Pulse-chase metabolic labeling studies of recombinant multimerin biosynthesis by AtT 20 cells. Radioimmunoprecipitates from a 30-minute pulse-labeling study (A; chase times indicated) and a 28-hour continuous labeling study (B) were analyzed using 4% to 8% reduced SDS-PAGE. In (B), labeled cells were consecutively chased for 12 hours and 1 hour in cold media before 60 minutes of incubation (postchase) with (+) or without (−) the secretagogue 10 mmol/L 8-Br-cAMP. (A) Multimerin was first synthesized as promultimerin (proM, M_r160 kD), which migrated with a higher apparent molecular mass (proM*; A) after additional processing of its N-linked carbohydrate. Multimerin was constitutively secreted as proM* and smaller proteolyzed subunits. (B) Proteolyzed multimerin, with a M_r of 122 kD (reduced), persisted in the cell lysate after there was no further constitutive secretion of radiolabeled multimerin during the chase and postchase periods and it was secreted into the postchase medium only when the cells were incubated with secretagogue.