β 1 -Adrenergic Receptors but not β 2 -Adrenergic or Vasopressin Receptors Regulate K + Secretion in Vestibular Dark Cells of the Inner Ear

Abstract

Receptors were identified pharmacologically in functional studies where K+ secretion was monitored as transepithelial current (Isc). Further, receptors were identified as transcripts by cloning and sequencing of reverse-transcriptase polymerase chain reaction (RT-PCR) products. Isc under control conditions was 796 +/- 15 microA/cm2 (n = 329) in gerbilline VDC and 900 +/- 75 microA/cm2 (n = 6) in murine VDC. Forskolin (10(-5) m) but not 1, 9-dideoxy-forskolin increased Isc by a factor of 1.42 +/- 0.05 (n = 7). 10(-9) m Arg8-vasopressin and 10(-9) m desmopressin had no significant effect in gerbilline and murine VDC. Isoproterenol, norepinephrine, epinephrine and prenalterol stimulated Isc maximally by a factor of 1.38 +/- 0.04 (n = 7), 1.59 +/- 0.06 (n = 6), 1.64 +/- 0.03 (n = 8) and 1.37 +/- 0.03 (n = 6), respectively. The EC50 values were (1.4 +/- 0.7) x 10(-8) m (n = 36), (2.5 +/- 1.0) x 10(-8) m (n = 31), (1.7 +/- 0.7) x 10(-7) m (n = 36) and (5 +/- 4) x 10(-7) m (n = 32), respectively. Propanolol inhibited isoproterenol-induced stimulation of Isc. Atenolol, ICI118551 and CGP20712A inhibited isoproterenol-induced stimulation of Isc with a pKDB of 5.0 x 10(-8) m (pKDB = 7.30 +/- 0.07, n = 38), 4.4 x 10(-8) m (pKDB = 7.36 +/- 0.14, n = 37) and 6.8 x 10(-12) m (pKDB = 11.17 +/- 0.12, n = 37), respectively. RT-PCR of total RNA isolated from microdissected vestibular labyrinth tissue using specific primers revealed products of the predicted sizes for beta1- and beta2-adrenergic receptors but not for beta3-adrenergic receptors. Sequence analysis of the amplified cDNA fragments from gerbilline tissues revealed a 96.4%, 91.5% and 89.6% identity compared to rat beta1-, beta2- and beta3-adrenergic receptors, respectively. These results demonstrate that K+ secretion in VDC is under the control of beta1- but not beta2- or beta3-adrenergic receptors or vasopressin-receptors.

Adrenergic activation of electrogenic K+ secretion in guinea pig distal colonic epithelium: desensitization via the Y2-neuropeptide receptor

“…11, A and B). ICI-118551 produced a shift in epi EC 50 consistent with a K d of ϳ130 nM, which was 10-to 100-fold less efficacious than expected for antagonism at ␤2-AdrR (6,29,46). The relative shifts in epi EC 50 were similar whether BIIE0246 or PYY was present, indicating that antagonist sensitivity was not influenced by Y2-NpR activation with PYY.…”

Localization of β1-Adrenergic Receptors in the Cochlea and the Vestibular Labyrinth

“…In addition, positive and negative controls were used to verify specificity. Staining of stria vascularis and vestibular dark cells was taken as a positive control since it is consistent with the pharmacological demonstration of functional β 1 -AR in these tissues [23,24]. Further, the absence of staining in the semicircular canal epithelium, which contains functional β 2 -AR but not β 2 -AR [15], was taken as a negative control.…”

“…The existence of oligomeric SDS-resistant complexes that contain dimeric heptahelical receptors has only recently been established [1,6,18]. [23,24]. Endolymph is the luminal fluid of the inner ear, which contains ~150 mM K + and provides a reservoir of K + , which serves as the charge carrier for sensory transduction in cochlea and vestibular labyrinth.…”

“…Recent pharmacological studies have demonstrated β 1 -AR in the cochlea and the vestibular labyrinth. β 1 -AR occur in strial marginal cells of the cochlea and in dark cells of the vestibular labyrinth [23,24]. These epithelial cells are responsible for K + secretion and essential for K + cycling, which is a prerequisite for sensory transduction [22].…”

“…These epithelial cells are responsible for K + secretion and essential for K + cycling, which is a prerequisite for sensory transduction [22]. Activation of β 1 -AR has been shown in both epithelial cell types to increase the rate of K + secretion [23,24]. One may speculate that an elevated rate of K + secretion is necessary to enable the cochlea and the vestibular labyrinth to perform in the presence of an increased stimulatory load.…”