BREFELDIN-A DEFINES DISTINCT PATHWAYS FOR ATRIAL-NATRIURETIC-FACTOR SECRETION IN NEONATAL RAT ATRIAL AND VENTRICULAR MYOCYTES

The intracellular pathways for basal atrial natriuretic factor (ANF) s ecretion from the heart and their correlation with ANF processing to t he active form were characterized in cultured neonatal rat atrial and ventricular myocytes. Brefeldin A, a fungal antimetabolite that blocks transport of newly synthesized proteins from the endoplasmic reticulu m, was used to inhibit nascent protein trafficking. Thus, release of n ewly synthesized hormone was blocked, but release of stored hormone wa s unaffected. Whereas brefeldin A inhibited basal ventricular ANF rele ase to 10% of the control value, basal ANF release from atrial cells w as enhanced. Furthermore, basal atrial ANF secretion was inhibited by agents preventing myocyte depolarization, Ca2+ influx, release of Ca2 from intracellular stores, or activation of protein kinase C, whereas ventricular ANF secretion was unaffected by these agents. Brefeldin A did not alter maturational processing of pro-ANF to ANF-(99-126) in e ither atrial or ventricular cultures. These findings indicate that (1) basal secretion of ANF from ventricular cells relies largely on newly synthesized hormone and is probably constitutive, (2) basal secretion of ANF from atrial cells is independent of transport of newly synthes ized protein and occurs via a regulated pathway controlled at least in part by signaling changes associated with myocyte beating, and (3) pr ocessing of pro-ANF occurs either with constitutive or regulated secre tion of hormone, which may indicate multiple cellular locations for th e processing enzyme.