Expression of multiple α-adrenoceptor isoforms in rat CCD

Abstract

In the rat cortical collecting duct (CCD), epinephrine inhibits vasopressin (AVP)-dependent water permeability and Na+ reabsorption. Although inhibition is reversed by the α2-adrenoceptor (AR) antagonist yohimbine, suggesting the epinephrine effect is primarily mediated by an α2-AR [C. T. Hawk, L. H. Kudo, A. J. Rouch, and J. A. Schafer. Am. J. Physiol. 265 ( Renal Fluid Electrolyte Physiol. 34): F449–F460, 1993], there are also suggestions of an effect at an additional receptor, perhaps an α1-AR. For the present experiments, we used RT-PCR of total RNA extracted from 1 to 5 mm of microdissected CCDs from rat kidney to identify the α-AR isoforms expressed. Specific primers for the α2-ARs amplifying from the 6th transmembrane (TM) to the 3′-untranslated regions, revealed the presence of α2A and α2B. Western blot analysis also indicated the presence of α2B-AR at the protein level. Degenerate α1-AR primers that amplify from conserved regions of TM-1 to TM-5, as well as specific primers that amplify either the same region (α1B), the carboxy terminus (α1A), or within the third cytoplasmic loop (α1D), indicated the presence of all three α1-ARs. Measurement of transepithelial voltage in isolated perfused renal tubules indicated a small inhibitory effect mediated by α1-ARs. Although the functional effects of epinephrine on AVP-dependent transport processes appear to be mediated predominantly by an α2-AR, a small contribution to the overall α-AR effect may be due to simultaneous activation of an α1-AR.