Expression and functional analysis of β-adrenoceptor subtypes in rabbit submandibular gland

Abstract

β-Adrenoceptors (β-ARs) mediate important physiological functions in salivary glands. Here we investigated the expression and function of β-AR subtypes in rabbit submandibular gland (SMG). Both β 1- and β 2-ARs, but not β 3-AR, were strongly expressed in rabbit SMG. β 1-AR proteins were widely expressed in acinar and ductal cells whereas β 2-AR proteins were mainly detected in ductal cells. A [ 3H]-dihydroalprenolol binding assay revealed that β-AR B max was 186 ± 11.9 fmol/mg protein and K d was 2.71 ± 0.23 nM. A competitive binding assay with CGP 20712A, a β 1-AR antagonist, indicated that the proportion of β 1-AR and β 2-AR was 71.9% and 28.1%, respectively. Gland perfusion with the β-AR agonist isoproterenol induced a significant increase in saliva secretion which was abolished by pretreatment with the non-selective β-AR antagonist propranolol. Pretreatment with β 1- or β 2-AR selective antagonists, CGP 20712A or ICI 118551, diminished isoproterenol-induced increase in saliva secretion by 71.2% and 28.8%, respectively. The expression of α-amylase mRNA was significantly stimulated by isoproterenol, which was eliminated by propranolol and CGP 20712A. Perfusion with isoproterenol decreased α-amylase protein storage in SMG and increased α-amylase activity in saliva. These alterations became less significant after pretreatment with propranolol and CGP 20712A. Our results suggest that both β 1- and β 2-ARs are expressed in rabbit SMG. β 1-AR is the predominant subtype and may play an important role in regulating saliva and α-amylase secretion.