An alternative pathway for secretion of lipoprotein particles in rat liver.

Vesicles containing single lipoprotein particles were observed in the perisinusoidal cytoplasm of rat hepatocytes in the proximity of particle-containing rough-smooth endoplasmic reticulum transition elements. Single particle vesicles were observed also in stages interpreted as fusion with the plasma membrane. The putative smooth endoplasmic reticulum-derived vesicles were clearly different from the multiparticle secretory vesicles derived from the Golgi apparatus. The findings suggest the operation of two pathways of lipoprotein secretion in rat liver. One follows the orthodox smooth endoplasmic reticulum Golgi apparatus secretory vesicle plasma membrane export route. The second seems to proceed via smooth endoplasmic reticulum secretory vesicle plasma membrane in a manner that bypasses the Golgi apparatus.