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

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 β1- and β2-ARs, but not β3-AR, were strongly expressed in rabbit SMG. β1-AR proteins were widely expressed in acinar and ductal cells whereas β2-AR proteins were mainly detected in ductal cells. A [3H]-dihydroalprenolol binding assay revealed that β-AR Bmax was 186 ± 11.9 fmol/mg protein and Kd was 2.71 ± 0.23 nM. A competitive binding assay with CGP 20712A, a β1-AR antagonist, indicated that the proportion of β1-AR and β2-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 β1- or β2-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 β1- and β2-ARs are expressed in rabbit SMG. β1-AR is the predominant subtype and may play an important role in regulating saliva and α-amylase secretion.