CD38/ADP-ribosyl cyclase in the rat sublingual gland: Subcellular localization under resting and saliva-secreting conditions

CD38 is a 42-45 kDa transmembrane glycoprotein that exhibits ADP-ribosyl cyclase enzyme activity. In the rat, we have previously reported strong ADP-ribosyl cyclase activity in the sublingual salivary gland (Masuda W. and Noguchi T. Biochem. Biophys. Res. Commun. (2000) 270, 469-472). Here, we have examined the specific localization of CD38/ADP-ribosyl cyclase activity in this gland and whether that localization changes upon saliva-secretary stimulation. Under resting conditions, CD38/ADP-ribosyl cyclase activity in the post-nuclear fraction of SLG homogenates was separated into two major peaks by sucrose density gradient centrifugation. The first peak included the plasma membrane proteins Na⁺/K⁺ ATPase and aquaporin 5, while the second peak included mucous secretory protein mucin and vesicle-associated membrane protein 2. When rats were subjected to the muscarinic agonist pilocarpine, the CD38/ADP-ribosyl cyclase activity disappeared from the second peak, as did mucin and vesicle-associated membrane protein 2. Pre-treatment of rats with the muscarinic antagonist atropine before pilocarpine administration, or adrenergic stimulation with isoproterenol, the sucrose density gradient separation profiles were same as that seen under resting condition. Using an immunofluorescent strategy, we observed the preferential localization of CD38 in the basolateral plasma membrane and intracellular granule-like membrane in sublingual acinar cells under resting conditions.