A stereologic study of the granule population of rat atrial myoendocrine cells: effects of chronic water/sodium restriction and acute water/sodium load.

The effects of chronic water/sodium restriction and acute water/sodium load on the "specific granule" population of rat atrial cardiocytes were investigated by stereology. The volume density (Vv) of atrial granules, which contain atrial natriuretic peptide (ANP), displayed a 135%-rise in water/sodium-restricted animals. The injection of 2.5 ml of isotonic saline to water/sodium-restricted rats provoked a striking degranulation of atrial myoendocrine cells (Vv of granules decreased by about 31%). The administration of an equal volume of hypertonic (5%) saline caused a more pronounced effect (Vv decreased by about 51%). These findings are in agreement with the view that chronic water/sodium restriction blocks the exocytotic release of atrial granules, without lowering the rate of their production. Moreover, they suggest that not only blood volume expansion, but also sodium ions can be a potent stimulating factor of ANP release.