Apocrine secretion of amylase and exocytosis of trypsin along the midgut of Tenebrio molitor larvae

Amylase and trypsin were purified from Tenebrio molitor midgut larvae and u sed to raise antibodies in a rabbit. A Western blot of T. molitor midgut ho mogenates, after sodium dodecyl sulfate-polyacrylamide gel electrophoresis using amylase and trypsin antisera, showed only bands co- migrating with th e purified enzymes. The antisera were used to localize the enzymes by immun ofluorescence and immunogold labeling. Amylase occurs in a few regularly di sposed anterior midgut cells. Non-amylase-secreting anterior midgut cells a re proposed to be water-absorbing cells based on morphology and dye movemen ts. Amylase is found inside vesicles originating from Golgi areas that seem to fuse together before their release along with the now disorganized apic al cytoplasm (apocrine secretion). Trypsin precursors are observed inside s mall vesicles near the apical plasma membrane of posterior midgut cells, su ggesting an exocytic mechanism of secretion, followed by putative trypsin a ctivation. Apocrine secretion is thought to be an adaptation to enhance the dispersion of secretory vesicle contents released from a water-absorbing e pithelium, whereas exocytosis is an efficient secretory mechanism in a wate r-secreting epithelium. (C) 2000 Elsevier Science Ltd. All rights reserved.