Constitutive and basal secretion from the endocrine cell line, AtT-20

The mouse pituitary cell line, AtT-20, expresses the hormone, ACTH, and packages it in electron-dense secretory granules (Mains and Eipper, 1976) . In addition to regulated secretion of ACTH, AtT-20 cells secrete proteins such as laminin from a constitutive secretory pathway (Burgess et al., 1985) . In this paper we have characterized a variant of AtT-20 cells that does not express several soluble markers of the dense core secretory vesicle, including ACTH, and does not have morphologically identifiable dense core secretory granules. We are now able to experimentally distinguish in endocrine cells, two types of unstimulated secretion, basal release and constitutive secretion. In earlier estimates of the efficiency of sorting in AtT-20 cells we ignored basal secretion when measuring sorting into the regulated pathway (Moore and Kelly, 1985) . The secretion properties of the variant cell line indicated that a considerable fraction of the protein sorted into the regulated pathway was secreted in the absence of stimulation by what we term basal secretion.