ETHACRYNIC ACID: DIURETIC PROPERTY COUPLED TO REACTION WITH SULFHYDRYL GROUPS OF RENAL CELLS

Effect of inhibitors and uncouplers on the Na pump of the Maia muscle fibre

1. A study has been made of sodium efflux from single muscle fibres of the crab Maia squinado, and of the effect of inhibitors and uncouplers on this efflux, using the microsyringe technique.2. (i) Externally applied ethacrynic acid or injected ethacrynic acid produces 50% inhibition. The inhibitory state due to externally applied ethacrynic acid is shown to be sensitive to 40 mM-K(+) in the bathing medium.(ii) Internally applied chlorpromazine causes 40% inhibition.(iii) Internally applied ethyl alcohol causes a slight increase or a slight decrease in Na efflux or no effect.(iv) Internally applied antimycin A produces a net inhibitory effect of almost 50%.(v) Internally applied rotenone produces a 60% inhibitory effect while fibres that fail to contract initially show about a 90% fall in efflux.(vi) The response pattern to internally applied oligomycin is similar to that of antimycin but the net inhibitory effect is only 25%.(vii) Internally applied atractylate produces 45% inhibition.(viii) Internally applied gramicidin A produces about 40% inhibition.(ix) Internally applied valinomycin is without an effect, but in the presence of 40 mM-K(+) in the external medium there is marked inhibition of Na efflux.3. The evidence put forward suggests that the Na pump of the Maia fibre is divided into two parts, one the Na(+)-K(+)-ATPase, driven by ATP and arginine phosphate (ArP), and the other, an electrogenic mechanism, driven by respiration and possibly direct coupling of glycolytic energy. The process of reverse electron flow can be invoked as a reasonable explanation of some of the observations made.

Purification and properties of coproporphyrinogenase

1. Coproporphyrinogenase has been prepared from rat-liver mitochondria and its properties have been studied. The isoelectric point was found to be around pH5.0 and the molecular weight to be 80000+/-8000. The pH optimum of the enzymic reaction was 7.4 and the apparent K(m) was of the order 0.03mm. The enzyme was destroyed by boiling and irreversible inactivation occurred below pH3.5. It could be stored at -10 degrees without loss of activity. The enzyme acts specifically on coproporphyrinogen III and does not form protoporphyrinogen from trans-2,4-diacrylicdeuteroporphyrin or its porphyrinogen. It was unaffected by prolonged dialysis and no cofactor requirement could be demonstrated. 2. Column chromatography of a partially purified enzyme preparation on Sephadex G-200 was found to be an improved method of purification, which gave a coproporphyrinogenase 58-fold purified. The purified enzyme was studied electrophoretically but no evidence was obtained to suggest that more than one enzyme was involved in the reaction. 3. The action was studied of various compounds added to the system. The presence of monothiol groups in the enzyme system was indicated, whereas vicinal dithiol groups were not involved in the reaction. Metal-chelating agents did not inhibit the reaction and no requirement for the presence of any essential metal has been found. All attempts to demonstrate the presence of a prosthetic group, in particular flavines, failed. Neither pyridoxal phosphate nor ATP was involved in the reaction, nor was a mitochondrial electron-transport chain required for the activity of the enzyme. Some circumstantial evidence was obtained to suggest that cis-2,4-diacrylicdeuteroporphyrin is an intermediate in the reaction.

Ethacrynic Acid

Ethacrynic acid, a chemically different diuretic, has been evaluated in 82 patients. The drug was remarkably potent when administered intravenously (0.5 to 1 mg./Kg.) or by mouth (doses to 50 mg. four times per day). It proved particularly effective in patients with refractory edema, and in edematous states accompanied by azotemia or electrolyte disturbances. Its natriuretic effects added to those of other diuretics and potentiation actually appeared to occur when it was combined with a carbonic-anhydrase inhibitor.

Renal clearance data indicated that ethacrynic acid has a unique mode of action. There was little effect on glomerular filtration or renal plasma flow and its major action was to block sodium and chloride reabsorption probably in both proximal and distal portions of the renal tubules.

Ion exchange mechanisms seemed little affected by the drug because it characteristically increased both hydrogen and potassium ion excretion. In general the degree of induced potassium loss correlated well with the rate of endogenous aldosterone secretion. Furthermore, ethacrynic acid did not cause potassium loss in one adrenalectomized subject nor in another subject pretreated with an aldosterone antagonist.

Diuresis induced by ethacrynic acid was often accompanied by the development of systemic metabolic alkalosis. Factors in the development of this alkalosis were (1) urinary K + loss with hypokalemia, (2) urinary H + loss, and (3) in edematous patients the rapid large loss of relatively HCO 3 - -free fluid from the extracellular fluid space. The systemic alkalosis was found to be accompanied by compensatory alveolar hypoventilation. These studies of the mechanisms of the alkalosis point to the occasional importance of the administration of chloride, of potassium, or of saline as corrective measures.

Diuresis with ethacrynic acid, like other diuretic agents, produced marked increases in the rate of aldosterone secretion of normal subjects. More variable effects were observed in patients with heart disease, possibly because improvement in their circulatory status operated to lower hormone secretion activity.

Ethacrynic acid had effects on urate excretion that resembled thiazide diuretics. Higher blood levels of intravenous administration were uricosuric. Lower blood levels of oral therapy caused renal urate retention and hyperuricemia.

Most of the problems associated with the use of ethacrynic acid appeared to be related more to its pharmacologic potency than to any truly toxic effect. Because of this it is suggested that the sensitivity and response pattern of each patient be determined in stepwise fashion. In most patients with edema an intermittent dosage schedule provides a more efficient diuretic response and allows time intervals for correction of any electrolyte imbalance. If used with a thorough understanding of its pharmacologic effects ethacrynic acid promises to be a most useful agent in the therapy of patients with difficult or refractory edema.