EXPRESSION OF MULTIPLE ALPHA-ADRENOCEPTOR ISOFORMS IN RAT CCD

In the rat cortical collecting duct (CCD), epinephrine inhibits vasopr essin (AVP)-dependent water permeability and Na+ reabsorption. Althoug h inhibition is reversed by the alpha(2)-adrenoceptor (AR) antagonist yohimbine, suggesting the epinephrine effect is primarily mediated by an alpha(2)-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 rece ptor, perhaps an alpha(1)-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 alpha-AR isoforms expressed. Specific prime rs for the alpha(2)-ARs amplifying from the 6th transmembrane (TMl) to the 3'-untranslated regions, revealed the presence of alpha(2A) and a (2B). Western blot analysis also indicated the presence of alpha(2B)-A R at the protein level. Degenerate alpha(1)-AR primers that amplify fr om conserved regions of TM-1 to TM-5, as well as specific primers that amplify either the same region (alpha(1B)), the carboxy terminus (alp ha(1A)), or within the third cytoplasmic loop (alpha(1D)), indicated t he presence of all three alpha(1)-ARs. Measurement of transepithelial voltage in isolated perfused renal tubules indicated a small inhibitor y effect mediated by alpha 1-ARs. Although the functional effects of e pinephrine on AVP-dependent transport processes appear to be mediated predominantly by an alpha(2)-AR, a small contribution to the overall a lpha-AR effect may be due to simultaneous activation of an alpha(1)-AR .