FIGURE 4.

Inhibition of phosphodiesterase activity, together with stimulation of D₁ dopamine receptors in Jurkat cells, is associated with significant elevation in intracellular cAMP level and inhibition of cell proliferation. A, the D₁ DA receptor agonist (SKF, SKF 82526) induced dose-dependent inhibition of proliferation with 1 μm concentration, showing the maximum inhibition observed in CD3/CD28-stimulated Jurkat T cells when pretreated with phosphodiesterase inhibitor (Theo, theophylline; 1 mm), but not in CD3/CD28-stimulated Jurkat cells similarly treated with D₁ agonist alone. Results are based on [³H]thymidine incorporation assay. Results are mean ± S.E. of six separate experiments (*, p < 0.05). B, inhibition of phosphodiesterase activity by theophylline (1 mm) or stimulation with D₁ DA receptor-specific agonist (1 μm) separately was not sufficient to inhibit the CD3/CD28-induced Jurkat T cell proliferation, but stimulation of D₁ DA receptors with specific agonist (1 μm) together with inhibition of phosphodiesterase activity (theophylline; 1 mm) significantly inhibited proliferation of these cells as evident from [³H]thymidine incorporation assay. Inhibition of D₁ DA receptor activity by specific D₁ DA receptor antagonist (SCH, SCH 23390; 100 μm) abrogated this above-mentioned proliferation inhibition in Jurkat, suggesting the functional competence of D₁ DA receptors, and both inhibition of phosphodiesterase activity and D₁ receptor stimulation are required for effective inhibition of proliferation in Jurkat cells. C, significant increase in intracellular cAMP observed in CD3/CD28-stimulated Jurkat cells treated with phosphodiesterase inhibitor (theophylline; 1 mm) and D₁ DA receptor agonist (SKF 82526; 1 μm), when compared with only theophylline-treated (1 mm) or D₁ DA receptor agonist (SKF 82526; 1 μm)-treated groups which showed little or no increase of cAMP, respectively. Inhibition of D₁ DA receptor activity by D₁ DA receptor-specific antagonist (SCH 23390; 100 μm) and then treatment with theophylline (1 mm) and D₁ DA receptor agonist (SKF 82526; 1 μm) also showed only negligible increase in intracellular cAMP in Jurkat cells. Results are mean ± S.E. of six separate experiments (*, p < 0.05). Culture and treatment protocols are as described under “Experimental Procedures.”