RENAL TUBULAR EFFECTS OF ETHACRYNIC ACID

Loop and distal actions of a novel diuretic, M17055

We investigated the mechanism of action of a novel 'high ceiling' diuretic, M17055, in in vivo clearance studies with anesthetized dogs during water diuresis and in vitro microperfusion studies of isolated rabbit renal tubules. In the clearance study, intravenous infusion of M17055 (1 mg/kg per h) decreased free water clearance and increased urinary excretion of Na+ and Cl- to a greater extent than did a maximum dose of furosemide (30 mg/kg per h). With the maximum dose of furosemide, an additional dose of M17055 or hydrochlorothiazide resulted in additional suppression of free water clearance. These results indicate that M17055 has some additional mechanisms of action in the distal nephron. In isolated rabbit cortical thick ascending limb of Henle's loop, M17055 applied to the lumen decreased the lumen positive transepithelial voltage at concentrations over 10(-6) M and suppressed the lumen-to-bath 36Cl- flux at 10(-5) M. In the connecting tubule, M17055 added to the lumen suppressed lumen negative transepithelial voltage in a concentration-dependent manner in a range from 10(-4) to 10(-3) M. The effect of M17055 on transepithelial voltage was also observed in the distal convoluted tubule and cortical collecting duct. Moreover, 10(-3) M of M17055 in the lumen significantly decreased the lumen-to-bath 22Na+ flux in the cortical collecting duct. From these observations, it appears that M17055 acts not only on the thick ascending limb of Henle's loop but also on the distal segments via inhibition of electrogenic Na+ transport.

An electrophysiological study of amiloride on sino-atrial node cells and ventricular muscle of rabbit and dog

Effects of amiloride were studied on canine and rabbit sino-atrial node cells and the canine ventricular cells. In the canine sino-atrial node selectively perfused with Tyrode solution through the sinus node artery, amiloride (4.4 X 10(-4)-1.3 X 10(-2)mol/l) induced a negative chronotropic and a positive inotropic effect in a concentration-dependent manner. In canine ventricular muscle, the amplitude of the action potential and the resting membrane potential were decreased in the presence of amiloride (4.4 X 10(-6)-8.7 X 10(-4) mol/l). The maximum rate of rise was also depressed in a use-dependent manner as well as a concentration-dependent manner. Simultaneously the action potential duration was markedly prolonged. Low concentrations of amiloride decreased the contractile force, but higher concentrations (over 8.7 X 10(-5) mol/l) enhanced it. In the spontaneously beating rabbit sino-atrial node, amiloride (8.7 X 10(-5)-2.2 X 10(-3) mol/l) lengthened the action potential duration and the cycle length accompanied with the decreases in the amplitude and the maximum rate of rise of the action potential. The maximum diastolic potential was slightly depolarized. In voltage clamp experiments of the rabbit sino-atrial node, amiloride (4.4 X 10(-4)-1.3 X 10(-3) mol/l) suppressed the slow inward current (Isi) and the outward current (Ik) concentration-dependently, without altering steady-state inactivation of Isi (f infinity) and the activation of Ik (p infinity). The hyperpolarization-activated inward current (Ih) was also reduced. These results suggest that amiloride decreased Isi, Ik and Ih by a reduction in the conductance of these current systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Diuretic potency of combined hydrochlorothiazide and furosemide therapy in patients with azotemia

The effect of combined hydrochlorothiazide and furosemide therapy was studied in eight hypertensive patients with renal insufficiency who had poor response to either furosemide or hydrochlorothiazide alone. The study was divided into two parts. In part A, five patients had an inadequate response to furosemide in doses of 160 to 240 mg/day followed a strict protocol in order to compare the effect of increased doses of furosemide with combined hydrochlorothiazide-furosemide administration. All had azotemia, presumable from nephrosclerosis, and had serum creatinine concentrations ranging from 2.3 to 4.9 mg/dl. Four of the five patients had inadequate arterial pressure control, and the remaining patients had fluid retention from the administration of minoxidil. In all five patients, plasma volume was either increased or normal, despite long-term treatment with furosemide. Increasing the dose of furosemide to between 320 and 480 mg/day had only a modest additional diuretic effect, and plasma volume and arterial pressure were not significantly changed. Adding hydrochlorothiazide, 25 to 50 mg twice a day, produced a marked diuresis, and a significant reduction in weight, plasma volume and mean arterial pressure (p less than 0.025 for all three patients). In part B, combined hydrochlorothiazide-furosemide therapy was used to treat three additional patients who had an inadequate response to either diuretic alone. The results indicate that combined hydrochlorothiazide-furosemide is a potent diuretic regimen and is effective in many patients wit chronic renal failure who have a poor response to furosemide alone.

Functional differences of ouabain and ethacrynic acid on renal potassium metabolism in dogs

Current concepts provide inadequate explanations for the effects of ethacrynic acid (ECA) and ouabain (OUA) on renal potassium metabolism because neither the site of action, nor the tubular effects have been agrreed upon. In the present study on anaesthetized dogs, in vivo inhibitory effects of ECA on mitochondria or Na-K-ATPase were excluded. Firstly, ECA hardly reduced outer medullary metabolic rates in kidneys exposed to OUA and 2,4-dinitrophenol. Secondly, whereas OUA reduced renal Na-KATPase activity by 78% and induced parenchymal potassium release into the renal vein, none of these effects were observed during injection of ECA. During free flow, sodium reabsorption was equally reduced by the two drugs, but fractional potassium excretion was 0.54 +/-0.04 after OUA and 0.99 +/-0.02 after ECA. OUA alone or combined with ECA inhibited almost all ion transport in the distal 60% of the tubular volume as revealed by stop-flow. During ECA alone, chloride reabsorption was reduced more than sodium reabsorption because sodium was exchanged for potassium. ECA and SO4(2-) infusion produced similar stop-flow patterns. Thus, OUA is bound to Na-K-ATPases functionally located to the peritubular cell membrane. ECA promotes potassium secretion probably by inhibiting cellular anion entry at the luminal membrane.

Influence of ethacrynic acid on intrarenal renin release mechanisms

Ethacrynic acid infused i.v. in anesthetized dogs after inhibiting sympathetic mechanisms of renin release increased renal blood flow rate (RBF) by 54% and practically abolished autoregulation of RBF; renin release increased from 0.8 +/- 0.9 (mean +/- SEM) to 16.4 +/- 3.7 mug/min (P less than 0.05). Without infusion of ethacrynic acid; constriction of the renal artery to a pressure below the range of autoregulation reduced renovascular resistance markedly and renin release rose to 27.2 +/- 5.5 mug/min (P less than 0.05). During arterial constriction, ethacrynic acid had no additional effect on renovascular resistance or renin release averaging 28.4 +/- 6.7 mug/min. Infusion of ethacrynic acid and saline at control pressure increased sodium excretion to about one-half of the filtrate and reduced rein release which did not, however, return to control. Infusion of hypertonic saline during autoregulated vasodilatation induced by arterial constriction had a similar effect, but again renin release continued to exceed control. We propose that ethacrynic acid increases renin release through a hemodynamic mechanism triggered by afferent arteriolar dilation and inhibits renin release by greatly increasing the delivery of sodium to the distal convoluted tubules.

Cardiac effect of diuretic drugs

Triamterene, amiloride, ethacrynic acid, and furosemide were studied to determine whether they modified the digitalis-induced egress of myocardial potassium which is thought to facilitate the development of digitalis arrhythmias. In a control group of 15 dogs, potassium was measured in samples obtained simultaneously from the femoral artery (FA) and the coronary sinus (CS) in a control period and at intervals after the administration of 1 mg. of acetylstrophanthidin. Acetylstrophanthidin caused a significant increase in cardiac A-V difference in the potassium concentration (CS-FA) averaging 0.47 mEq. per liter. In a group of 10 dogs, when 175 mg. of triamterene was infused prior to the acetylstrophanthidin, the rise in A-V differnece was abolished and the arrhythmias often aborted. In contrast, the infusion of potent diuretics (40 mg. of furosemide in five dogs and 100 mg. of ethacrynic acid in another five dogs) prior to acetylstrophanthidin, caused a doubling of the maximal A-V potassium difference. This study suggests that the clinical administration of antikaliuretic drugs may prevent the arrhythmias of digitalis toxicity not only by reducing kaliuresis and subsequent hypokalemia, but by a myocardial effect which antagonized the digitalis-induced loss of myocardial potassium. Contrariwise, potent diuretics may facilitate digitalis arrhythmias through a myocardial action causing a greater egress of myocardial potassium, thus explaining the development of arrhythmias despite normal serum potassium levels. These potent diuretics should be used cautiously, especially when given intravenously to patients receiving digitalis.

Effects of ethacrynic acid on the isolated collecting tubule

Effects of the diuretic ethacrynic acid on osmotic water permeability were investigated in the isolated perfused collecting tubule of the rabbit kidney. The base-line water permeability of the collecting tubule was not affected when the drug (10(-4)M) alone was added to the bathing medium. Vasopressin alone in the bathing medium (2, 5 muU/ml) elicited a significant increase in osmotic water absorption. With vasopressin kept in the bathing medium, the addition of 10(-5)M ethacrynic acid depressed the hydro-osmotic effect of vasopressin by 50%. This inhibitory effect of low concentrations of ethacrynic acid could be surmounted by high, supramaximal dosage levels of vasopressin. When 10(-4)M ethacrynic acid was added to the bathing medium before vasopressin, the hydro-osmotic effect of vasopressin and the diuretic in combination was insignificant.Dibutyryl adenosine 3'5'-monophosphate (10(-4)-10(-2)M) alone in the bathing medium significantly increased baseline osmotic water flow, mimicing the effect of antidiuretic hormone. When ethacrynic acid was added together with the nucleotide, the permeability remained at the same high level. Theophylline, like the nucleotide and vasopressin, produced a significant hydro-osmotic effect. The magnitude of this response was not affected by further addition of ethacrynic acid (10(-4)M). It was concluded that ethacrynic acid is an antagonist of antidiuretic hormone. The antagonism probably occurs at the level of the receptor site of the hormone on the peritubular membrane. Antagonism to circulating antidiuretic hormone may therefore be one of the factors involved in the loss of renal concentrating ability brought about by ethacrynic acid diuresis.

The postobstructive kidney. Observations on nephron function after the relief of 24 hr of ureteral ligation in the dog

After the relief of 24 hr of complete unilateral ureteral obstruction in the dog, the experimental kidney is characterized by a decrease in filtration rate and an increase in fractional and often absolute excretion of sodium before and after the administration of mannitol. In the hydrated state, the failure to conserve sodium is associated with increases in fractional free water clearance and fractional sodium supply to water-freeing sites signifying that the augmented sodium excretion is derived from a proximal source. In the hydropenic state there is decreased fractional free water reabsorption, and sometimes free water excretion, in the postobstructive kidney. An early plateau in free water reabsorption is associated with an increased fractional excretion of sodium. These findings are attributed to the early development of distal nephron impermeability to water as a result of enhanced distal tubular supply and transport of sodium. There is a decrease in maximal tubular reabsorptive capacity (Tm) of glucose in the post-obstructive kidney which is, however, less marked than the decrease in filtration rate. The fall in filtration rate is to some extent likely due to a dropping out of nephrons from the circulation while the remaining nephrons are hypoperfused. The magnitude of the sodium reabsorptive defect is markedly exaggerated as the concentration of nonreabsorbable solute (mannitol) in the glomerular perfusate is increased. It is concluded that the postobstructive increase in sodium excretion during mannitol administration is in part due to a limit in the capacity to reabsorb sodium against a concentration gradient in the proximal tubule.

The effect of altered sodium concentration in the distal nephron segments on renin release

Ethacrynic acid, a potent inhibitor of sodium reabsorption in the ascending limb of Henle's loop, produces a sharp rise in renal venous renin activity within 5 min after intravenous administration in anesthetized dogs. This response persists when volume depletion is prevented by returning urinary outflow to the femoral vein. Comparable studies with chlorothiazide, a diuretic with little or no effect on the medullary portion of the ascending limb of the loop of Henle, failed to produce a significant increase in renal venous renin activity.When administered during ureteral occlusion, ethacrynic acid produced no change in renal venous renin activity until ureteral occlusion was released and flow restored. Following release of the ureters, a prompt rise in renal venous renin was again observed within 5 min of release. Control studies of ureteral occlusion yielded a fall in renal venous renin activity following release of the ureter without administration of ethacrynic acid. These studies identify a prompt stimulatory effect of ethacrynic acid on renin release that is unrelated to volume depletion but dependent upon the presence of tubular urine flow. Although further definition of the site and characteristics of the distal tubular mechanism for stimulation of renin release requires more direct study, the data presented here indicate that changes in sodium concentration in distal tubular fluid serve as a stimulus for renin release.

Influence of sodium balance on the ability of diuretics to inhibit tubular reabsorption. A study of factors that influence renal tubular sodium reabsorption in man

Chlorothiazide and ethacrynic acid were given to normal humans and patients with cirrhosis and ascites, with replacement of urinary losses, to obtain information on (1) the influence of different states of sodium balance and excretion on the effectiveness of the drugs in blocking tubular reabsorption and promoting excretion of sodium, and (2) the influence of different states of sodium balance on reabsorption of sodium by different segments of the nephron of man. The results demonstrated that in normal man the natriuretic response to these agents is markedly influenced by the existing state of sodium balance. The differences in response to the drugs related both to differences in the filtered load of sodium and to differences in the fraction of filtered sodium reabsorbed at the diuretic-sensitive distal tubular sites. Sodium loading in the presence of a mineralocorticoid was associated with a thiazideinduced increment in the excretion of sodium which was 10 times greater than that observed during sodium depletion. This effect is suggestive of decreased proximal tubular reabsorption. In patients with cirrhosis and sodium retention the fraction of filtered sodium reabsorbed at both diuretic-sensitive tubular sites was diminished, suggesting increased proximal tubular reabsorption. In addition to demonstrating a striking influence of sodium balance on the natriuretic response to the diuretics, the results suggest that in man sodium diuresis is associated with decreased fractional reabsorption by the proximal tubule and increased fractional and absolute sodium reabsorption by the distal tubule. In patients accumulating edema and ascites, an increase in the fractional reabsorption of sodium appears to occur in the proximal tubule and could account for limited effectiveness of the diuretic agents.

The effects of infusion of water on renal hemodynamics and the tubular reabsorption of sodium

Anesthetized dogs receiving an infusion of chlorothiazide and ethacrynic acid were given 600-ml infusions of distilled water or dilute dextrose solutions. The absolute rate of tubular sodium reabsorption was depressed, and the glomerular filtration rate was increased during the water loading, despite the associated decreases in plasma sodium concentration and decreases in the filtered load of sodium. The extent to which fractional sodium reabsorption decreased and the excretion of sodium increased was inversely related to the degree to which the filtered load of sodium was depressed as a result of the decreased plasma sodium concentration. We conclude that, in the presence of the diuretic blockade of distal tubular sodium reabsorption, infusion of water depresses proximal tubular reabsorption of sodium and that these changes are qualitatively similar to those previously observed during infusions of saline. Similar depression of tubular reabsorption of sodium and increased excretion of sodium occurred during water loading in the absence of diuretics in dogs undergoing saline diuresis, which presumably provided a high rate of distal sodium reabsorption before water loading. We suggest that volume expansion with water depresses proximal tubular reabsorption of sodium in a manner qualitatively similar to infusions of saline and that the extent to which sodium excretion is increased during water loading is dependent upon 1) the absolute extent to which proximal reabsorption is depressed, 2) the extent to which the filtered load of sodium is maintained in the presence of a falling concentration of sodium in plasma, and 3) the extent to which increased distal reabsorption compensates for the depressed proximal reabsorption of sodium. Mechanisms are suggested whereby the previously reported inverse relationship between plasma concentration of sodium and over-all tubular reabsorption of sodium may be only apparent, and could be the result of physiologic "glomerulotubular balance" during the specific experimental maneuvers.

Effects of hypotonic saline loading in hydrated dog: evidence for a saline-induced limit on distal tubular sodium transport

We performed studies on dogs under hydrated conditions, utilizing the rate of free water formation (C(H2O)) as an index of the rate of distal tubular sodium transport. Since C(H2O) could be progressively increased with no evidence of a maximal rate during loading with hypotonic (2.5%) mannitol, it was concluded that there is no limit on distal tubular sodium transport during mannitol loading. In contrast, during hypotonic (0.45%) saline loading C(H2O) rose initially, but as urine flow (V) exceeded 25% of the filtered load C(H2O) attained maximal levels (up to 20% of the filtered load) and remained stable as V increased to 50% of the filtered load. It was concluded that saline loading progressively inhibits proximal sodium reabsorption. Initially, the distal tubule absorbes a large fraction of the proximal rejectate and sodium excretion rises slightly. Eventually, an alteration in distal sodium transport appears which culminates in a maximal rate or transport limit. This distal transport limit provoked by saline loading could not be characterized by a classical Tm as seen with glucose and does not seem to be consequent to high rates of flow through the distal tubule. Regardless of the precise nature of this limit, the major increment in sodium excretion develops during saline loading only after saline alters the capacity of the distal tubule to transport sodium.

Uphill transport of urea in the dog kidney: effects of certain inhibitors

To study the renal medullary transport and accumulation of urea in dogs independent of water transport, we obliterated the medullary electrolyte gradient by a sustained ethacrynic acid diuresis. Infusions of urea were also given at various rates to vary urinary urea concentration. In the steady state, the kidneys were removed, and slices were analyzed for water, urea, and electrolytes. In every experiment in 15 dogs over a range of urinary urea concentration from 19 to 230 mmoles per L and urine flow from 0.5 to 9.7 ml per minute per kidney, an intrarenal urea gradient persisted, and urinary urea concentration was always lower than papillary water urea concentration. The magnitude of this uphill urinary-papillary gradient (mean +/- SE = - 21 +/- 2.9 mmoles per L) was not affected by hemorrhagic hypotension or a nonprotein diet. In 12 additional experiments begun similarly, inhibitors were infused into one renal artery. Both iodoacetate, an inhibitor of anaerobic glycolysis, and acetamide, an analogue of urea, markedly and significantly reduced both the intrarenal urea gradient and the uphill urinary-papillary gradient. In contrast, cyanide, an inhibitor of oxidative metabolism, had no observable effect on the urea gradients. The data are best explained by postulating an active transport system for urea in the medullary collecting duct deriving its energy from anaerobic glycolysis.

Factors affecting sodium reabsorption by the proximal tubule as determined during blockade of distal sodium reabsorption

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Ethacrynic acid: two years' experience with a new diuretic

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