Muscarinic, α₁ and peptidergic agonists stimulate phosphoinositide hydrolysis and regulate mucin secretion in rat submandibular gland cells

Abstract

Three classes of agonists, associated with Ca²⁺-mobilization — α₁-adrenergic (methoxamine), muscarinic (carbachol) and peptidergic (substance P, SP) — significantly stimulated the secretion of mucin from enzymatically-dispersed rat submandibular gland acinar cells. The same three secretagogues also caused the hydrolysis of membrane inositol phospholipids, resulting in elevated cellular levels of inositol phosphates, particularly inositol 1,4,5-trisphosphate (IP₃). Exogenous IP₃ elicited the dose-dependent release of mucin in dispersed cells suggesting that agonist-generated endogenous IP₃ may provoke a secretory response. IP₃ and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) in combination, stimulated an additive secretion of mucin in the model. The potential use of these two agents as specific probes of the IP₃- and diacylglycerol-associated legs of the polyphosphoinositide (PPI) breakdown pathway is indicated. Although all three agonists shared a common action in stimulating PPI hydrolysis, their effects on the β-adrenergic mucosecretory response were inconsistent. A brief preincubation of cells with carbachol or SP significantly reduced the subsequent isoproterenol (IPR)-provoked secretion of mucin, whereas methoxamine plus IPR stimulated an additive response. The mechanisms underlying these opposite effects are not known. Failure of IP₃ or TPA to modify IPR responses suggests that modulation of the β response may operate at a locus before the generation of diacylglycerol and IP₃, possibily at the level of signal transduction. The study indicates a role for Ca²⁺-mobilizing, agonists in controlling submandibular mucin secretion and provides evidence that receptor-linked phosphoinositide hydrolysis is an early stage in their stimulus-secretion coupling mechanism.