Spironolactone and its active metabolites (including 7a-thiomethyl-spironolactone and canrenone) act as specific antagonists of aldosterone, and exert their effects by binding competitively to the mineralocorticoid receptor located in the kidneys, heart and blood vessels.

Spironolactone is a natriuretic drug (historically described as a soft diuretic). In the kidney, spironolactone inhibits the aldosterone-induced sodium retention leading to increase in sodium and subsequently water excretion, and potassium retention.

The renal effects of spironolactone and its metabolites lead to a decrease in extracellular volume and consequently in a decrease of cardiac preload and left atrial pressure. The result is an improvement in heart function.

In the cardiovascular system, spironolactone prevents the detrimental effects of aldosterone. Although the precise mechanism of action is not yet clearly defined, aldosterone promotes myocardial fibrosis, myocardial and vascular remodelling and endothelial dysfunction.

In experimental models in dogs, it was shown that long term therapy with an aldosterone antagonist prevents progressive left ventricle dysfunction and attenuates left ventricle remodelling in dogs with chronic heart failure.

In a clinical trial, dogs treated with spironolactone in addition to standard therapy (ACE-inhibitors) demonstrated a reduction in cardiovascular disease deterioration compared to dogs treated with standard therapy alone.

When used in combination with ACE-inhibitors, spironolactone may counteract the effects of “aldosterone escape”.

A slight increase in aldosterone blood levels may be observed in animals on treatment. This is thought to be due to activation of feedback mechanisms and is without adverse clinical consequence. There may be a dose related hypertrophy of the adrenal zona glomerulosa at high dose rates