Atropine treatment induced cholinergic supersensitivity at receptor and second messenger levels in the rat salivary gland

Physiological, biochemical and morphological correlates of chronic treatment of rats with the classical muscarinic antagonist atropine for 14 days (20 mg kg-1 day-1 s.c.) were studied in submandibular salivary glands. The amount of saliva collected from submandibular glands following a single injection of isoproterenol (30 mg kg-1 i.p.) was significantly larger and had higher protein concentration in rats treated with atropine than in saline-treated animals. In the glands of atropine-treated rats a conspicuous increase in the amount of rough endoplasmic reticulum (RER) along with a decrease in the mucous volume was observed in the acinus when examined by light microscopy. Several biochemical changes were observed in an enriched plasma membrane fraction from the submandibular gland of the atropine-treated rats: (1) an increase in the number of muscarinic antagonist binding sites (31 + 3.4%), (2) a decrease in the specific activity of basal adenylate cyclase, (3) a significantly lower Vmax of the adenylate cyclase in the presence of GTP (10 microM) and varying concentrations of Mg2+ (0-22.5 mM) with no apparent change in affinity of the enzyme for Mg2+ but (4) higher magnitude of stimulation in the presence of GTP (100 microM), vasoactive intestinal polypeptide (5 microM), isoproterenol (100 microM), NaF (10 microM) and forskolin (10 microM). There was however no change in the density of beta-adrenergic receptors upon atropine treatment. In tissue slices from the submandibular glands of atropine-treated rats we found lower basal cAMP levels (decrease 29 +/- 6.9%) and no significant change in the phosphatidylinositol breakdown stimulated by carbachol (10(-6) to 10(-4) M). It appears that chronic blockade of an inhibitory muscarinic input to the adenylate cyclase system is compensated by lowered adenylate cyclase activity. Phosphoinositide metabolism is not subject to the same adaptation, suggesting that cAMP may be the pivotal second messenger in the supersensitive salivary response.