Effects of reserpine treatment on beta-adrenergic/adenylate cyclase modulate secretion and resynthesis by the rat submandibular gland

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.

Effects of thyroxine and dexamethasone on rat submandibular glands

Glucocorticoids and thyroxine are known to have a marked effect on the flow rate and protein composition of rat parotid saliva in hormonally intact animals. In the present study, the effects of a one-week treatment of male rats with dexamethasone and thyroxine were studied by electron microscopy and x-ray micro-analysis, and by measurement of the flow rate and determination of the chemical composition of pilocarpine-induced submandibular saliva. Thyroxine had the most extensive effects on the submandibular gland. The acinar cells were enlarged and filled with mucus; the cellular calcium concentration was significantly increased. The flow rate of the submandibular saliva was significantly reduced compared with that in saline-injected control animals. Thyroxine caused an increase in the concentrations of protein, total calcium, and potassium in the saliva. Dexamethasone had no significant effects on gland ultrastructure or on the elemental composition of the acinar cells; flow rate was not affected, but the concentrations of protein, calcium, and potassium were significantly increased. The effects of dexamethasone and thyroxine on the flow rate and protein composition of pilocarpine-induced rat submandibular saliva differ from those reported earlier for rat parotid saliva after simultaneous stimulation with pilocarpine and isoproterenol.

Adenylate cyclase and beta-receptors in salivary glands of rats fed diets containing trans fatty acids

The effects of trans fatty acids on adenylate cyclase were determined in the submandibular salivary glands (SMSG) of rats fed diets containing 20% corn oil, 20% partially hydrogenated soybean oil (PHSBO) or 18% PHSBO + 2% corn oil. The fluoride- and isoproterenol-stimulated adenylate cyclase activities were higher in the SMSG from rats fed 20% PHSBO than in the control group fed 20% corn oil. The feeding of 2% corn oil with the diet containing 18% PHSBO resulted in a complete restoration of isoproterenol-stimulated and a partial restoration of fluoride-stimulated adenylate cyclase activity. There was no significant difference in the concentration of the beta-adrenergic receptor or in the receptor-binding affinity constants among the three dietary groups as measured by [3H]dihydroalprenolol (DHA)-binding. Higher fluorescence polarization of diphenyl hexatriene (DPH) was observed in SMSG membranes of rats fed diet containing 20% PHSBO than in the other two oils, suggesting that membrane fluidity may play a role in adenylate cyclase activity.

Effects of chemical sympathectomy on postnatal development of the rat submandibular gland

Temporary chemical sympathectomy was accomplished by daily injection of the catecholamine-depleting agent, reserpine, for the first six days of life. The rat pups were then allowed to mature, and their submandibular glands (SMG) sequentially examined by light and electron microscopy. Reserpine retarded SMG development by about two weeks when compared to saline-treated litter-mates. One day after the end of reserpine treatment, glands appeared rudimentary, similar to those of newborns. With age the glands matured, and seven weeks after treatment (eight weeks of age), there were few structural differences between glands from experimental or control animals. In a separate experiment, 12 newborn rats were treated with reserpine for six days and then allowed to mature to 12 weeks. Three developed an obstructive sialadenitis. Several experiments were run to see if this lesion could be replicated. Only three of 36 more animals developed sialadenitis, so its induction was neither consistent nor reproducible.

Short-term inhibition of cellular autophagy by isoproterenol in the submandibular gland

The present study examines the short-term (i.e. 10 min after injection) influence of isoproterenol on cellular autophagy in the rat submandibular gland. The volume fraction of autophagic vacuoles was significantly reduced, suggesting that an anticatabolic reaction, namely inhibition of cellular autophagy, is an early and significant event in the growth of the submandibular gland which is known to occur with long-term isoproterenol treatment.

Heterogeneity of beta-adrenoreceptors in guinea pig alveolar type II cells

[3H]Dihydroalprenolol ([3H]DHA) binding to guinea pig alveolar type II cell membrane revealed the presence of both high (KD = 0.38 nM) and low (KD = 4.2 nM) affinity beta-adrenoreceptors. The low affinity site had a higher binding capacity (Bmax = 245.6 fmol/mg protein) than the high affinity site (Bmax = 71.7 fmol/mg protein). Displacement of [3H]DHA by practolol, a selective beta 1 agent, confirmed the existence of two species of adrenoreceptors, corresponding to 21% high affinity (beta 1) and 79% low affinity (beta 2), respectively.

The influence of the sympathetic nervous system on the development of beta-adrenergic receptors in the rat submandibular salivary gland

During the development of the rat submandibular gland (SMG) there is a clearly-defined sequence in the maturation of the beta-adrenergic receptor/adenylate cyclase-linked stimulus-secretion coupling system. The sympathetic nervous system does not become functionally linked to the exocrine process in the SMG until approximately six days after birth. The temporal correlation of the ingrowth of catecholamine-containing nerve processes, the appearance of beta-adrenergic receptors and the functional coupling of the stimulus-secretion system suggested the possibility of a cause and effect relationship between the appearance of the catecholamine-containing nerves in the gland and the maturational increase in the number of beta-adrenergic receptors. Chemical sympathectomy in neonates did not effect the time of appearance or the number of beta-adrenergic receptors seen in the developing gland. However, chronic isoproterenol treatment resulted in accelerated maturation of the gland with a concomitant premature appearance of the beta-receptors. These data suggest that the increase in the number of beta-adrenergic receptors which normally occurs in the developing gland at 5-6 days after birth is a specifically-programmed step closely associated with the degree of maturation attained by the cells and is independent from the ingrowth of catecholamine-containing nerves.

Adrenergic regulation of protein synthesis in rat submandibular salivary gland cells

Epinephrine, a mixed alpha- and beta-adrenergic agonist, modulated new protein synthesis in dispersed rat submandibular cells. Both adrenoreceptors separately could increase protein synthesis after cells were exposed to [14C]-leucine for 10 min and then with 1 mM unlabelled leucine for 10-30 min. The extent of response was dependent on the concentration of epinephrine to which cells were exposed. Alteration in the sequence of exposure of cells to tracer and agonist changed the direction of agonist response.