Permeability of submandibular salivary glands in dogs to blood-borne horseradish peroxidase (HRP). I. Perarterial bolus injection in "resting" glands.

Horseradish peroxidase (HRP) was administered to the submandibular glands of dogs by close-arterial bolus-type injections, and its localisation was examined histochemically by light and electron microscopy. The HRP became widespread in the interstices of the glands and reached many central acinar lumina via scattered localised parts of their "tight" junctional complexes. Reaction product was less often found in the lumina of demilunes, which suggested that the intercellular junctions there were less "leaky". HRP was often found in sizeable spaces between myoepithelial cells and the underlying parenchymal cells; such large spaces have not been observed in this situation in other species. The possibility that permeability pathways may arise intermittently at different sites in the adhering mechanisms between the acinar cells is discussed. It is concluded that potential paracellular permeability pathways for macromolecules exist in these glands and, if the concentration gradient is sufficiently high, molecules even as large as those of HRP can to some extent permeate passively from the interstices to the saliva. In "resting" glands the principal permeability site is between the central acinar cells.