Evidence of prostacyclin deficiency in the syndrome of hyporeninemic hypoaldosteronism

Pathophysiology and clinical management of hyperkalemia in chronic kidney disease

Adaptive increases in kidney and gastrointestinal excretion of K+ help to prevent hyperkalemia in patients with chronic kidney disease (CKD) as long as the glomerular filtration rate (GFR) remains >15-20 mL/min. K+ balance is maintained by increased secretion per functioning nephron, which is mediated by elevated plasma K+ concentration, aldosterone, increased flow rate, and enhanced Na+-K+-ATPase activity. Fecal losses of potassium also increase in CKD. These mechanisms are effective in preventing hyperkalemia if urine output is in excess of 600 mL/day and the GFR exceeds 15 mL/min. Development of hyperkalemia with only mild to moderate reductions in GFR should prompt a search for intrinsic disease of the collecting duct, disturbances in mineralocorticoid activity, and/or decreased delivery of sodium to the distal nephron. The initial approach to treatment is to review the patient's medication profile and whenever possible discontinue drugs that impair kidney K+ excretion. Patients should be educated on sources of K+ in the diet and should be strongly encouraged to avoid the use of K+ containing salt substitutes as well as herbal remedies since herbs may be a hidden source of dietary K+. Effective diuretic therapy and correction of metabolic acidosis are effective strategies to minimize the potential for hyperkalemia. Discontinuation or use of submaximal doses of renin-angiotensin blockers should be discouraged given the cardiovascular protective effect these drugs provide. Potassium binding drugs can be useful to enable use of these drugs and potentially allow liberalization of the diet in CKD patients.

The pathophysiology of fluid and electrolyte balance in the older adult surgical patient

BACKGROUND & AIMS

Age-related physiological changes predispose even the healthy older adult to fluid and electrolyte abnormalities which can cause morbidity and mortality. The aim of this narrative review is to highlight key aspects of age-related pathophysiological changes that affect fluid and electrolyte balance in older adults and underpin their importance in the perioperative period.

METHODS

The Web of Science, MEDLINE, PubMed and Google Scholar databases were searched using key terms for relevant studies published in English on fluid balance in older adults during the 15 years preceding June 2013. Randomised controlled trials and large cohort studies were sought; other studies were used when these were not available. The bibliographies of extracted papers were also searched for relevant articles.

RESULTS

Older adults are susceptible to dehydration and electrolyte abnormalities, with causes ranging from physical disability restricting access to fluid intake to iatrogenic causes including polypharmacy and unmonitored diuretic usage. Renal senescence, as well as physical and mental decline, increase this susceptibility. Older adults are also predisposed to water retention and related electrolyte abnormalities, exacerbated at times of physiological stress. Positive fluid balance has been shown to be an independent risk factor for morbidity and mortality in critically ill patients with acute kidney injury.

CONCLUSIONS

Age-related pathophysiological changes in the handling of fluid and electrolytes make older adults undergoing surgery a high-risk group and an understanding of these changes will enable better management of fluid and electrolyte therapy in the older adult.

Tubular changes in early diabetic nephropathy

Far from being bystanders in diabetic nephropathy, changes in the proximal tubule are important for the development of progressive diabetic kidney disease. The proximal tubule is uniquely susceptible to a variety of metabolic and hemodynamic factors associated with diabetes. Renal function and prognosis correlate better with structural lesions in the tubuli and cortical interstitium than with classical glomerular changes of diabetic nephropathy. The proximal tubules show a variety of poorly characterized changes, which have led to the notion that tubular damage represents a "final common pathway" for proteinuric renal injury. However, tubular hypertrophy, reduced organic ion transport, and other tubular changes reviewed in this paper, are already apparent before the onset of proteinuria in diabetes. Indeed, increased tubuloglomerular feedback and defective uptake and lysosomal processing may independently contribute to hyperfiltration and urinary protein loss, respectively. This finding does not mean that glomerular or vascular dysfunction do not contribute to progressive nephropathy. However, although subdividing the nephron for the purposes of analysis and scientific discovery may be useful, the interactions between tubule, glomerulus, and interstitium are likely key to the understanding of complex disorders such as diabetic nephropathy. From this "holonephric" point of view, an understanding of the changes in the diabetic tubule forms an important component to the understanding of kidney disease in diabetes.

Hyper- and hypoaldosteronism

Aldosterone participates in blood volume and serum potassium homeostasis, which in turn regulate aldosterone secretion by the zona glomerulosa of the adrenal cortex. Autonomous aldosterone hypersecretion leads to hypertension and hypokalemia. Improved screening techniques have led to a re-evaluation of the frequency of primary aldosteronism among adults with hypertension, recognizing that normokalemic cases are more frequent than was previously appreciated. The genetic basis of glucocorticoid remediable aldosteronism has been elucidated and adequately explains most of the pathophysiologic features of this disorder. A new form of familial aldosteronism has been described, familial hyperaldosteronism type II; linkage analysis and direct mutation screening has shown that this disorder is unrelated to mutations in the genes for aldosterone synthase or the angiotensin II receptor. The features of aldosterone hypersecretion may be due to non-aldosterone-mediated mineralocorticoid excess. These include two causes of congenital adrenal hyperplasia (11 beta-hydroxylase deficiency and 17 alpha-hydroxylase deficiency), the syndrome of apparent mineralocorticoid excess (AME) due to 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) deficiency, primary glucocorticoid resistance, Liddle's syndrome due to activating mutations of the renal epithelial sodium channel, and exogenous sources of mineralocorticoid, such as licorice, or drugs, such as carbenoxolone. The features of mineralocorticoid excess are also often seen in Cushing's syndrome. Hypoaldosteronism may lead to hypotension and hyperkalemia. Hypoaldosteronism may be due to inadequate stimulation of aldosterone secretion (hyporeninemic hypoaldosteronism), defects in adrenal synthesis of aldosterone, or resistance to the ion transport effects of aldosterone, such as are seen in pseudohypoaldosteronism type I (PHA I). PHA I is frequently due to mutations involving the amiloride sensitive epithelial sodium channel. Gordon's syndrome (PHA type II) is due to resistance to the kaliuretic but not sodium reabsorptive effects of aldosterone for which the genetic basis is still unknown. This review aims to provide a survey of the clinical disorders of aldosterone excess and deficiency and their clinical management, with a focus on primary aldosteronism and isolated aldosterone deficiency.

Renin-aldosterone system can respond to furosemide in patients with hyperkalemic hyporeninism

Thirty-four patients (65.3+/-3.3 years of age, mean+/-SEM) with hyperkalemia (serum potassium >5.0 mEq/L) had measurement of their renin-aldosterone system. Nineteen patients (56%) had plasma renin activity (PRA) >1.5 ng/mL/h, which was not low, while 15 (44%) had PRA <1.5. Twelve of the 15 hyporeninemic hyperkalemic patients were studied to determine whether their renin-aldosterone system responded to 2 weeks of furosemide, 20 mg daily. Four were nonresponders: PRA averaged 0.3+/-0.1 ng/mL/h, and it did not increase with furosemide or respond to captopril before or after furosemide. Eight patients were responders: PRA averaged 0.6+/-0.2 ng/mL/h and increased with furosemide to 5.5+/-3.4 ng/mL/h. Captopril failed to increase PRA before furosemide, but PRA increased to 15.3+/-8.4 ng/mL/h after furosemide. Plasma aldosterone was low in both nonresponders and responders (3.5+/-1.2 ng/dL vs 5.8+/-2.5 ng/dL) and did not increase significantly with furosemide (4.3+/-1.7 ng/dL vs 8.7+/-2.5 ng/dL). Serum potassium did not fall and therefore did not limit the rise in aldosterone. Renin responders had greater body weight, were predominantly female (6/8 vs 2/4) and were more likely to have diabetes mellitus (7/8 vs 0/4). Plasma atrial natriuretic peptide (ANP) fell with furosemide in 8 of 8 responders and in 1 of the 2 nonresponders in whom it was measured. Neither group had suppressed plasma prorenin levels, indicating no suppression of renin gene expression. These results indicate that many hyperkalemic patients do not have suppressed PRA. Further, a majority of patients with suppressed PRA have high levels of ANP and can respond to diuretic therapy with a rise in PRA and a fall in ANP, suggesting physiologic suppression of the renin system by volume expansion. A minority of hyperkalemic patients with suppressed PRA had PRA that did not increase under these study conditions.

Calcium-dependent low renin syndrome in a diabetic patient with prostaglandin deficiency

Calcium and prostaglandin are supposed to play a critical role in the renin-angiotensin aldosterone system. Calcium has been described as an inhibitory second messenger for renin exocytosis whereas vasodilatory prostaglandins, such as PGE2, are known to stimulate the production of renin. These factors are probably interrelated since calcium also enhances urinary prostaglandin release. We report the case of a 52 year-old diabetic patient treated with insulin injections with intestinal malabsorption leading to chronic hyperkalemia and hypocalcemia in whom a low renin syndrome and low levels of urinary prostaglandins were observed. The correction of the hypocalcemia was able to improve plasma renin as well as urinary prostaglandin levels. This observation suggests a prominent role played by calcium on the in vivo regulation of renin and prostaglandin release. These results illustrate the closed loop between plasma calcium level, urinary prostaglandins production and renin release.

Accidental shock during epidural anesthesia in a patient with NSAID-induced hyporeninemic hypoaldosteronism

An obese man suffered cardiac arrest twenty minutes after receiving epidural anesthesia for incision and debridement of wound over the right leg. The patient's condition stabilized after emergent cardiopulmonary resuscitation. It was found that the patient had been self-administering an herbal drug continuously for a year and a half, and that this drug contained ethoxybenzamide, which is a nonsteroidal anti-inflammatory drug (NSAID). Low plasma renin and aldosterone levels were noted from the blood sample taken at the time of the cardiac arrest. The cardiac arrest was believed to be related to NSAID-induced hyporeninemic hypoaldosteronism, superimposed with epidural anesthesia-induced sympathectomy.

Assessment of adrenal function in women heterozygous for adrenoleukodystrophy

Adrenoleukodystrophy (ALD) is an X-linked recessive disorder that destroys the white matter of the brain and is associated with adrenal insufficiency. The prevalence of adrenal dysfunction in 71 women carriers of the X-linked ALD gene was studied. These subjects were identified initially on the basis of being obligate carriers of the X-linked trait by pedigree analysis and were confirmed by plasma very long chain fatty acid levels consistent with a heterozygote status. One subject had well documented overt adrenal insufficiency, diagnosed and treated since age 9 yr. Among the remaining women, the mean serum 0800 h and 1 h post-ACTH cortisol concentrations [16 +/- 7 (+/-SD) and 34 +/- 8 micrograms/dL, respectively] were normal. All subjects had normal ACTH-stimulated serum cortisol levels, i.e. more than 20 micrograms/dL. However, 4 subjects (6%) had subnormal ACTH-stimulated aldosterone concentrations (mean, 9 +/- 6 vs. 42 +/- 16 ng/dL for other subjects; P = 0.001, by Mann Whitney rank sum test). Three of these women (75%) were taking nonsteroidal antiinflammatory agents (NSAIDs), whereas only 4 of 67 (6%) subjects with normal aldosterone responsiveness were NSAIDs users (P < 0.01, by Fisher's exact test). Thus, NSAIDs use was associated with increased risk of hypoaldosteronism (odds ratio, 50.2; 95% confidence interval, 3.3-266; P < 0.002). Three of these four women had symptoms consistent with mineralocorticoid deficiency. Serum sodium and potassium concentrations were normal in all subjects. Basal and metyrapone-stimulated plasma ACTH concentrations were also normal in adequately tested subjects with and without mineralocorticoid insufficiency. Five of eight subjects (63%) who underwent testing with synthetic ovine CRH (oCRH) had abnormalities. Three did not meet the criteria for adequate cortisol stimulation (i.e. > 20 micrograms/dL) and had peak ACTH levels greater than 30 pg/mL. Two other subjects had exaggerated ACTH responses with normal cortisol levels. There were no significant differences in the mean or median levels of very long chain fatty acid, C26:0, C24/22 ratios, or C26/22 ratios among the entire subject group, the subgroup with blunted aldosterone responses to ACTH, and the subgroup with blunted responses to oCRH (P > 0.05, by ANOVA and Kruskall-Wallis test for C26, C24/22 ratio, and C26/22 ratio). We conclude that 1) adrenal cortical insufficiency rarely develops in ALD heterozygotes; 2) isolated mineralocorticoid insufficiency can occur in ALD heterozygotes, as has been previously reported to occur with autoimmune and acquired immunodeficiency syndrome-related adrenal dysfunction; 3) ALD heterozygosity may predispose these individuals to NSAID-related hypoaldosteronism; and 4) a subclinical decrease in glucocorticoid reserve, as measured by oCRH testing, may be present in a majority of these women. Aldosterone levels should be included in the ACTH stimulation testing when seeking evidence of adrenal insufficiency in affected women. NSAIDs should be considered a risk factor for the development of hypoaldosteronism in women heterozygous for ALD.

Magnesium deficiency produces insulin resistance and increased thromboxane synthesis

Evidence suggests that magnesium deficiency may play an important role in cardiovascular disease. In this study, we evaluated the effects of a magnesium infusion and dietary-induced isolated magnesium deficiency on the production of thromboxane and on angiotensin II-mediated aldosterone synthesis in normal human subjects. Because insulin resistance may be associated with altered blood pressure, we also measured insulin sensitivity using an intravenous glucose tolerance test with minimal model analysis in six subjects. The magnesium infusion reduced urinary thromboxane concentration and angiotensin II-induced plasma aldosterone levels. The low magnesium diet reduced both serum magnesium and intracellular free magnesium in red blood cells as determined by nuclear magnetic resonance (186 +/- 10 [SEM] to 127 +/- 9 mM, p < 0.01). Urinary thromboxane concentration measured by radioimmunoassay increased after magnesium deficiency. Similarly, angiotensin II-induced plasma aldosterone concentration increased after magnesium deficiency. Analysis showed that all subjects studied had a decrease in insulin sensitivity after magnesium deficiency (3.69 +/- 0.6 to 2.75 +/- 0.5 min-1 per microunit per milliliter x 10(-4), p < 0.03). We conclude that dietary-induced magnesium deficiency 1) increases thromboxane urinary concentration and 2) enhances angiotensin-induced aldosterone synthesis. These effects are associated with a decrease in insulin action, suggesting that magnesium deficiency may be a common factor associated with insulin resistance and vascular disease.

Paracrine regulation of the renin-aldosterone system

A number of clinical states have been described where there are derangements or discrepancies between renin-angiotensin and aldosterone secretion. We have studied the potential effect of some cytokines or growth factors (peptide regulatory factors) on this system in vitro. Both tumor necrosis factor/cachectin and interleukin I are potent regulators acting as renin secretagogues and inhibitors of aldosterone synthesis. These actions are mediated by prostaglandin cyclooxygenase products and their actions mimic the syndrome of hyperreninemic hypoaldosteronism in critical illness. Insulin and insulin-like growth factor I are also renin secretagogues in vitro However in a diabetic model (streptozotocin rat), there is resistance to both agonists as well as enhanced feedback suppression to angiotensin. A third peptide, transforming growth factor (TGF beta) has even more complex actions, acting as a secretagogue at low doses (10(-12) M) but inhibiting renin at higher doses. TGF beta production is increased in the diabetic state so that this peptide as well as the insulin family may be involved in hyporeninemic hypoaldosteronism.

Effect of the renin-angiotensin system in the vascular disease of type II diabetes mellitus

A universal underlying abnormality in the pathogenesis of hypertension, atherosclerosis, myocardial dysfunction, and diabetic glomerulosclerosis involves alteration in smooth muscle cell structure, function, and growth. Angiotensin II, through its effects on contractility, growth, and the sympathetic nervous system, may potentially play a key role in this pathologic process and, thus, contribute to the development of these cardiovascular and renal complications of diabetes mellitus. Angiotensin-converting enzyme inhibitors and some direct renin inhibitors prevent or slow the progression of some of these complications, which further suggests a pathologic role for the reninangiotensin system in diabetes mellitus.

Potassium homeostasis with indomethacin therapy in normal subjects

In an attempt to delineate effects of prostaglandin (PG) synthesis inhibition on potassium metabolism in normal subjects, we challenged 13 young, healthy volunteers with a potassium chloride infusion before and after a 4-day course of indomethacin (25 mg orally, three times a day). The plasma potassium level was monitored at 10-minute intervals throughout the 50-minute infusion and for a total of 180 minutes. The maximal increment in plasma potassium level was 0.82 +/- 0.07 mmol/L (mEq/L) in the untreated state, and 0.86 +/- 0.08 mmol/L with indomethacin treatment. The basal potassium level before infusion was higher in the indomethacin-treated than the control state (3.83 +/- 0.07 v 3.68 +/- 0.07 mmol/L; P less than 0.01). Urinary potassium excretion over the 3-hour study period equalled the potassium load administered, and was unaffected by indomethacin therapy. Indomethacin did not alter insulin or aldosterone levels during the study. PGE2 excretion over the 3 hours was lower in the indomethacin than the control phase, although it was higher than normal in both phases. In an additional experiment, the comparative effects of a saline versus saline-potassium infusion on PG excretion were studied. No differences were seen between the excretion patterns of PGE2 or 6-keto-PGF1a with the two infusions. We conclude that (1) although basal serum potassium level is slightly higher in healthy young people during indomethacin treatment, there is little effect on handling of an acute potassium load; (2) the aldosterone response to hyperkalemia is PG-independent; (3) urinary PG excretion increases in response to a saline-based infusion, but the effect is not enhanced by acute potassium loading.

Effects of renin inhibition in systemic hypertension

The effect of the direct renin inhibitor enalkiren (Abbott Laboratories) was examined in 8 healthy patients with essential hypertension. With an unrestricted sodium diet, plasma renin concentration was inhibited within 10 minutes by intravenous enalkiren and remained essentially undetectable for greater than or equal to 6 hours (11.9 +/- 4 to 1.0 +/- 0.6 ng angiotensin I/ml/hour, p less than 0.05). Mean arterial blood pressure declined gradually (108 +/- 5 to 84 +/- 4 mm Hg, p = 0.02), as did plasma aldosterone concentration (14.4 +/- 3.8 to 4.4 +/- 0.8 ng/dl, p = 0.03), whereas plasma immunoreactive active renin concentration increased progressively (35 +/- 14 to 160 +/- 60 pg/ml, p greater than 0.05). Urinary excretion of the stable metabolite of prostacyclin (6-keto-prostaglandin F1 alpha) decreased slightly, but not significantly (42 +/- 10 to 33 +/- 11 ng/g creatinine, p = 0.13). The addition of a diuretic decreased baseline blood pressure and increased baseline plasma renin and aldosterone values. Blood pressure responses to enalkiren were slightly (though not significantly) greater than those observed before diuretic administration. We conclude that enalkiren is effective in decreasing blood pressure and in inhibiting the renin system, without significantly altering urinary prostacyclin excretion, in patients with essential hypertension. These results suggest that the renin system contributes to the maintenance of elevated blood pressure in some patients with essential hypertension.

Hyperkalemia in the elderly

Physiologic and pathologic events that occur in patients as they grow older may result in distal renal tubular dysfunction, as well as decreased levels of plasma renin activity and plasma aldosterone. Such alterations result in a tendency toward hyperkalemia. A syndrome termed hyporeninemic hypoaldosteronism, associated with hyperkalemia, has been frequently described in elderly patients. The common occurrence of hyperkalemia in the elderly may be aggravated by the use of drugs that either further suppress renin and/or aldosterone or interfere with distal tubular potassium excretion. Some patients with hyporeninemic hypoaldosteronism respond to diuretic therapy. The recognition of the possible development of severe hyperkalemia in the elderly patient may avoid serious and even fatal complications of this electrolyte disorder.

Hyperkalemia in diabetes mellitus

Potassium filtered at the glomerulus is almost completely reabsorbed before the distal tubule; it must therefore be secreted into the collecting duct. The rate of potassium secretion is determined by a number of factors, notably aldosterone, distal sodium delivery, and serum potassium. Normal serum potassium is maintained by the interplay of passive leak of potassium from the cells and its active return to the cells. Transmembrane potassium distribution is influenced largely by acid-base equilibrium and hormones including insulin and catecholamines. In the diabetic with ketoacidosis hyperkalemia, in the face of potassium depletion, is attributable to reduced renal function, acidosis, release of potassium from cells due to glycogenolysis, and lack of insulin. Chronic hyperkalemia in diabetics is most often attributable to hyporeninemic hypoaldosteronism but other conditions including urinary tract obstruction may also contribute. A variety of clinical situations (e.g., volume depletion) and drugs (e.g., nonsteroidal antiinflammatory agents, and heparin) may acutely provoke hyperkalemia in susceptible individuals.

Prostaglandin deficiency

Healthy cells from virtually all tissues synthesize a variety of prostaglandins, autacoids which can significantly alter cellular functions. An absolute or relative deficiency of prostaglandins has now been demonstrated in many diseases or clinical conditions. These include 'natural' disorders such as peptic ulcer disease and diabetes mellitus. These also include 'acquired' or iatrogenic conditions such as cyclosporine nephrotoxicity and the gastropathy induced by nonsteroidal anti-inflammatory drugs. We believe that the diversity of the disorders associated with prostaglandin deficiency may be wider and of greater pathogenetic importance than is currently recognized. We propose: 1) that prostaglandin deficiency will be demonstrated in many abnormalities which are now described as of uncertain etiology; and 2) that adverse effects from many commonly prescribed drugs may also be related to an unrecognized and unfavorable alteration in prostaglandin synthesis, disposal, or activity.

Alterations in blood pressure by derangement of the mechanisms that regulate sodium excretion

Understanding the sequence of events responsible for pressure-related natriuresis and their pathophysiologic alterations may be useful in distinguishing various types of essential hypertension of renal origin. The perturbation of a distal step in the sequence is likely to be reflected in a simple physiologic defect. For instance, pathophysiologic alterations in the medullary production of prostaglandin E2 might directly influence natriuresis and diuresis because of its modulatory effect on tubular reabsorption of sodium and water. Perturbation of more proximal steps in the sequence could influence all the distal events as well. For instance, prostaglandin I2 and endothelium-derived relaxing factor may be produced by the preglomerular vasculature in response to alterations in renal perfusion pressure and may modulate the release of renin from the juxtaglomerular cells. Thus, variations in the production of prostaglandin I2 or endothelium-derived relaxing factor may be reflected by various renal vascular, tubular, and systemic homeostatic events related to the renin-angiotensin system.

Diseases of the adrenal cortex

The adrenal cortex is functionally a three-dimensional gland that secretes glucocorticoids, mineralocorticoids, and sex steroids. Of these three classes of steroids only the gluco- and mineralocorticoid hormones are necessary to sustain life. The availability of sensitive and specific radioimmunoassays has permitted accurate measurement of practically every steroid hormone secreted by the adrenal cortex. As in other endocrinopathies, suppression studies are employed when hyperfunction is suspected, while provocative tests are used to detect hypofunction. These dynamic studies enable the clinician to evaluate the functional status of the adrenal cortex. The anatomic configuration of the adrenal cortices is delineated by high-resolution computed tomography (and magnetic resonance imaging), obviating the need for invasive procedures such as venography or arteriography. The disorders of the adrenal cortex can be viewed from the dual perspectives of hyperfunction and hypofunction. Clinical expressions of hyperfunctional adrenocortical syndromes include Cushing's syndrome, primary hyperaldosteronism, and the adrenogenital syndrome. The expressions of hypofunctional syndromes include Addison's disease and selective hypoaldosteronism. The diagnosis and treatment of these disorders are outlined in this issue.

Renal tubular hyperkalaemia in childhood

Potassium output from the body is regulated by renal excretion, which takes place predominantly in the late distal and cortical collecting tubules. The accepted model for potassium secretion implies the accumulation of potassium into the cell by the activity of basolateral Na-K-ATPase and its exit through voltage-dependent conductive channels. The factors regulating renal potassium secretion are potassium intake, distal urinary flow, systemic acid-base equilibrium, aldosterone, antidiuretic hormone and, probably, epinephrine. Renal handling of potassium is best studied by the response to the acute administration of furosemide. This loop diuretic not only increases sodium and chloride excretion but also enhances potassium and hydrogen ion excretion and stimulates the renin-aldosterone axis. The term "renal tubular hyperkalaemia" refers to a tubular dysfunction where the hyperkalaemia is disproportionate to any reduction in glomerular filtration rate (GFR) and not due primarily or solely to aldosterone deficiency or to drugs impairing either mineralocorticoid action or tubular transport. The syndromes of renal tubular hyperkalaemia mainly observed in childhood are "chloride shunt" syndrome, hyporeninaemic hypoaldosteronism and primary or secondary pseudohypoaldosteronism. Differential diagnosis between these conditions is easily made if attention is paid to the level of GFR, presence of sodium wasting, activity of the renin-aldosterone axis and renal response to acute administration of furosemide.

Mechanisms underlying pressure-related natriuresis: the role of the renin-angiotensin and prostaglandin systems. State of the art lecture

It has long been known that increments in renal perfusion pressure can induce an elevation of urine sodium excretion without changing renal blood flow or glomerular filtration rate. The mechanism underlying this pressure-related natriuresis remains undefined, although the interest in its elucidation has been stimulated by the notion that it may constitute the central phenomenon through which the kidney regulates blood volume and, thereby, blood pressure. Recently, the use of novel experimental techniques has disclosed some important clues about changes in renal hemodynamics that, along with changes in renal humoral regulators, allow us to visualize a possible sequence of events responsible for pressure-related natriuresis. According to this hypothesis, the autoregulatory responses responsible for maintaining glomerular filtration rate are elicited in preglomerular vasculature by changes in renal perfusion pressure. These myogenic responses are coupled through Ca2+ entry in juxtaglomerular cells with inversely related changes in the release of renin and, consequently, with the amount of angiotensin II generated in renal interstitium. The release of renin from juxtaglomerular cells is modulated by the synthesis of prostaglandin I2 from the adjacent endothelial cells. Interstitial angiotensin II could influence sodium tubular reabsorption directly by stimulating sodium transport in proximal renal tubules and indirectly by altering medullary blood flow and, thereby, medullary interstitial pressure. In the renal medulla, the effects of interstitial pressure on sodium reabsorption can be amplified by the release of prostaglandin E2 from interstitial cells. A deficient regulation of this relationship could result in a shift of the pressure-natriuresis curve, leading to hypertension.

Antihypertensive effect of enalapril in essential hypertension: role of prostacyclin

The effects of enalapril alone and in combination with the cyclooxygenase inhibitors sulindac and indomethacin on blood pressure (BP), plasma aldosterone, renin activity and converting enzyme activity were evaluated in 29 patients with mild to moderate essential hypertension, 26 of whom had low plasma renin activity. Patients were randomly assigned to one of three treatment groups. All patients underwent a 4-week placebo phase (phase I), then received enalapril (20 mg BID) for 4 weeks (phase II). In phase III, group I (n = 10) continued on enalapril alone; group II (n = 9) received sulindac 200 mg BID plus enalapril, and group III (n = 10) received indomethacin 50 mg BID plus enalapril, all for 4 weeks. Enalapril lowered BP significantly (mean supine BP 149/100 in phase I vs. 134/90 in phase II, p less than 0.05) without inhibiting aldosterone production. The BP effect was not blunted by concomitant administration of sulindac or indomethacin. Enalapril lowered converting enzyme activity to 25% to 30% of baseline and tended to increase renin activity. In the 10 patients who received indomethacin (group III), the effects of enalapril alone and enalapril plus indomethacin on urinary excretion of 6-keto prostaglandin F1 alpha (PGF1 alpha), a stable metabolite of prostacyclin (PGI2), were examined. Enalapril increased urinary 6-keto PGF1 alpha in group III from 118 +/- 23 to 194 +/- 38 ng/g creatinine (p less than 0.05), while addition of indomethacin reduced 6-keto PGF1 alpha to basal levels (138 +/- 26 ng/g creatinine).(ABSTRACT TRUNCATED AT 250 WORDS)

The inhibitory role of 12- and 15-lipoxygenase products on renin release

Release of arachidonic acid from membrane phospholipids is a limiting step in the synthesis of both cyclooxygenase products and lipoxygenase products. The direct effects of prostacyclin and some lipoxygenase products on renin release were studied using rat renal cortical slices. Prostacyclin, at concentrations of 10(-5) M, stimulated renin secretion, but this effect was short-lived. Leukotrienes or their precursor, 5-hydroperoxyeicosatetraenoic acid, did not affect basal renin release. In contrast, 10(-9) M 12-hydroperoxyeicosatetraenoic acid and 10(-8) M 12-hydroxyeicosatetraenoic acid were potent inhibitors of renin secretion. Similarly, 15-hydroperoxyeicosatetraenoic acid and its hydroxy derivative, 15-hydroxyeicosatetraenoic acid, at somewhat higher molar concentrations (10(-6) M) also reduced basal renin. These studies confirm prostacyclin as a potential renin secretagogue; however, its action in vitro is transient, probably because of its rapid degradation. Our studies provide new evidence that products of the 12-lipoxygenase and 15-lipoxygenase pathways, reported to be present in renal vascular tissue, are potent inhibitors of renin secretion and much more active on a molar basis on renin secretion than is prostacyclin. These studies suggest the potential presence of a dual system of stimulation and suppression that may regulate renin secretion in normal and clinical states.

Hyporeninemic hypoaldosteronism associated with acquired immune deficiency syndrome

Four patients with the acquired immune deficiency syndrome (AIDS) and persistent unexplained hyperkalemia were studied. Testing with cosyntropin (0.25 mg intravenously) revealed normal baseline and stimulated cortisol levels and adequate aldosterone stimulation. The baseline aldosterone level was low for the degree of hyperkalemia. Renin/aldosterone stimulation testing was performed by intravenous injection of 80 mg of furosemide followed by four hours of upright posture. This study showed low baseline renin and aldosterone levels and inadequate renin and aldosterone stimulation. Three patients were subsequently treated with fludrocortisone (0.1 to 0.2 mg per day), with normalization of serum potassium levels. It is concluded that hyporeninemic hypoaldosteronism is responsible for hyperkalemia in some patients with AIDS and that treatment with fludrocortisone is effective in these cases.

Effects of nonsteroidal anti-inflammatory drugs in healthy subjects

Nonsteroidal anti-inflammatory drugs inhibit cyclo-oxygenase activity and thereby reduce prostaglandin synthesis. Studies in humans have used these cyclo-oxygenase inhibitors to examine the role of prostaglandins in controlling renal function. Although short-term studies have demonstrated reductions in effective renal plasma flow, glomerular filtration rate, urinary sodium excretion, and plasma renin activity, long-term administration of nonsteroidal anti-inflammatory drugs does not result in significant or clinically important changes in renal function in normal human subjects. If healthy volunteers are placed on low-sodium diets or treated with diuretics, both renal hemodynamics and salt and water excretion can become prostaglandin-dependent. Studies in normal subjects suggest that sulindac, a nonsteroidal anti-inflammatory drug that undergoes biotransformation in the kidney, does not inhibit renal prostaglandin synthesis or urinary sodium excretion under basal conditions but may impair furosemide-stimulated prostaglandin synthesis and changes in renal function. Doses of sulindac that spare basal renal cyclo-oxygenase do inhibit extrarenal cyclo-oxygenase. The mechanism responsible for this biochemical selectivity of sulindac is not related to a differential sensitivity of the renal cyclo-oxygenase to the active metabolite of sulindac, sulindac sulfide. Sulindac sulfide, in concentrations as low as 1 nM, was equipotent to indomethacin as an inhibitor of prostaglandin E2 synthesis in primary cultures of three renal cell lines. Appropriate clinical use of all nonsteroidal anti-inflammatory drugs, including sulindac, requires careful consideration of risk factors that predispose to nephrotoxicity and careful monitoring when administered to patients at risk.