Inhibition of sodium- and potassium-dependent adenosine triphosphatase by ethacrynic acid: ligand-induced modifications

Central effect of agents which alter sodium transport on water drinking in the rat

The central effect of ouabain (OUA), ethacrynic acid (EA) and diphenylhydantoin (DPH) on water drinking was studied in rats. OUA and EA inhibit cellular Na efflux, increasing intracellular (IC) Na content, while DPH inhibits cellular Na influx lowering IC sodium content. The animals were injected into the third ventricle (3V), fourth ventricle (4V), lateral ventricle (LV), or lateral hypothalamus (LH). Ouabain (50 pg) and EA (50 ng) when injected into the 3V or LV significantly decreased the water intake induced by 24 hr water deprivation. No effect was observed after injections in other loci, thus supporting the hypothesis that sensors could be located in the circumventricular organs, in an area without the blood brain barrier. DPH (270 ng) injected into the 3V or LV significantly enhanced water intake in rats deprived of water for 14 hr, but it did not increase water intake in water-repleted rats. DPH also enhanced water intake induced by cellular-dehydration (2 M NaCl IP). These results suggest that changes in the Na content of sensor cells located on or near the walls of the 3V could be responsible for the movement of water between the cells and their medium, in agreement with the osmoreceptor hypothesis for thirst.

Effects of pharmacological and physiological modulators on Ca-ATPase and alkaline phosphatase activities from guinea-pig placenta in vitro

In a search for modulators of Ca-ATPase and AP activities, we examined three pharmacological agents and the cations Ca2+ and Zn2+. Placental Ca-ATPase specific activity was uncompetitively inhibited in vitro by millimolar concentrations of the diuretics ethacrynic acid and furosemide. Cysteine, a sulphydryl donor, partially reversed the ethacrynic acid inhibition but enhanced the furosemide inhibition, indicating that sulphydryl-binding may be part of the mechanism of the inhibition of Ca-ATPase by ethacrynic acid but not by furosemide. In contrast to Ca-ATPase, AP activity was enhanced by both ethacrynic acid and furosemide. Zinc inhibited Ca-ATPase activity at all concentrations tested, but enhanced and, at higher concentrations, inhibited AP activity. The inhibition of AP activity by D-penicillamine was reversed by Zn, supporting the view that this drug acts by chelating Zn which is essential for AP activity. D-penicillamine had no significant effect on Ca-ATPase activity. Calcium activated both enzyme activities but inhibited only AP activity at higher concentrations. These results indicate that placental Ca-ATPase and AP activities may be distinct and dissociable based on responses to various pharmacological and physiological modulators.

Effects of ethacrynic acid and furosemide on phosphorylation reactions of kidney mitochondria. Inhibition of the adenine nucleotide translocase

Previous reports that ethacrynic acid and furosemide diminish mitochondrial P : O ratios and reduce (Na+ + K+)-ATPase activity suggested that these diuretics may inhibit mitochondrial phosphorylation reactions. This possibility was initially studied by determining the effects of ethacrynic acid and furosemide on [32P]ATP exchange activity of rat kidney mitochondria. Concentrations of both drugs at 10(-4) M or greater, significantly inhibited [32P]ATP exchange. To investigate the mechanism of this inhibition, the effects of ethacrynic acid and furosemide on the ATPase activity of intract mitochondria and sonicated submitochondrial particles were determined. Both diuretics inhibited ATPase activity of intact mitochondria at 10(-4) M. In contrast, ATPase of submitochondrial particles was significantly less susceptible to inhibition by the diuretics. These results suggested that ethacrynic acid anf furosemide inhibit adenine nucleotide transport across the mitochondrial membrane. This was directly tested by determining the effects of the diretics on the mitochondrial adenine nucleotide translocase. At 5-10(-4) M, both ethacrynic acid and furosemide significantly inhibited adenine nucleotide transport. These findings suggest that ethacrynic acid and furosemide may diminish renal tubular solute reabsorption by direct inhibition of adenine nucleotide transport across the mitochondrial inner membrane.

Renal adenylate cyclase-effects of diuretics

The in vitro effect of various diuretics on rat kidney adenylate cyclase was investigated in crude homogenates of the cortex, the outer and inner medulla. 10-3 M furosemide inhibited adenylate cyclase by 40% in the cortex, by 16% in the outer medulla and by 43% in the inner medulla. 10-3 M ethacrynic acid inhibited adenylate cyclase activity by 65% in the cortex, 59% in the outer medulla and by 57% in the inner medulla. Amiloride produced no significant inhibition of the adenylate cyclase reaction. In the cortex, furosemide partially inhibited adenylate cyclase under basal, fluoride-stimulated and parathyroid hormone-stimulated conditions. Ethacrynic acid produced a strong inhibition of adenylate cyclase activation by F- and parathyroid hormone. In the inner medulla 10-2 M F- and 1 mU antidiuretic hormone reversed the furosemide effect on adenylate cyclase. Ethacrynic acid produced a strong inhibition of adenylate cyclase in the presence of F- and antidiuretic hormone. It is suggested that inhibition of renal adenylate cyclase might be a possible mode of action of certain diuretics.

Reversal of cyclic AMP-mediated intestinal secretion by ethacrynic acid

Ethacrynic acid (EA) has been reported to reduce cholera toxin-induced intestinal fluid secretion in the intact animal. We explored the nature of this inhibition in vitro by measuring unidirectional, transmural fluxes of (22)Na and (36)Cl across isolated rabbit ileal mucosa. Under control conditions (short-circuited mucosa bathed in bicarbonate-Ringer), there was net absorption of Na and Cl. Theophylline (10 mM), cyclic AMP (5 mM), and cholera toxin (added in vivo) abolished net Na flux and produced net Cl secretion. In the presence of either theophylline or cAMP, addition of 0.1 mM EA to the serosal bathing solution abolished net Cl secretion and restored net Na absorption. Cholera toxin-treated mucosa was exposed to 0.05 and 1.0 mM EA. The lower concentration restored net Na absorption but did not significantly reduce Cl secretion. The higher concentration abolished net transport of both Na and Cl. Short-circuit current and Na flux measurements in the presence and absence of glucose indicated that 0.1 mM EA does not inhibit glucose-coupled Na transport. Short-circuit current measurements in the presence of 1.0 mM EA suggested that even this concentration of EA does not inhibit glucose-coupled Na transport. Thus EA appears to specifically inhibit Cl (or NaCl) secretion without inhibiting the absorptive Na "pump." The anti-secretory effect of 0.1 mM EA does not appear to result from inhibition of adenylate cyclase since secretion stimulated by addition of 5 mM cAMP was abolished. Furthermore, 0.1 mM EA did not significantly reduce theophylline-augmented and cholera toxin-augmented cAMP levels in ileal mucosa. We conclude that EA interacts specifically with the active Cl (or NaCl) secretory mechanism of the small intestine at a step beyond generation of cAMP.