Chlorpromazine inhibition of muscarinic-cholinergic responses in the rat parotid gland

Neurotransmitter Control of Secretion

It is very well established that the principal control of salivary secretion is derived from autonomic innervation. Transmission of a neural signal to a salivary gland acinar cell occurs chemically via neurotransmitters, the first messengers of a secretory response. Neurotransmitters bind to specific cell surface receptor proteins, an event which activates precise transduction mechanisms which then transfer the neural signal to the inside of the cell. There are two major transduction mechanisms operative in salivary gland acinar cells. One involves the generation of cAMP, the other involves the breakdown of plasma membrane polyphosphoinositides. For both mechanisms, the appropriate stimulated receptor activates a second plasma membrane protein, termed an N (or G) protein. The N protein requires GTP to activate an enzyme (adenylate cyclase or phospholipase C), which then catalyzes the formation of a second messenger (cAMP and inositol trisphosphate/diacylglycerol, respectively). This action provides the intracellular signal for secretory events (protein, fluid, electrolyte secretion) to begin.

Beta-adrenergic-induced cytosolic redistribution of Rap1 in rat parotid acini: role in secretion

Rap1 has recently been identified on the secretory granule membrane and plasma membrane of rat parotid acinar cells (N. J. D'Silva, D. DiJulio, C. B. Belton, K. L. Jacobson, and E. L. Watson. J. Histochem. Cytochem. 45: 965-973, 1997). In the present study, we examined the cellular redistribution of Rap1 following treatment of acini with isoproterenol (ISO), the beta-adrenergic agonist, and determined the relationship between translocation and amylase release. In the presence of ISO, Rap1 translocated to the cytosol in a concentration- and time-dependent manner; this effect was not mimicked by the muscarinic agonist, carbachol. Translocation was maximal at 1 microM ISO and paralleled amylase release immediately after ISO stimulation. Rap1 translocation and amylase release were blocked by the beta-adrenergic antagonist, propranolol, whereas okadaic acid, a downstream secretory inhibitor, significantly blocked amylase release but did not inhibit Rap1 redistribution. Results suggest that the translocation of Rap1 is causally related to secretion and that the role of Rap1 in secretion is at a site proximal to the exocytotic event.

Altered responses to agonists after chronic in vivo atropine administration in rat parotid acini

Salivary gland hypofunction, resulting from a variety of perturbations including prescribed medications, is associated with adverse effects on the health of the oral cavity. In the present study, we investigated the in vivo effects of chronic administration of atropine, a muscarinic antagonist, on the acute response of rat parotid acini to alpha-adrenergic and muscarinic stimulation. The regulation of intracellular pH (pHi) and cytosolic free Ca2+ ([Ca2+]i) were monitored using dual wavelength microfluorometry of the ion-sensitive fluorescent dyes, BCECF and fura-2, respectively. Chronic atropine treatment (40 mg/kg/d for 4 weeks) significantly increased the magnitude of the initial (< 30 s) agonist-induced rise in [Ca2+]i, but did not alter the sustained increase in [Ca2+]i (> 2 min). The generation of inositol trisphosphates and inositol tetrakisphosphates after 30 s of muscarinic stimulation was not significantly altered. The resting Cl- content as well as the stimulated Cl- loss, were reduced in parotid acini after chronic atropine administration. In addition, the muscarinic- and alpha-adrenergic-induced intracellular acidification was blunted, suggesting that reduced HCO3- efflux occurs in acini isolated from atropine-treated animals. Our results indicate (1) that chronic atropine treatment does not inhibit the receptor-coupled generation of inositol phosphates or the resulting rise in [Ca2+]i and (2) chronic treatment may prevent the production of saliva either by reducing the driving force for anion-dependent fluid secretion or by preventing the activation of the anion efflux pathway.

Interaction between the calcium and cyclic AMP messenger systems in perifused rat parotid acinar cells

Potentiation of amylase secretion induced by a combination of isoproterenol and carbamylcholine was examined in perifused rat parotid acinar cells. The time course of changes in the augmented amylase secretion induced by isoproterenol plus carbamylcholine was similar to that induced by carbamylcholine alone, but not to that caused by isoproterenol. Concentration-response analysis showed that isoproterenol increased the apparent affinity for carbamylcholine to stimulate amylase secretion with the maximum effect attained by isoproterenol plus carbamylcholine being higher than that attained by isoproterenol or carbamylcholine. 8-Bromo cyclic AMP, forskolin and 3-isobutyl-1-methylxanthine mimicked the effect of isoproterenol. Calcium ionophores (A23187 and ionomycin), but not phorbol 12,13-dibutyrate, mimicked the effect of carbamylcholine. Chelation of intracellular free calcium with 1,2-bis-[2-aminophenoxyl]-ethane-N,N,N',N'-tetraacetic acid, but not that of extracellular calcium with [ethylenebis(oxyethylenenitrile)]-tetraacetic acid (EGTA), abolished the potentiation. Calmodulin antagonists inhibited amylase secretion induced by isoproterenol plus carbamylcholine or carbamylcholine alone, but not that induced by isoproterenol alone. These results suggest that the potentiation is mainly, if not completely, caused by a coordinated interaction between the cyclic AMP system and the Ca2+ system at a step distal to second messenger generation, probably via a cyclic AMP-induced increase in the sensitivity of the Ca2+ response element to calcium.

In vitro methods for the assessment of the inhibitory effects of antidepressants in rat parotid glands

The present study evaluates suitable in vitro methods for the assessment of the inhibiting properties of four principally different antidepressant drugs. This was done by comparing the acute effects of antidepressants on autonomic receptor binding (homogenates) together with parallel tests evaluating the biological activities of the receptor systems in collagenase-isolated rat parotid acini. The responses were measured as receptor-activated changes in cyclic nucleotide formation and acinar oxygen consumption. Muscarinic acetylcholine receptor binding, carbachol-induced cGMP formation, and oxygen consumption all reflected the various inhibiting effects of the antidepressants tested. Measurements of the carbachol-induced O2 consumption was however, the most sensitive method and may be considered a well-suited and reliable parameter concerning the expected severity of anticholinergic side-effects caused by medication. The disturbing 'dry mouth' symptoms following treatment with amitriptyline or mianserin are however, also attributed to their substantial adrenoceptor-blocking effects, which are best demonstrated by alpha 1-adrenoceptor binding studies in combination with measurements of the adrenaline-induced O2 consumption in the rat parotid gland.

High affinity quinuclidinyl benzilate binding to rat parotid membranes requires muscarinic receptor. G protein interactions

The binding of the non-selective muscarinic antagonist [3H]quinuclidinyl benzilate (QNB) to rat parotid membranes was characterized. Under equilibrium conditions, [3H]QNB bound to a homogenous population of muscarinic receptors (Kd, 118 +/- 19 pM; Bmax, 572 +/- 42 fmol/mg membrane protein, n = 12). The addition of G protein activators AlF4- or guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) + Mg2+ increased the Kd by 77 +/- 7% (n = 4, P less than 0.05) and 83 +/- 27% (n = 7, P less than 0.05), respectively, without a change in the Bmax or homogeneity of the binding site. GTP gamma S added without exogenous Mg2+ did not affect [3H]QNB binding. Thus, optimal QNB binding requires a muscarinic receptor/G protein interaction.

Anticholinergic effects of cis-chlorprothixene characterized in rat parotid acini

The anticholinergic effects of the antipsychotic drug, cis-chlorprothixene, on the secretory events underlying the formation of primary saliva were investigated. The neuroleptic, cis-chlorprothixene, is used extensively as a major tranquillizer but shares side-effects such as xerostomia with most antidepressants. The inhibitory effects of cis-chlorprothixene upon the cholinergic-induced rise in Ca2+ as well as on O2 consumption and Cl- loss were investigated in isolated rat parotid acini in order to characterize its anticholinergic effects quantitatively. The cholinergic-induced rise in cytosolic, free Ca2+ was inhibited by cis-chlorprothixene with half-maximal effect at 1.9 microM and maximal inhibition at 10 microM. When the cytosolic, free Ca2+ was enhanced in the presence of 10 microM cis-chlorprothixene by means of the Ca2+ ionophore A23187, a loss of Cl- was observed similar to that observed during cholinergic stimulation in the absence of cis-chlorprothixene. The findings are consistent with the possibility that cis-chlorprothixene exerts its effects on the steps leading from agonist binding to the acetylcholine receptor and to the increase of cytosolic free Ca2+. Thus, measurement of the stimulation-induced rise in cytosolic, free Ca2+ in the presence of neuroleptics such as the thioxanthenes represents a fast and reliable method for detecting inhibitory effects on autonomic receptor activation.

Salivary gland fluid secretion during aging

Salivary gland function is generally well-preserved in healthy older persons. Similar results are observed in the laboratory rat. Older people are, however, more likely to experience salivary disorders due to disease or its treatment. For many patients with remaining salivary gland parenchymal tissue, improved function may result from pharmacological therapy.

Functional characterization of muscarinic receptors in rat parotid acini

The muscarinic agonist, carbamylcholine, stimulated amylase secretion in rat parotid acini 6-fold, the 86Rb efflux 5-fold, the 45Ca efflux 5-fold and the accumulation of inositol monophosphate, bisphosphate, trisphosphate and tetrakisphosphate 4-, 4-, 3- and 3-fold, respectively. The EC50 of carbamylcholine on these parameters were 0.4, 0.5, 1.3, 12, 12, 6 and 9 microM, suggesting spareness between phospholipase C activation and amylase secretion. These muscarinic responses were inhibited by four muscarinic antagonists with an order of potency on all parameters and on receptor occupancy (using N-[methyl-3H]scopolamine as a tracer): atropine greater than hexahydrosiladifenidol greater than pirenzepine greater than AF-DX 116. The pA2 of these antagonists on carbamylcholine-stimulated amylase secretion were 9.72 for atropine, 8.14 for hexahydrosiladifenidol, 7.16 for pirenzepine and 6.22 for AF-DX 116, indicating that the parotid muscarinic receptors were of an M2 subtype 83-fold more sensitive to hexahydrosiladifenidol than to AF-DX 116.

Neurotransmitter control of secretion

It is very well established that the principal control of salivary secretion is derived from autonomic innervation. Transmission of a neural signal to a salivary gland acinar cell occurs chemically via neurotransmitters, the first messengers of a secretory response. Neurotransmitters bind to specific cell surface receptor proteins, an event which activates precise transduction mechanisms which then transfer the neural signal to the inside of the cell. There are two major transduction mechanisms operative in salivary gland acinar cells. One involves the generation of cAMP, the other involves the breakdown of plasma membrane polyphosphoinositides. For both mechanisms, the appropriate stimulated receptor activates a second plasma membrane protein, termed an N (or G) protein. The N protein requires GTP to activate an enzyme (adenylate cyclase or phospholipase C), which then catalyzes the formation of a second messenger (cAMP and inositol trisphosphate/diacylglycerol, respectively). This action provides the intracellular signal for secretory events (protein, fluid, electrolyte secretion) to begin.