Ouabain induces secretion of proatrial natriuretic factor by rat atrial cardiocytes

Neonatal rat atrial and ventricular cardiocytes in primary culture secrete proatrial natriuretic factor (proANF), the 126-amino acid precursor of atrial natriuretic factor (ANF). The cellular mechanisms of proANF secretion differ in the two cell types: atrial cardiocytes store proANF in abundant secretory granules before secretion, whereas ventricular cardiocytes secrete the precursor rapidly after synthesis. In this study, we used the Na⁺-K⁺-adenosinetriphosphatase (ATPase) inhibitor ouabain to investigate the functional significance of these differing secretory mechanisms. Ouabain increased immunoreactive ANF (iANF) secretion by atrial cardiocytes 1.5- to 2.4-fold. Metabolic labeling studies demonstrated that proANF was the form of iANF released in response to ouabain. Ventricular secretion of iANF was unaffected by the Na⁺-K⁺-ATPase inhibitor. These observations are consistent with a model in which atrial cardiocytes are able to release proANF via a regulated secretory pathway, whereas ventricular cardiocytes utilize a constitutive secretory pathway. The ability of ouabain to stimulate proANF secretion suggests that an increase in intracellular calcium concentration may promote fusion of secretory granules with atrial cell membranes thereby mediating exocytosis of stored proANF.