ALPHA(2)-ADRENOCEPTOR REGULATION OF ADENYLYL-CYCLASE IN CHO CELLS - DEPENDENCE ON RECEPTOR DENSITY, RECEPTOR SUBTYPE AND CURRENT ACTIVITY OF ADENYLYL-CYCLASE
K. Pohjanoksa et al., ALPHA(2)-ADRENOCEPTOR REGULATION OF ADENYLYL-CYCLASE IN CHO CELLS - DEPENDENCE ON RECEPTOR DENSITY, RECEPTOR SUBTYPE AND CURRENT ACTIVITY OF ADENYLYL-CYCLASE, European journal of pharmacology, 335(1), 1997, pp. 53-63
Chinese hamster ovary (CHO) cells stably transfected to express differ
ent densities of the human alpha(2A)-, alpha(2B)- and alpha(2C)-adreno
ceptor subtypes, were used to characterize the regulation of adenylyl
cyclase activity by alpha(2)-adrenoceptor agonists. In isolated cell m
embranes, activation of alpha(2A)- and alpha(2C)-adrenoceptors did not
affect basal enzyme activity, but activation of alpha(2B)-adrenocepto
rs stimulated adenylyl cyclase activity, The extent of stimulation was
dependent on the receptor density and was insensitive to pertussis to
xin treatment. In the presence of 10 mu M forskolin all three receptor
subtypes mediated inhibition of adenylyl cyclase activity in a pertus
sis toxin-sensitive manner. In experiments performed with intact cells
the same pattern could be seen: the basal production of cAMP was not
affected when alpha(2C)-adrenoceptors were activated, but activated al
pha(2B)-adrenoceptors mediated stimulation of cAMP production. In the
presence of forskolin, both receptor subtypes mediated inhibition of c
AMP production. Our results suggest that alpha(2B)-adrenoceptors are c
oupled to both G(i) and G(s) proteins. The signal transduction pathway
to which the receptor is coupled is not dependent on receptor density
, but its effect on adenylyl cyclase regulation is dependent on the cu
rrent activity of adenylyl cyclase. The results also suggest that the
alpha(2A)- and alpha(2C)-subtypes are preferentially coupled to G(i) a
nd transduce only inhibition of adenylyl cyclase activity in transfect
ed CHO cells. At low densities of alpha(2C)-adrenoceptors, clonidine w
as a partial agonist, but in clones expressing high levels of alpha(2C
)-adrenoceptors, clonidine acted as a full agonist by inhibiting cAMP
accumulation with the same efficacy as (-)-noradrenaline. This demonst
rates that receptor reserve can mask partial agonist activity. (C) 199
7 Elsevier Science B.V.