A comparison of the binding characteristics of the alpha 2-adrenoceptor antagonists 3H-yohimbine and 3H-rauwolscine in bovine teat muscles.

3H-Yohimbine and 3H-rauwolscine, both potent and selective alpha 2-adrenergic antagonists, were used to identify alpha 2-adrenoceptors in smooth muscles of the cistern wall of teats of lactating cows. Binding of these radioligands was rapid and readily reversed by 10 microM phentolamine. Saturation experiments in the presence of 3H-yohimbine showed an equilibrium KD value of 6.23 +/- 0.74 nM and a maximum number of sites of 81 +/- 7 fmol/mg of membrane protein. In the presence of 3H-rauwolscine, however, a higher density of alpha 2-receptors (164 +/- 12 fmol/mg protein) with a KD of 6.16 +/- 0.64 nM was found. No cooperative interactions among both binding sites were observed. Both 3H-yohimbine and 3H-rauwolscine binding sites showed alpha 2-adrenergic specificity. On the basis of its higher affinity to the alpha 2-adrenoceptor sites, better ratio of specific to non-specific binding and for other reasons, 3H-rauwolscine appears to be the ligand of choice.