Abstract

The D₂-like dopamine receptors couple to a variety of signal transduction pathways, including inhibition of adenylate cyclase, mitogenesis, and activation of potassium channels. Although these effects are mediated via pertussis toxin-sensitive G proteins, G_i/o, it is likely that some of these effects are influenced by the release of G protein βγ subunits. Type II adenylate cyclase (ACII) is highly regulated by multiple biochemical stimuli, including protein kinase C, forskolin, G protein α subunits, and G protein βγ subunits. The ability of βγ subunits to activate this enzyme in the presence of activated α_s has been particularly well characterized. Although stimulation by βγ subunits has been described as conditional on the presence of activated α_s, βγ subunits also potentiate ACII activity after activation of protein kinase C. We created stable cell lines expressing ACII and the D_2L receptor, the D₃ receptor, or the D_4.4 receptor. Activation of D_2L or D_4.4 receptors, but not D₃ receptors, potentiated β-adrenergic receptor/G_s-stimulated activity of ACII, as measured by the intracellular accumulation of cAMP. Similarly, stimulation of D_2L or D_4.4 receptors potentiated phorbol ester-stimulated ACII activity in the absence of activated α_s, whereas stimulation of D₃receptors did not. The effect of D₂-like receptor stimulation was blocked by pretreatment with pertussis toxin and by inhibition of protein kinase C. We propose that activation of both D_2L and D_4.4 dopamine receptors potentiated phorbol-12-myristate-13-acetate-stimulated ACII activity through the release of βγ subunits from pertussis toxin-sensitive G proteins. In contrast, the lack of D₃ receptor-mediated effects suggests that stimulation of D₃ receptors does not result in an appreciable release of βγ subunits.