6 - The Actions of Aldosterone

Abstract

This chapter discusses the effects, actions, and mechanism of action of aldosterone. Aldosterone, from phyletic evidence, appears to be the most recently evolved adrenocortical hormone. It is present in all groups of tetrapods starting with Amphibia but is found in only a few of the fish where it appears to be only systematically present in the lungfish. Adrenalectomy is shown to have comparable effects, including anorexia, muscular weakness, hypoglycaemia, hypotension, decreased blood volume, and disturbances in blood electrolyte levels. The most prominent of the latter are hyponatraemia and hyperkalaemia, which in mammals, principally reflects an excessive loss of sodium in the urine, accompanied by a failure to excrete sufficient potassium. Many of the systemic effects of adrenocortical insufficiency are related to these disturbances in electrolyte metabolism, and indeed the survival of many mammals can be substantially prolonged by providing additional sodium in the drinking water. Glucocorticoids are the only essential hormones for maintaining adrenalectomized mammals. In mammals, adrenocortical insufficiency, such as that occurring in Addison's disease or following adrenalectomy, apart from altering intermediary metabolism produces disturbances in renal electrolyte and water metabolism. The difficulties in excreting excess water are related to a deficiency of glucocorticoid hormones, while the excessive loss of sodium and retention of potassium primarily reflect the absence of aldosterone.