α₂-Adrenoceptor regulation of adenylyl cyclase in CHO cells: Dependence on receptor density, receptor subtype and current activity of adenylyl cyclase

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• 作者：Pohjanoksa ; Katariina ; Jansson ; Christian c. ; Luomala ; Kirsti ; Marjamä ; ki ; Anne ; Savola ; Juha-Matti ; et. al.
• 关键词：α ; ₂-Adrenoceptor subtypes ; CHO cells ; [³H]RX821002 binding ; Adenylyl cyclase ; cAMP ; CHO ; Chinese hamster ovary cells ; PC12 ; rat pheochromocytoma cells ; JEG-3 ; human chromocytoma cells ; Sf-9 ; Spodoptera frugiperda insect ov
• 刊名：European Journal of Pharmacology
• 出版年：1997
• 出版时间：September 17, 1997
• 年：1997
• 卷：335
• 期：1
• 页码：53-63
• 全文大小：1.05 M

文摘

Chinese hamster ovary (CHO) cells stably transfected to express different densities of the human α_2A-, α_2B- and α_2C-adrenoceptor subtypes, were used to characterize the regulation of adenylyl cyclase activity by α₂-adrenoceptor agonists. In isolated cell membranes, activation of α_2A- and α_2C-adrenoceptors did not affect basal enzyme activity, but activation of α_2B-adrenoceptors stimulated adenylyl cyclase activity. The extent of stimulation was dependent on the receptor density and was insensitive to pertussis toxin treatment. In the presence of 5 μM forskolin all three receptor subtypes mediated inhibition of adenylyl cyclase activity in a pertussis toxin-sensitive manner. In experiments performed with intact cells the same pattern could be seen: the basal production of cAMP was not affected when α_2C-adrenoceptors were activated, but activated α_2B-adrenoceptors mediated stimulation of cAMP production. In the presence of forskolin, both receptor subtypes mediated inhibition of cAMP production. Our results suggest that α_2B-adrenoceptors are coupled 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 current activity of adenylyl cyclase. The results also suggest that the α_2A- and α_2C-subtypes are preferentially coupled to G_i and transduce only inhibition of adenylyl cyclase activity in transfected CHO cells. At low densities of α_2C-adrenoceptors, clonidine was a partial agonist, but in clones expressing high levels of α_2C-adrenoceptors, clonidine acted as a full agonist by inhibiting cAMP accumulation with the same efficacy as (−)-noradrenaline. This demonstrates that receptor reserve can mask partial agonist activity.