Abstract
Transport of polymeric immunoglobulin A (plgA) in rat salivary glands has been investigated by combined morphological and biochemical techniques in vivo and in vitro. The distribution of IgA and its cellular receptor secretory component (SC) was observed by immunoperoxidase staining of cryosections from parotid and submaxillary gland, showing serous acinar cells are the site of IgA transport into saliva. Binding of horse radish peroxidase specific IgA to parotid serous acinar cells in vitro, observed by electron microscopy, shows that only the basolateral domain of acinar cells possesses exposed SC. A combination of new cell fractionation methods and standard western blotting techniques shows that SC present on basolateral plasma membrane of parotid acinar cells has a molecular weight (mwt) >100,000 and shows a high affinity for plgA in vitro. The existence of a 73,000 mwt SC occurring with plgA in cellular fractions of parotid gland suggest cleavage of SC occurs prior to secretion. The kinetics of plgA trancytosis was studied using isolated parotid acini. Bound plgA was secreted into the incubation medium as slgA, within thirty minutes of incubation at 37°C. Secretion of plgA was initially rapid but slowed over a 2hr period of incubation at 37°C. In addition to facilitating plgA transport serous acinar cells also synthesise and secrete a diverse range of other salivary proteins which are packaged into secretion granules and secreted directly through the apical plasma membrane. It is improbable that one complex secretory pathway facilitates both bulk secretion of salivary protein and transport of plgA. Therefore secreted proteins must be selectively segregated during secretion into saliva. Secretion of proteins from acinar cells in vitro shows proteins are released at two distinct rates.