COUPLING OF HUMAN ALPHA(2)-ADRENOCEPTOR SUBTYPES TO REGULATION OF CAMP PRODUCTION IN TRANSFECTED S115 CELLS

Stable S115 mouse mammary tumour cell lines, expressing separately alp ha(2)A-C10, alpha(2)B-C2 and alpha(2)C-C4 adrenoceptors were used to c ompare the receptor binding properties of alpha(2)-adrenoceptor agonis ts with their potency in inhibiting cAMP production. All tested agonis ts detected high and low affinity binding sites in all three receptor subtypes. In the presence of the GTP analogue Gpp(NH)p (10 mu M), all displacement curves were shifted to the right and were best modelled b y one-site fits, suggesting that the receptor subtypes are coupled to G-proteins. The extent of the Gpp(NH)p-induced shift was greatest in t he alpha(2)A-C10 subtype, smaller in alpha(2)C-C4, and minimal in alph a(2)B-C2. All three receptor subtypes were also coupled to inhibition of forskolin-stimulated cAMP production through pertussis toxin-sensit ive G-proteins. For the the full agonists noradrenaline, UK 14,304, an d dexmedetomidine, the maximal inhibitory effect on cAMP production wa s smaller in the alpha(2)B-C2 subtype (35%) than in the alpha(2)A-C10 and alpha(2)C-C4 subtypes (50-70%). After treatment of cells expressin g alpha(2)B-C2 receptors with pertussis toxin, cAMP production was inc reased by up to 58% by alpha(2)-adrenoceptor agonists. Similar stimula tion of adenylyl cyclase activity could not be demonstrated at the oth er two receptor subtypes. In conclusion, these results demonstrate tha t (1) alpha(2)-adrenoceptor agonists may be characterized by an agonis t-type binding pattern in homogenates of transfected S115 cells, (2) a ll three alpha(2)-adrenoceptor subtypes are coupled to inhibition of a denylyl cyclase in S115 cells through pertussis toxin-sensitive G-prot eins, (3) the receptor-effector coupling in S115 cells is different am ong the subtypes so that the alpha(2)A-C10 subtype is coupled with hig h efficacy but with low sensitivity, the alpha(2)B-C2 subtype with low efficacy but high sensitivity, and the alpha(2)C-C4 subtype with both high efficacy and high sensitivity, and (4) at least alpha(2)B-C2 rec eptors may also be coupled to stimulation of adenylyl cyclase activity , presumably through G(s).