Studies on Secretory Glycoproteins in the Rat Exocrine Pancreas. I. Fine Structure of the Golgi Complex and Release of Fucose-labeled Proteins After in Vivo Stimulation with Caerulein

Using a double-label technique on isolated rat pancreatic lobules, the rate of synthesis and discharge of regular and fucosylated secretory proteins was studied under control conditions and after in vivo prestimulation with caerulein. Both labeled leucine and fucose were incorporated into pancreatic proteins at a linear rate, which was potentiated by in vivo stimulation. In pulse-chase experiments both regular and fucosylated secretory proteins were discharged into the medium in parallel. The in vivo pretreatment with caerulein caused an earlier discharge and increased the total amount released. Kinetic analysis of unstimulated (baseline) discharge of both classes of secretory proteins indicated in vitro sensitivity by the previous in vivo treatment with caerulein. The biochemical data were compared to the fine structure of the Golgi complex under both control and prestimulated ocnditions. The Golgi stacks were composed of four to six individual cisternae which in some cases were connected by intercisternal pores. Transporting vesicles were observed fusing along the total length of the outermost cisterna on both the cis- and trans-side and with the lateral ends of the intermediate cisternae. Under control conditions only the last trans-cisterna contained some electron opaque material; in vivo prestimulation led to distension and filling of all disternae in an individual Golgi-unit. Numerous stages of transformation of the last transcisterna into condensing vacuoles were observed, lending support to the hypothesis that during packaging of secretory products the membranes of the Golgi complex undergo a continuous turnover.