INDUCIBLE EXPRESSION OF BETA(1)-ADRENERGIC AND BETA(2)-ADRENERGIC RECEPTORS IN RAT C-6 GLIOMA-CELLS - FUNCTIONAL INTERACTIONS BETWEEN CLOSELY-RELATED SUBTYPES

We examined the role of beta(1)- and beta(2)-adrenergic receptor (AR) density and ratio in catecholamine-stimulated cAMP responses in rat C- 6 glioma cells. These cells, which normally express both subtypes, wer e stably transfected with an isopropylthio-beta-D-galactoside-inducibl e vector containing either beta(1)AR or beta(2)AR coding sequences, an d receptor expression was controlled by the time and concentration of isopropylthio-beta-D-galactoside exposure. Induction of the dominant b eta(1)AR subtype increased the potencies of isoproterenol (ISO) and ot her agonists in stimulating cAMP accumulation by 20-40-fold without ch anging maximal response. Induction of beta(2)AR expression caused 7-13 -fold increases in the potency of lSO, epinephrine, and zinterol, but not of norepinephrine, and a 20-40% loss in maximal response to all ag onists. Selective antagonists showed that both subtypes contributed in a nonadditive manner in the response to ISO under different condition s. After beta(2)AR induction, the effects of ISO were not blocked by t he beta(1)-selective antagonist CGP 20712A but were shifted 100-fold t o the right by the beta(2)-selective antagonist ICI 118,551. However, in the presence of ICI 118,551, CGP 20712A caused an additional 100-fo ld decrease in ISO potency, and Schild analysis revealed complex inter actions between the two subtypes. Each antagonist alone caused smaller shifts to the right in the dose-response curve to NE and, when presen t simultaneously, completely abolished the NE response, We conclude th at beta(1)ARs and beta(2)ARs have different efficiencies in activating cAMP accumulation in C-6 glioma cells. Activation of coexisting subty pes results in complex and sometimes synergistic interactions between the two subtypes, which vary with agonist concentration, selectivity, subtype density, and ratio.