K+ secretion in strial marginal cells is stimulated via beta 1-adrenergic receptors but not via beta 2-adrenergic or vasopressin receptors.

Abstract

Pharmacologic tools were used to identify receptors in functional studies by measuring either transepithelial current (I(sc)) in strial marginal cells (SMC) or cAMP production in stria vascularis (SV). Further, receptors were identified in SV as transcripts by cloning and sequencing of reverse-transcriptase polymerase chain reaction (RT-PCR) products. Experiments were performed using tissues isolated from gerbils unless specified otherwise. I(sc) under control conditions was 1090 +/- 21 microA/cm(2) (n = 213) in gerbil SMC and 2001 +/- 95 microA/cm(2) (n = 6) in murine SMC. Direct stimulation of adenylate cyclase with 10(-5) m forskolin but not with 10(-5) m 1,9-dideoxy-forskolin resulted in an increase in the I(sc) by a factor of 1.14 +/- 0.01 (n = 6). The vasopressin-receptor agonist 10(-8) m Arg(8)-vasopressin had no significant effect on I(sc) in gerbil and murine SMC. The beta-adrenergic agonists isoproterenol, norepinephrine and epinephrine stimulated I(sc) with an EC(50) of (6 +/- 2) x 10(-7) m (n = 28), (3 +/- 1) x 10(-6) m (n = 40) and (7 +/- 2) x 10(-6) m (n = 38), respectively. Isoproterenol stimulated cAMP production in SV with an EC(50) of (5 +/- 2) x 10(-7) m (n = 8). The beta-antagonist 10(-4) m propanolol completely inhibited 2 x 10(-5) m isoproterenol-induced stimulation of I(sc). The beta-antagonists atenolol, ICI118551 and CGP20712A inhibited isoproterenol-induced stimulation of I(sc) with a K(DB) of 1 x 10(-7) m (pK(DB) = 6.96 +/- 0.15, n = 14), 1 x 10(-7) m (pK(DB) = 7. 01 +/- 0.14, n = 15), 2 x 10(-9) m (pK(DB) = 8.73 +/- 0.13, n = 19), respectively. CGP20712A inhibited isoproterenol-induced cAMP production with a K(DB) of 1 x 10(-10) m (pK(DB) = 9.94 +/- 0.55, n = 9). RT-PCR of total RNA isolated from SV using primers specific for the beta(1)-, beta(2)- and beta(3)-adrenergic receptors revealed products of the predicted sizes for the beta(1)- and beta(2)- but not the beta(3)-adrenergic receptor. Sequence analysis confirmed that amplified cDNA fragments encoded gene-specific nucleotide sequences. These results demonstrate that K(+) secretion in SMC is under the control of beta(1)-adrenergic receptors but not beta(2)-adrenergic or vasopressin-receptors and that the beta(1)-subtype is the primary beta-adrenergic receptor in SV although SV contains transcripts for both beta(1)- and beta(2)-adrenergic receptors.