Fig. 1. Discharge of the minor regulated pathway precedes granule exocytosis. (A) Coomassie staining profiles for parotid secretory proteins released at early time points after pulse-chase biosynthetic labeling with [³⁵S]amino acids upon stimulation of lobules by 10 µM isoproterenol. At each time point, the medium was removed and replaced with fresh medium containing freshly added stimulant. Amylase (Amy.), proline-rich protein (PRP), and parotid secretory protein (PSP) are identified. The composition of secretion elicited by 40 nM carbachol (CCh) and of the content of isolated secretory granules (Gr) is also shown. (B) Radiochemical composition of protein profiles illustrated in panel A. PRP contains no methionine or cysteine and is unlabeled. (C) Specific radioactivity of amylase for the samples shown in panels A and B obtained by normalizing the intensities of the amylase bands in B (quantified by phosphorimaging) to the amylase enzyme activity in each sample.