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

I s cells that have specialized regulated secretory vesicles, called secretory granules, sorting of the regulated secretory products away from constitutively secreted products and from lysosomal enzymes occurs in the trans-Golgi network (Griffiths and Simons, 1986; Tooze et al., 1987; Orci et al., 1987) . The concentrated contents of the secretory granules are released from cells in response to external signals, whereas no signals are required for constitutive secretion. The mechanism of protein sorting between these two secretory pathways is not known; both a hormonespecific receptor mechanism (Chung et al., 1989) and selective condensation of secretory granule contents (Kelly, 1985; Gerdes et al., 1989) have been proposed.