The pharmacology of the alpha-2 adrenoceptor of the human adipocyte was improved by using some new alpha-2 antagonists from different chemical families (imidazolines, benzazepines and benzofuroquinolizines) in biological and binding assays. Moreover, investigations were also carried out to define the binding properties of a new imidazolinic antagonist, RX821002 [2-(2-methoxy-1,4-benzodioxan-2yl)-2-imidazoline], which could be a potential radioligand. [3H]RX821002 binding was very rapid and reversible. Saturation isotherms indicated that [3H]RX821002 labeled, with high affinity, a homogeneous population of noninteracting binding sites with a mean Kd of 0.98 +/- 0.05 nM (n = 6). The binding of [3H]RX821002 on the human fat cell alpha-2 adrenoceptor displayed a specificity which is strictly similar to that obtained with [3H]rauwolscine and which is classical for an alpha-2 A adrenoceptor. The binding parameters of [3H]RX821002 were compared with those obtained with the classical alpha-2 antagonist [3H]yohimbine. Analysis of the data indicate: 1) that [3H]RX821002 exhibited higher affinity; 2) that the nonspecific binding of [3H]RX821002 was very low; 3) that the total number of sites (maximum binding values) defined with [3H]RX821002 was significantly higher than that defined with [3H]yohimbine. This difference was not due to a specific preferential labeling of one of the two affinity states of the receptor, but suggested that [3H]yohimbine does not label the whole receptor population; 4) that [3H]RX821002 specific binding was less sensitive to magnesium chloride and GTP than [3H]yohimbine binding; and 5) that [3H]RX821002 can be used suitably for identification of alpha-2 adrenoceptors on the intact adipocyte.(ABSTRACT TRUNCATED AT 250 WORDS)