Expression and postnatal changes of adrenergic receptor subtype mRNA in rat submandibular glands

Adrenergic receptors (ARs) are involved in regulating saliva secretion and composition in salivary glands. Nine AR subtypes, including three α1-ARs (α1a-, α1b- and α1d-ARs), three α2-ARs (α2A-, α2B- and α2C-ARs) and three β-ARs (β1,β2- and β3-ARs), have been identified through molecular cloning. The five subtype genes, α1a-, α1b-, α2A-, β1-, and β2-ARs, were expressed in rat submandibular glands. In contrast, the other four subtype mRNAs, α1d-, α2B-, α2C- and β3-ARs, were not detected by reverse transcription-polymerase chain reaction (RT-PCR). The steady-state mRNA expression for the five AR subtypes in rat submandibular glands was measured by quantitative competitive RT-PCR using synthetic DNA as internal standard at different stages of postnatal development. The relative rank order of AR subtype mRNA expression was α1a>β2>β1>α2A>α1b at all stages except that β1- and α2A-subtypes were reversed at 2 weeks of age. The gene expression of α1a-AR subtype relative to total AR was low at 2 weeks of age and increased and reached a maximum at 6 weeks of age, whereas those patterns of α2A-, β1- and β2-AR subtypes were similar to each other and their gene expressions were high at 2 weeks of age and then decreased. On the other hand, the gene expression of α1b-AR subtype did not change over the different stages in relation to that of a housekeeping gene, glyceraldehyde 3-phosphate dehydrogenase, and to total AR. Although rat submandibular glands contain the five AR subtype mRNAs, distinct subtype-specific expression is evident.