ABSTRACT
~3H-Clonidine, a potent and selective alpha2-adrenergic agonist was used to label alpha2-a-drenoceptors in intact lymphocytes isolated from rat spleen. Binding of ~3H-Clo-nidine was rapid(t1/2: 2min)and readily reversed by 10umol?L~(-1) clonidine(t1/2: 3-4min). ~3H-Clonidine saturationexperiments indicated a single c1ass of site with a K_D of 6.57?1.63nM and Bmax of 72.4?13.4 fmol/10~7 lymphocytes. Adrenergic agonists competed for ~3H-clonidine binding site with anorder of potency:epinephrine)norepinephrine)isoproterenol. These results show the presence ofalpha2-adrenoceptors in splenic lymphocytes. Computer analysis of competition experiments withadrenergic agonists revealed three classes of sites: high affinity site, medium affinity site and lowaffinity site. The affinity of high affinity site is 2-3 orders of magnitude higher than the one ofmedium affinity site, whereas the latter is the same orders higher than low affinity site Using im-proved Mishell-Dutton method, 10umol?L~(-1) clonidine suppressed the Ig M synthesis and thesuppression was blocked by 10 umol?L~(-1) phentolamine. These results indicate the suppression ofIg M antibody response to SRBC in vitro by clonidine is mediated by the alpha-adrenoceptors.