Cc. Jansson et al., COUPLING OF HUMAN ALPHA(2)-ADRENOCEPTOR SUBTYPES TO REGULATION OF CAMP PRODUCTION IN TRANSFECTED S115 CELLS, European journal of pharmacology. Molecular pharmacology section, 266(2), 1994, pp. 165-174
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).