DECREASED PLASMA CLEARANCE AND HEPATIC EXTRACTION OF ALDOSTERONE IN PATIENTS WITH HEART FAILURE

An Abbreviated History of Aldosterone Metabolism, Current and Future Challenges

The initial isolation of adrenal steroids from large quantities of animal adrenals resulted in an amorphous fraction resistant to crystallization and identification and had potent effects on electrolyte transport. Aldosterone was eventually isolated and identified in the fraction and was soon shown to cause hypertension when in excess. The autonomous and excessive production of aldosterone, primary aldosteronism, is the most common cause of secondary hypertension. Aldosterone is metabolized in the liver and kidney, and its metabolites are conjugated with glucuronic acid for excretion. The most common liver metabolite is 3α,5β-tetrahydroaldosterone-3-glucuronide, while that of the kidney is aldosterone-18-oxo-glucuronide. In terms of their value, especially the aldosterone-18-oxo-glucuronide, is commonly used for the diagnosis of primary aldosteronism because they provide an integrated value of the total daily production of aldosterone. Conversion of aldosterone to 18-oxo-glucuronide is impeded by drugs, like some common non-steroidal anti-inflammatory drugs that compete for UDP-glucuronosyltransferase-2B7, the most important glucuronosyltransferase for aldosterone metabolism. Tetrahydroaldosterone is the most abundant metabolite and the most reliable for the diagnosis of primary aldosteronism, but it is not commonly measured.

The discovery, isolation and identification of aldosterone: reflections on emerging regulation and function

This paper has a focus on the early history of aldosterone. The Taits take us on a chronological trawl through the history in which they had a first hand role and made a major contribution-their bioassay was in many ways the key. The gifted Swiss chemists made a critical contribution to the scale and isolation of larger amounts. This was international collaboration at its best. Developing technologies were utilised as crucial cutting edge applications in the advancing front, technology transfer before the word was invented. Measurement of aldosterone and angiotensin were crucial advances to the understanding of the regulation of the hormone. In the period 1960-2003, some 30,000 papers mentioned aldosterone as a keyword, even so advances on a larger scale were slow. I have indicated some of my own work with the Howard Florey team using the adrenal autotransplant in the conscious sheep. Recently, the understanding of the role of induced proteins, the flow on from the RALES trial and the development of eplerenone has revitalised the aldosterone field.

Studies on the activity of the renin-angiotensin-aldosterone system (RAAS) in patients with cirrhosis of the liver

Plasma renin activity (PRA), plasma renin concentration (PRC), angiotensinogen, angiotensin II (AT II) and plasma aldosterone were determined by radioimmunoassay in 77 patients with cirrhosis of the liver [group I: with ascites, untreated (n=23); group II: patients with ascites during treatment (n=32); group III: after removal of fluids, but under further spironolactone therapy (n=10); group IV: untreated subjects without ascites (n=12)]. With the exception of decreased angiotensinogen values in all groups ranging between 39% (group IV) and 73% (group III) no significant changes of the other parameters of the RAAS were found in untreated patients. A highly significant increase of PRA, PRC, AT II and plasma aldosterone was observed in treated cirrhotics with (group II) or without (group III) ascites. In the total series of patients AT II was closely related to PRA, PRC and aldosterone emphasizing aldosterone secretion. Plasma sodium was inversely correlated to PRA, PRC, AT II and aldosterone, but no relationship was detected between these parameters of the RAAS and plasma potassium. Our results indicate that hyperaldosteronism in cirrhosis appears unlikely to be the major determinant of avid renal sodium retention and ascites formation. An increased activity of the RAAS is most often initiated by therapeutic factors and/or markedly altered electrolyte metabolism. Therefore, basal conditions of the patients to be studied must be well defined to exclude any artificially induced stimulation of the RAAS.

Altered hepatic blood flow and drug disposition

For some drugs, delivery to the liver by the hepatic circulation is an important determinant of removal by this organ. Classical pharmacokinetic analyses cannot predict the changes produced by altering any of the biological determinants of drug elimination by the liver; hepatic blood flow, metabolic enzyme activity, drug binding and route of administration. However, with the use of a physiological model of hepatic drug elimination, such predictions can be made. This model has been tested experimentally and appears to be valid. Hepatic blood flow can vary over about a 4-fold range from half normal flow to twice logical changes affecting the circulation. For drug clearance to be affected significantly by these changes in flow, the drug must be avidly removed by the liver as reflected in a high hepatic extraction ratio and intrinsic hepatic clearance. This latter term is a useful way to characterise the ability of the liver to irreversibly remove drug from the circulation in the absence of any flow limitation. The clearance of drugs with low intrinsic clearance will not be affected significantly by changes in liver blood flow.

Increased clearance of antipyrine and d-propranolol after phenobarbital treatment in the monkey. Relative contributions of enzyme induction and increased hepatic blood flow

The effects of phenobarbital treatment for 12 days on the regional distribution of blood flow and on the disposition of two model drugs, antipyrine and d-propranolol, have been determined in six unanesthetized rhesus monkeys. Phenobarbital significantly increased total hepatic blood flow from 179+/-15 to 239+/-27 ml/min. Liver weight was increased to a similar degree (34%) in phenobarbital-treated animals as compared to control monkeys. The clearance of both antipyrine and d-propranolol was increased and the half-life decreased significantly by phenobarbital. Analysis of the data by a perfusion-limited pharmacokinetic model showed that the changes in antipyrine clearance were due almost entirely to enzyme induction. On the other hand, with d-propranolol, the increase in liver blood flow contributed as much to the enhanced clearance as did the stimulation of drug metabolism. The mechanism by which phenobarbital produces the frequently observed increase in drug clearance, therefore, depends upon the initial clearance value of the drug. For low clearance drugs like antipyrine, clearance changes occur largely as a result of enzyme induction. With higher clearance drugs, the effects of increased hepatic blood flow become progressively more important the greater the initial clearance value.

Pathophysiology of intense physical conditioning in a hot climate. I. Mechanisms of potassium depletion

Serial estimations of exchangeable (42)K showed that six volunteer subjects undergoing intensive physical conditioning in a hot climate sustained a mean deficit of 517 mEq. This deficit occurred despite a daily potassium intake of 100 mEq. Simultaneous values for lean body mass rose suggesting that potassium deficiency was not the result of catabolism. Although sweating was the major avenue by which the deficit occurred, daily excretion of potassium into the urine when each subject was maximally deficient ranged from 46 to 75 mEq and thus was inappropriately high for potassium-depleted subjects. Despite high intakes of sodium and excretion of corresponding quantities into the urine, Na/K ratios in sweat were low thus indicating unsuppressed activity of aldosterone on sweat glands. Moreover, excretion and secretion of aldosterone and in many instances, plasma renin activity, appeared to be high with respect to sodium intake. These findings suggest that intense physical work in the heat stimulates higher production of aldosterone than would occur in nonexercising subjects on similar sodium intakes. Similar to the phenomenon of mineralocorticoid escape, such overproduction of aldosterone in the presence of conditions permitting excretion of sodium into the urine could facilitate continued excretion of potassium by the kidney despite serious potassium depletion. As a consequence, the kidney played a role in the genesis of potassium depletion in these subjects. In contrast to subjects undergoing conditioning in the summer months, potassium depletion did not occur in 16 subjects during identical training under cooler environmental conditions.

Pathogenesis of cardiac oedema

When the heart ceases to meet the requirements of the body for oxygen, the sympathetic-adrenal system is activated. This occurs in people with a healthy heart when the demands for oxygen are excessive-for example, in heavy muscular work-and in subjects with a failing heart when the demands are normal or small. Eventually, when the heart is unable to meet even the ordinary requirements of everyday life, the sympathetic activity becomes more or less continuous. It may lessen during rest at night, but with a further failing of the heart its output may become inadequate even in complete rest.The sympathetically-medicated renal vasoconstriction, with reduction of the glomerular sodium load, redistribution of the blood flow in the renal cortex to the juxtamedullary glomeruli, and the mobilization of the renin-angiotensin-aldosterone system, is responsible for the salt and water retention which will ultimately become clinically manifest as oedema-especially when it is no longer counteracted by the tidal output of water and sodium at night. A by-product of this continuing dehydration reaction is a cumulative potassium loss which may lead to disastrous consequences if untreated.

Plasma renin in chronic experimental heart failure and during renal sodium "escape" from mineralocorticoids

A striking increase in the plasma renin level occurred in dogs with low output right heart failure secondary to tricuspid insufficiency and pulmonic stenosis and in three of five animals with high output failure produced by a large arteriovenous fistula. When dogs with a small arteriovenous fistula were given daily injections of DOCA, the renal sodium "escape" phenomenon occurred. In these animals, the level of plasma renin was suppressed during DOCA administration both during the initial period of sodium retention and also later when sodium balance was normal or negative. In contrast, when dogs with a larger arteriovenous fistula but without evidence of cardiac failure were given DOCA, they retained sodium and developed signs of congestive heart failure. However, in these animals with congestion and ascites, in contrast to the dogs that developed spontaneous high output failure, the plasma renin was low. Renin-substrate was unaltered in all of the experimental situations studied except for the decrease observed in dogs with low output right heart failure. In these animals, it seems likely that decreased renin-substrate was secondary to hepatic congestion and liver damage. The renin-angiotensin system does not seem to be related to the "escape" phenomenon, and renin does not appear to be the factor that makes the kidney unusually responsive to mineralocorticoids. Thus, in experimental heart failure the renin-angiotensin system was activated, but in the congestive syndrome produced by DOCA the plasma renin level was suppressed.

The metabolism and secretion of aldosterone in elderly subjects

The secretion rates [34 +/- 6 (SE) mug per day, 9 subjects] and metabolic clearance rates (MCR) [1,288 +/- 120 (SE) L of plasma per day, 9 subjects] of aldosterone in elderly subjects are significantly lower than those of young subjects [77 +/- 7 (SE) mug per day and 1,631 +/- 106 (SE) L per day, respectively]. There is a correlation of the MCR and secretion rate values (p = 0.02), but the calculated plasma concentrations (secretion rate/MCR) are also significantly low in the elderly subjects [2.6 +/- 0.3 (SE) compared with concentrations in the plasma from young subjects of 4.7 +/- 0.6 (SE) mug per 100 ml plasma]. The urinary excretion of radioactivity from oral and intravenously administered labeled aldosterone as aldosterone in the neutral extract, as aldosterone released by acid hydrolysis, and as tetrahydroaldosterone released by incubation with beta-glucuronidase is generally similar for young and elderly subjects except that a larger portion of the oral compared with the intravenous dose is excreted as free aldosterone in the elderly subjects, indicating that the splanchnic extraction is reduced. The calculated splanchnic blood flow (assuming no alteration in extrasplanchnic metabolism) is also slightly lowered. Therefore, as in patients with mild cardiac dysfunction, the lowered MCR of subjects is due to both reduced splanchnic extraction and blood flow. However, unlike the heart failure patients, in the elderly subjects the plasma concentration of aldosterone is also reduced.

The effect of acute haemorrhage in the dog and man on plasma-renin concentration

1. The effect of acute haemorrhage on the plasma renin concentration was studied in the dog and man.2. Plasma-renin concentration was regularly increased after the larger bleeds; after the smaller haemorrhages plasma-renin concentration remained unchanged.3. The results are discussed in relation to current hypotheses concerning the control of renin and aldosterone secretion.