The control of sodium metabolism to maintain osmo- and volumehomeostasis

Chronically instrumented female beagles were maintained in standardized environmental and dietary conditions allowing careful examination of the mechanisms governing sodium homeostasis. The experimental increase in left atrial pressure (obtained by a reversible mitral stenosis) is accompanied by an increase in sodium excretion (atrial natriuresis, AN). AN served as an experimental manoeuvre from which the mechanisms governing sodium homeostasis could be elucidated. The results allow the following conclusions: (1) The 'signals' arising from distension of the left atrium (e.g. expansion of the extracellular fluid volume) appear not to be a necessary prerequisite for the maintenance of sodium homeostasis. (2) The control mechanisms seem to be very sensitive to changes in total body sodium (TBS). A small reduction in TBS abolishes sodium eliminating processes e.g. saline diuresis, osmotic diuresis, AN. (3) It is probable that a natriuretic factor exists for sodium elimination. In summary, total body sodium appears to be controlled by a series of 'redundant' mechanisms which guarantee an appropriate strategy for the comfort and ultimate survival of the organism. At the moment it is impossible to quantitate the contributions made by the various mechanisms in the control of sodium metabolism.