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Summary Groups of male rats were treated with antihypertensive drugs (alpha-methyl-dopa, clonidine, propranolol, reserpine, diazoxide) which were administered under conditions causing the onset of high plasma renin activity (PRA) and high plasma and urine aldosterone levels, i.e. together with distilled water load (5% of body weight). Alpha-methyl-dopa and propranolol, which fail to significantly alter PRA and plasma aldosterone levels when administered without distilled water load, cause a marked decrease of plasma and urine aldosterone levels and of PRA when administered together with distilled water load, while diazoxide, and reserpine cause a marked increase of these values. Clonidine causes significant decreases of PRA and plasma aldosterone levels in both experimental situations. Increases of PRA and of plasma aldosterone levels occur also in rats treated with diazoxide associated with propranolol and in those treated with an association of reserpine and propranolol. The highest values of plasma aldosterone levels were found in rats treated with reserpine for two days and receiving a further dose of reserpine associated with propranolol on the third day, which fact was ascribed to the reduction of peripheral vascular resistance and of plasma flow in the kidney (as shown by decreased diuresis) brought about by propranolol and followed by a marked compensatory activation of the renin-angiotensin-aldosterone system. Moreover there is no apparent correlation between the antihypertensive effect and the variations of the renin-angiotensin-aldosterone system.

PRA, plasma and urine aldosterone levels and plasma digoxin were measured in rats in which digoxin had been administered under conditions of high PRA and high aldosterone levels experimentally induced by administering distilled water load and in rats in which digoxin had been administered without distilled water load. Results show that under conditions of high PRA and high aldosterone levels, plasma digoxin concentrations as measured 6 h after treatment were higher (45,3%) than in rats having received digoxin without water load. In assays carried out on rats sacrificed 12 h after digoxin treatment (with or without water load) all values approach basic levels again, thus suggesting that in rats too aldosterone might compete with digoxin at the level of tubular excretion.

Summary The data reported in the present paper refer to quantitative variations of plasma renin activity and plasma aldosterone levels after administration of alpha-blocking agents, i.e. phentolamine, phenoxybenzamine and gihydroergotoxin derivatives, either to rats kept on a standard diet with water ad libitum or to rats receiving distilled water load, the latter treatment causing an increase of both plasma renin activity and plasma aldosterone levels if compared to control values. No strict correlation between (a) drug-induced modifications of plasma renin activity and plasma aldosterone levels and (b) blood pressure drop caused by the same drugs was shown to occur under either experimental condition. Infact phentolamine and dihydroergotoxin derivatives exert all a more or less marked hypotensive action, which may be associated with relatively high plasma renin activita and plasma aldosterone levels, as is the case with phentolamine and phenoxybenzamine, or with low PRA and low plasma aldosterone levels, as is the case with dihydroergotoxin derivatives.

Summary Groups of 10 rats were treated with diuretics (furosemide, hydrochlorotiazide, spironolactone, triamterene) under conditions of high or low plasma renin activity (PRA) brought about by administering distilled water or saline load, respectively. PRA, plasma aldosterone levels, urine aldosterone concentration as well as diuresis and Na+ and K+ excretion were