Saliva contains various components, including mucins, which function to protect the oral mucosa and the tooth surface. Mucins are synthesized and secreted in large part by mucous glands (e.g., sublingual glands and labial glands) which are composed primarily of mucous acinar cells. Study of the regulation of mucous cell secretion of mucins has been hindered by both the small size of mucous glands and by their small volume of highly viscous secretions. Using a new in vitro model system consisting of enzymatically dispersed acinar structures from rat sublingual glands, we found that mucin secretion is primarily under muscarinic cholinergic control but is also stimulated (to a lesser extent) by vasoactive intestinal peptide (VIP), a neurotransmitter co- localized with acetylcholine. We also found evidence for cross-talk between these two stimulatory pathways. At low concentrations of muscarinic agonist, mucin secretion is potentiated; whereas at high concentrations of agonist, both the VIP-stimulated component of secretion and potentiated secretion are inhibited. Inhibition of VIP-induced secretion is correlated with attenuation of VIP-stimulated cellular cAMP levels. Evidence from radioligand binding studies and immunoprecipitation experiments suggests sublingual glands contain primarily equal portions of the m1 and m3 muscarinic receptor subtypes. Questions raised by these preliminary studies (which we plan to address using our in vitro model system) include: What are the specific muscarinic receptor subtypes associated with sublingual acinar structures and what is their cellular distribution? Is a single muscarinic receptor subtype responsible for both the stimulation of mucin secretion and inhibition of VIP-induced cAMP accumulation, or do both receptor subtypes act in concert to regulate each function? Results from these studies will provide new insights into the muscarinic cholinergic regulation of mucous cell secretion and may assist in the development of more specific pharmacological agents to activate mucous gland secretion in patients suffering from hyposalivary function and also in the design of therapeutic agents without hyposalivary side effects.
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