Functional β2-Adrenergic Receptors Are Present in Nonstrial Tissues of the Lateral Wall in the Gerbil Cochlea

Publisher Website
Google Scholar
Abstract
Recently, we have demonstrated that functional β1-adrenergic receptors are the dominant β-adrenergic receptor subtype in the stria vascularis and that β1-adrenergic receptors stimulate K+ secretion in strial marginal cells. The goal of the present study was to determine whether nonstrial tissues in the cochlear lateral wall contain β-adrenergic receptors and if so which subtype is present. Pharmacological tools were used to identify receptors in functional studies where cAMP production was measured. Further, receptors were identified as transcripts by cloning and sequencing of reverse-transcriptase polymerase chain reaction (RT-PCR) products. Experiments were performed on gerbil nonstrial lateral wall tissues. Tissues stimulated with 10–5M isoproterenol produced 0.42 ± 0.22 pmol cAMP per ear within 12 min (n = 14). The selective β-adrenergic receptor agonist isoproterenol stimulated cAMP production with an EC50 of (2 ± 3) × 10–7M (n = 7). Isoproterenol-stimulated cAMP production was inhibited by the β2-adrenergic receptor antagonist ICI 118551 with an IC50 of (7 ± 7) × 10–6M, which corresponds to an affinity constant of 1 × 10–7M (pKDB = 6.89 ± 0.23, n = 3). Isoproterenol-stimulated cAMP production was not inhibited by the highly selective β1-adrenergic receptor antagonist CGP 20712A. The IC50 and the affinity constant for CGP 20712A were estimated to be >3 × 10–4 and >6 × 10–6M, respectively. RT-PCR of total RNA isolated from nonstrial lateral wall tissues using primers specific for β1-, β2- and β3-adrenergic receptors revealed products of the predicted sizes for the β1- and β2- but not for the β3-subtype. Sequence analysis confirmed that amplified cDNA fragments encoded gene-specific nucleotide sequences. These results demonstrate that nonstrial lateral wall tissues contain transcripts for β1- and β2- but not for β3-adrenergic receptors and that the β2-adrenergic receptor is the dominant functional receptor subtype. The cellular localization and function of the β2-adrenergic receptors remains to be determined.