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 alpha(2)A-C10, alpha(2)B-C2 and alpha(2)C-C4 adrenoceptors were used to compare the receptor binding properties of alpha(2)-adrenoceptor agonists with their potency in inhibiting cAMP production. All tested agonists 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 by 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 the alpha(2)A-C10 subtype, smaller in alpha(2)C-C4, and minimal in alpha(2)B-C2. All three receptor subtypes were also coupled to inhibition of forskolin-stimulated cAMP production through pertussis toxin-sensitive G-proteins. For the the full agonists noradrenaline, UK 14,304, and dexmedetomidine, the maximal inhibitory effect on cAMP production was 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 expressing alpha(2)B-C2 receptors with pertussis toxin, cAMP production was increased by up to 58% by alpha(2)-adrenoceptor agonists. Similar stimulation of adenylyl cyclase activity could not be demonstrated at the other two receptor subtypes. In conclusion, these results demonstrate that (1) alpha(2)-adrenoceptor agonists may be characterized by an agonist-type binding pattern in homogenates of transfected S115 cells, (2) all three alpha(2)-adrenoceptor subtypes are coupled to inhibition of adenylyl cyclase in S115 cells through pertussis toxin-sensitive G-proteins, (3) the receptor-effector coupling in S115 cells is different among the subtypes so that the alpha(2)A-C10 subtype is coupled with high 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 receptors may also be coupled to stimulation of adenylyl cyclase activity, presumably through G(s).