Diuretics and the renal adenylate cyclase system

1 The relationship between the diuretic effectiveness and the effect on the renal adenylate cyclase of three diuretics, acetazolamide, frusemide and ethacrynic acid, was examined. The hypothesis that acetazolamide and parathyroid hormone (PTH), inhibit renal carbonic anhydrase by a cyclic adenosine 3′,5′-monophosphate (cyclic AMP)-dependent mechanism was also tested.

2 In vitro, acetazolamide, frusemide and ethacrynic acid at high concentrations (10^-3M) all produced some inhibition of basal and stimulated rat kidney plasma membrane adenylate cyclase. The effect of acetazolamide was much less than that of frusemide and ethacrynic acid. These plasma membrane effects were reproduced in studies of cyclic AMP formation in isolated kidney tubules of rats.

3 Intravenous injections of acetazolamide did not change the total cyclic AMP content of the kidneys of rats killed by microwave irradiation.

4 Acetazolamide produced a diuresis in the rat and a slight inhibition of the antidiuretic effect of Pitressin. Frusemide produced a diuresis and greatly reduced the antidiuretic response to Pitressin. Ethacrynic acid was ineffective as a diuretic in the rat and actually enhanced the antidiuretic response to Pitressin.

5 In investigating the possible influence of diuretics and PTH on the activity and state of phosphorylation of carbonic anhydrase it was found that: there was no correlation between the ability of diuretics to inhibit carbonic anhydrase activity and to inhibit carbonic anhydrase phosphorylation; neither PTH nor cyclic AMP (in the presence of adenosine triphosphate, Mg²⁺, K⁺ and incubation at 37°C) inhibited rat cortex homogenate carbonic anhydrase activity.

6 It seems unlikely that any of the tested diuretics exerts its pharmacological effect by means of changes in kidney cyclic AMP metabolism.