[Alpha]-2 And [Beta]-adrenergic receptors mediate NE's biphasic effects on rat thick ascending limb chloride flux

Plato, Craig F. [Alpha]-2 And [Beta]-adrenergic receptors mediate NE's biphasic effects on rat thick ascending limb chloride flux. Am J Physiol Regulatory Integrative Comp Physiol 281: R979-R986, 2001.--The sympathetic neurotransmitter norepinephrine (NE) influences renal sodium excretion via activation of adrenergic receptors. The thick ascending limb (THAL) possesses both [Alpha]-2 and [Beta]-adrenergic receptors. However, the role(s) different adrenergic receptors play in how isolated THALs respond to NE are unclear. We tested the hypothesis that both [Alpha]-2 and [Beta]-adrenergic receptors are responsive to NE in the isolated THAL, with [Alpha]-2 receptors inhibiting and [Beta]-receptors stimulating chloride flux ([J.sub.Cl]). THALs from male Sprague-Dawley rats were perfused in vitro, and the effects of 1) incremental NE, 2) the [Alpha]-2 agonist clonidine, and 3) the [Beta]-agonist isoproterenol on [J.sub.Cl] were measured. Low concentrations (0.1 nM) of NE decreased [J.sub.Cl] from a rate of 114.2 [+ or -] 8.1 to 93.5 [+ or -] 14.6 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1] (P [is less than] 0.05), with the nadir occurring at 1 nM (67.7 [+ or -] 8.8 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1]; P [is less than] 0.05). In contrast, greater concentrations of NE significantly increased [J.sub.Cl] from the nadir to a maximal rate of 131.0 [+ or -] 28.5 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1] at 10 [micro]M (P [is less than] 0.05). To evaluate the adrenergic receptors mediating these responses, the THAL [J.sub.Cl] response to NE was measured in the presence of selective antagonists of [Beta]- and [Alpha]-2 receptors. A concentration of NE (1 [micro]M), which alone tended to increase [J.sub.Cl], decreased THAL [J.sub.Cl] (from 148.9 [+ or -] 16.4 to 76.2 [+ or -] 13.6 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1]; P [is less than] 0.01) in the presence of the [Beta]-antagonist propranolol. In contrast, a concentration of NE (0.1 [micro]M), which alone tended to decrease [J.sub.Cl], increased THAL [J.sub.Cl] (from 85.5 [+ or -] 20.1 to 111.8 [+ or -] 20.1 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1]; P [is less than] 0.05) in the presence of the [Alpha]-2 antagonist rauwolscine. To further clarify the role of different adrenergic receptors, selective adrenergic agonists were used. The [Alpha]-2 agonist clonidine decreased [J.sub.Cl] from 102.4 [+ or -] 9.9 to 54.0 [+ or -] 15.7 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1], a reduction of 49.1 [+ or -] 11.0% (P [is less than] 0.02). In contrast, the [Beta]-agonist isoproterenol stimulated [J.sub.Cl] from 95.3 [+ or -] 11.6 to 144.1 [+ or -] 15.0 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1], an increase of 56 [+ or -] 14% (P [is less than] 0.01). We conclude that 1) the sympathetic neurotransmitter NE exerts concentration-dependent effects on [J.sub.Cl] in the isolated rat THAL, 2) selective [Alpha]-2 receptor activation inhibits THAL [J.sub.Cl], and 3) selective [Beta]-receptor activation stimulates THAL [J.sub.Cl]. These data indicate the response elicited by the isolated rat THAL to NE is dependent on the neurotransmitter concentration, such that application of NE in vitro biphasically modulates [J.sub.Cl] via differential activation of [Alpha]-2 and [Beta]-adrenergic receptors in a concentration-dependent manner. kidney tubule; clonidine; isoproterenol; phenylephrine; sympathetic nervous system; norepinephrine