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TA Esbenshade, C Han, TL Theroux, JG Granneman and KP Minneman
Department of Pharmacology, Emory University, Altanta, Georgia 30322.
In SK-N-MC human neuroblastoma cells, the cAMP response to 10 nM isoproterenol (ISO) is mediated primarily by beta 1-adrenergic receptors. However, responses to higher concentrations of ISO (100-1000 nM) were only weakly blocked by beta 1- and beta 2-selective antagonists. When beta 1 receptors were blocked with 10 microM CGP 20712A, catecholamines still maximally activated cAMP accumulation, with only small decreases in potency. In the presence of CGP 20712A, beta blockers inhibited the response to ISO stereoselectively but with relatively low potencies. Pindolol derivatives were partial agonists with low potencies, and the atypical agonist BRL 37344 was a partial agonist with an intermediate potency. All binding sites in these cells labeled by 125I-cyanopindolol were of the beta 1 subtype. Nuclease protection assays indicated that SK-N-MC cells contain mRNA for both the human beta 1- and beta 3-adrenergic receptors, with the beta 3 subtype mRNA being expressed 25-50% more abundantly than that for the beta 1 subtype. Northern blot hybridizations showed the presence of two beta 3 mRNA transcripts of 3.1 and 2.4 kilobases. These results suggest that beta 1- and atypical beta-adrenergic receptors coexist in these cells and cause redundant increases in cAMP formation. Although molecular approaches suggest that the atypical subtype is the beta 3, the observed drug specificity differs from that reported for the expressed recombinant human beta 3 receptor.
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