In the rat CCD, epinephrine at 1 to 100 nM reversibly inhibits AVPdependent Na+ and water transport. This inhibition is prevented by yonimbine, and is almost absent wnen cAMP rather than AVP stimulates transport, implying an α2 receptor is involved (Hawk et al., Am. J. Physiol. 265:F449, 1993). In contrast, a adrenergic agonists stimulate volume reabsorption in the proximal tubule through ot] receptors. We examined the Ot receptor isoforms expressed in rat PST and CCD by extraction of RNA, foJlowed by RT/PCR amplification. Restriction enzyme analysis of PCR products from degenerate primers to transmembrane regions allowed detection of the cci isoforms. Specific primers designed to 3′ cytoplasmic and noncoding regions were used to detect the α2 isoforms. PCR cycle numbers were 43 for immunodissected CCD (IDCCD), and 48 for microdissected CCD and PST. Tissue α1A α1B α1C α2A α2B α2C PST 0/4 4/4 4/4 3/3 3/3 3/3 CCD 0/5 5/5 5/5 4/6 6/6 2/6 IDCCP - - 6/6 6/6 2/6 Data are presented as the (# positive/total # of tissue samples). All control experiments (no RT enzyme, primers alone) were negative. Whenever CX2C message was observed in the CCD, it was negligible in comparison with α2A or α2B. From these data it seems that more than one aj or 0:3 adrenergic receptor isoform may be linked to the effects of adrenergic agonists in the PST and CCD. Further experiments to establish relative isoform expression at the protein level are planned.