treatment with antihypertensive agents (THT: n = 37) in comparison to normotensive subjects (NT: n = 14).

RESULTS

deltaHX, as well as deltaAmm and deltaLAC, were significantly higher in UHT and THT (P< 0.01) than in NT. This release of deltaHX from exercising skeletal muscles correlated significantly with the elevation of lactate in NT, UHT and THT (y = 0.209 + 0.031x; R2 = 0.222, n = 139: P < 0.01). Administration of doxazosin (n = 4), bevantolol (n = 5) and alacepil (n = 8) for 1 month significantly suppressed the ratio of percentage changes in deltaHX by -38.4 +/- 55.3%, -51.3 +/- 47.3% and -76.3 +/- 52.2%, respectively (P< 0.05) but losartan (n = 3), atenolol (n = 7) and manidipine (n = 10) did not reduce the ratio of changes; on the contrary, they increased it in deltaHX by +188.2 +/- 331%, +96.2 +/- 192.2% and +42.6 +/- 137.3%, respectively. The elevation of deltaHX after exercise correlated significantly with the serum concentration of uric acid at rest in untreated hypertensive patients (y = 0.194 - 0.255x; R2 = 0.185, n = 30: P < 0.05). The prevalence of reduction of both deltaHX and serum uric acid was significantly higher in the patients treated with alacepril, bevantolol and doxazosin (67%: P < 0.02) than in the patients treated with losartan, atenolol and manidipine (12%).

CONCLUSIONS

It is concluded that the skeletal muscles of hypertensive patients released deltaHX in excess by activation of muscle-type adenosine monophosphate (AMP) deaminase, depending on the degree of hypoxia. The modification of deltaHX by angiotensin-converting enzyme inhibitors and alpha1-blockers influenced the level of serum uric acid, suggesting that the skeletal muscles may be an important source of uric acid as well as of the substrate of xanthine oxidase in hypertension.

Adverse effects of angiotensin converting enzyme (ACE) inhibitors. An update

The angiotensin converting enzyme (ACE) inhibitors are a group of effective drugs with a unique mechanism of action. These drugs have proven to be useful for hypertension and congestive heart failure. Early clinical trials of captopril used doses that are now known to be inappropriately high, and dose-related adverse effects were observed frequently. The recognition that lower doses are effective has reduced the incidence of adverse reactions and resulted in improved patient tolerance. When patients are properly selected and correctable risk factors are removed, serious side effects are uncommon. Unfortunately, the early reputation of nephrotoxicity persists, as does the belief that significant blood dyscrasias, endocrine effects and rash are serious risks for the average patient. After wide use of captopril, enalapril and lisinopril, and investigational trials of nearly a dozen newer agents, a sufficiency of clinical observation, experimental evidence and accurate postmarketing recording of events is accumulating to allow insight into the major toxicities with regard to more intelligent patient selection, more rational dosing and proper identification of risk factors. The most common adverse reactions are cough and skin rash. It appears that the agents are generally not cross-reactive with regard to skin rash, although it is not clear whether this effect is drug-specific or class-specific with regard to cough. Statistically but not clinically significant lowering of haemoglobin and hematocrit is common; these effects are inconsequential in most patients. Neutropenia, once thought to be prevalent, now appears to be so only in patients with autoimmune or collagen-vascular disease; the majority of patients outside these groups are at low risk. Hyperkalaemia is a frequent occurrence. This should not be surprising in view of the effect of the ACE inhibitors on plasma aldosterone. When dietary potassium intake is regulated and sources of altered potassium excretion are identified, hyperkalaemia is seldom a serious problem. Identification of sodium and water deficits allows correction before the drugs are started, and the frequency of hypotension and hyperkalaemia caused by the drugs is quite low if these factors are properly managed. An unexpected finding emerging in recent years is the dry cough associated with ACE inhibitor therapy. Its mechanism is not definitely known. Nonsteroidal anti-inflammatory drugs may control this symptom in some patients. The frequent observation of proteinuria in patients taking ACE inhibitors has gained notice and sometimes caused undue alarm. It is difficult to separate disease effects in diabetes and hypertension from true drug effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiovascular risk factors and antihypertensive therapy

Reduction of elevated blood pressure is effective in reducing morbidity and mortality from cardiovascular disease in general. Striking decreases in stroke, congestive heart failure, and renal impairment have been observed when blood pressure is reduced. However, the ability of traditional, diuretic-first, stepped-care therapeutic algorithms to reduce the occurrence of myocardial infarction or angina has been exceedingly difficult to demonstrate. An increased frequency of sudden death among hypertensive men with electrocardiographic abnormalities has been observed in some diuretic-based treatment trials. The failure of conventional therapy to reduce coronary artery disease and death has forced a re-appraisal of antihypertensive treatment. The risk factors for coronary disease are reviewed in detail with an emphasis on the impact of various classes of antihypertensive drugs on these factors. The emergence of effective antihypertensive agents that lower blood pressure without adversely affecting cardiovascular risk factors provides a new opportunity to further extend the benefit of treatment by reducing such risk.

The metabolic effects of enalapril

The metabolic effects of Enalapril (EN) were compared in a cross over study with those of Propranolol (PPL). Long term treatment with the ACEI did not modify the lipid and glucose profile and increased urate excretion, while PPL increased triglycerides and decreased urate clearance.

Side effects and metabolic effects of converting-enzyme inhibitors

Side effects of angiotensin converting enzyme (ACE) inhibitors are not common with currently recommended doses. Hypotension, hyperkalemia and renal impairment may occur under special circumstances, and relate directly to blockade of ACE, in particular when pre-treatment renin levels are high. Other adverse effects, once quite common when excessive doses of captopril were prescribed, are now rarely seen. A possible exception is cough. Though not dangerous, cough is not infrequently seen during treatment with ACE inhibitors. Experience with these drugs is insufficient to recommend their use in the hypertension of pregnancy. The "metabolic-profile" with ACE inhibitors appears favourable, since they can increase uric acid excretion and lower plasma urate levels, carbohydrate tolerance is unaltered or improved, and lipid levels are unchanged. Electrolyte and metabolic effects of thiazide diuretics are blunted by addition of an ACE inhibitor.