The endocrine control of blood volume, blood pressure and sodium balance: atrial hormone and renin system interactions.

The atrial peptide hormone exhibits remarkable vasorelaxant, natriuretic and hypovolaemic actions, suggesting that it is part of a cardiovascular control mechanism which operates to regulate blood pressure, blood volume and sodium balance. In this endocrine control, the renin axis provides the primary defence against sodium volume depletion and hypotension while atrial hormone plays an increasingly active counter-role for coping with situations that involve a sodium-volume surfeit or rising blood volume or blood pressure levels. For this assignment, the atrial hormone acts in four different ways to counter or oppose the renin-angiotensin-aldosterone axis. Thus, it promptly reduces renin secretion, it relaxes angiotensin-constricted vessels, it blocks angiotensin-induced aldosterone synthesis, and its natriuresis opposes aldosterone-induced sodium retention. Another special action of atrial hormone seems to be rapidly adjust central blood volumes by promoting the rapid transfer of fluid to the extracellular spaces. This hypovolaemic action probably reflects an action on capillary fluid exchange. It is reflected by a prompt and sustained rise in haematocrit levels. A similar action on the glomerular capillaries may be involved in causing natriuresis. These effects serve to rapidly decompress blood volumes and thereby reduce cardiac work. More work is needed to define the afferent stimuli to the right and left heart which elicit atrial hormone release. The possible role(s) of this new hormonal system in hypertensive and oedematous cardiovascular disorders also remains to be defined.