Regulatory subunits of cyclic AMP-dependent protein kinase: presence in granules and secretion by exocrine and endocrine cells

Cyclic AMP-dependent protein kinase (cAPK) is the intracellular mediator of signal transduction events involving the adenylate cyclase-cyclic AMP system. A monoclonal antibody (MAb BB1) to the type II regulatory subunit (RII) of cAPK was used in a post-embedding immunogold-labeling procedure to determine the ultrastructural localization of RII in several different secretory cells of the rat. Label was present in nuclei, especially over the heterochromatin, and in the cytoplasm, particularly in areas containing rough endoplasmic reticulum. Immunolabeled RII was also present in secretory granules of the parotid gland, exocrine and endocrine pancreas, seminal vesicle, anterior and intermediate pituitary, and intestinal endocrine cells. Photoaffinity labeling of parotid saliva, pancreatic and seminal fluids with the cyclic AMP analogue, 32P-labeled-8-azido-cyclic AMP, revealed the presence of cyclic AMP-binding proteins with electrophoretic mobilities similar to those of authentic cAPK regulatory subunits. These results confirm our previous observations on the localization of cAPK regulatory subunits in the rat parotid using polyclonal antibodies, and extend them to a number of other exocrine and endocrine cells. The apparent widespread occurrence of cAPK subunits in secretory granules and secretory fluids suggests that cAPK may be involved in specific intragranular regulatory and/or phosphorylation events, or that it has an unidentified extracellular function.