Characterization of natriuretic peptide production by adult heart atria

The cardiac polypeptide hormones atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP) are synthesized and costored by atrial cardiocyte s and share receptors and many biologic properties. Although some aspects o f their synthesis and release are specific for each peptide, it is not clea r whether they share intracellular sorting and secretory mechanisms. In the present work we take advantage of a stable isolated rat atrial preparation that allows, for the first time, long-term study of synthesis, trafficking , targeting, and secretion of ANF and BNP by adult atrial muscle. Three mod el stimuli of secretion were used: increased intra-atrial pressure, endothe lin-1 (ET-1), and phenylephrine (PE), representing mechanical, hormonal, an d alpha(1)-adrenergic stimuli, respectively. To gain further insight into t he secretory process under basal and agonist-induced secretion, we employed agents known to inhibit protein synthesis (cycloheximide) or to interfere with the vectorial transport of protein targeted for secretion (brefeldin A and monensin). All these agents induced significant changes in ANF and BNP release. Cycloheximide decreased natriuretic peptide secretion under basal and stimulated conditions. Brefeldin A dramatically increased basal as wel l as stimulated secretion of ANF and BNP. Monensin partially decreased basa l ANF and BNP secretion and completely blocked stimulated secretion. None o f these agents modified proteolytic processing as assessed by reverse-phase HPLC analysis. Double-label pulse-chase experiments using [H-3]- and [C-14 ]leucine demonstrated that the secretory response to ET-1, in contrast to t he response to muscle stretch, is based on peptide other than newly synthes ized or relatively newly stored ANF. It is concluded that, in adult atrial cardiocytes, ANF and BNP are sorted to constitutive and regulated pathways in a manner that is substantially unique for atrial cardiocytes. In particu lar, it appears that basal and stimulated ANF and BNP secretion may have a large "constitutive-like" component, as previously defined in other endocri ne systems. This type of secretion is based on the preferential release of hormone through vesicles arising from immature secretory granules. The capa city of the atria to release ANF and BNP in response to stimuli, therefore, may depend more on stimulation of the rate of formation of immature granul es than on the amount of stored hormone.