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.

The present study was designed to clarify the chronological developmental process of monovalent ions (Na⁺, K⁺, Cl^–) in the endolymph of the mouse in relation to the development of the endocochlear potential (EP). The EP and ionic concentrations were measured simultaneously with the ion-sensitive double-barreled microelectrodes from the scala media of the basal turn. The EP increased abruptly 7 days after birth (DAB) and reached approximately 80 mV 14 DAB. In the earliest postnatal days, the endolymphatic Na⁺ concentration was significantly higher than that in adult mice, however, the K⁺ and the Cl^– concentrations were lower. The concentrations of all the monovalent ions in endolymph reached adult levels at 7 DAB when the EP was still under 20 mV. These data strongly suggest the presence of a different mechanism between the production of monovalent ions, especially of high K⁺ in the endolymph and that of EP.

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K+ secretion in strial marginal cells (SMC) of stria vascularis (SV) is stimulated by beta1-adrenergic receptors. The aim of the present study was to determine, whether SMC from the gerbil inner ear contain muscarinic receptors that inhibit K+ secretion. Receptors were identified with pharmacological tools in functional studies where K+ secretion was monitored as transepithelial current (Isc). The cytosolic Ca2+ concentration ([Ca2+]i) was measured as fluo-4 fluorescence and cAMP production with a colorimetric immunoassay. Further, receptors were identified in SV as transcripts by cloning and sequencing of reverse-transcriptase polymerase chain reaction (RT-PCR) products. The cholinergic receptor agonist carbachol (CCh) caused a transient increase in [Ca2+]i with a half-maximal concentration value (EC50) of (5 +/- 6) x 10(-6) m (n = 29) and a decrease in basal and stimulated cAMP production. Apical CCh had no effect on Isc but basolateral CCh caused a transient increase in Isc with an EC50 of (3 +/- 1) x 10(-6) m and a sustained decrease of Isc with an EC50 of (1.2 +/- 0.2) x 10(-5) m (n = 129). The effects of CCh on Isc and [Ca2+]i were inhibited in the presence of muscarinic antagonist 10(-6) m atropine. Further, the muscarinic antagonists pirenzipine, methoctramine and para-fluoro-hexahydo-sila-defenidol (pFHHSiD) inhibited the CCh-induced transient increase of Isc with affinity constants (KDB) of 3 x 10(-8) m (pKDB = 7.54 +/- 0.19, n = 17), 2 x 10(-6) m (pKDB = 5.71 +/- 0.26, n = 19) and 2 x 10(-8) m (pKDB = 7.65 +/- 0.28, n = 19) and the sustained decrease of Isc with KDB of 7 x 10(-8) m (pKDB = 7.05 +/- 0.09, n = 33), 6 x 10(-6) m (pKDB = 5.21 +/- 0.13, n = 23), 5 x 10(-8) m (pKDB = 7.34 +/- 0.13, n = 31), respectively. RT-PCR of total RNA isolated from SV using primers specific for the M1-M5 muscarinic receptors revealed products of the predicted sizes for the M3- and M4- but not the M1-, M2- and M5-muscarinic receptor subtypes. Sequence analysis confirmed that amplified cDNA fragments encoded gene-specific nucleotide sequences. These results suggest that K+ secretion in SMC is under the control of M3- and M4-muscarinic receptors that may be located in the basolateral membrane of strial marginal cells.