Figure 5.

RGS4 diminished the efficacy, but not the potency, of [D‐Ala², N‐Me‐Phe⁴, Gly⁵‐ol] enkephalin (DAMGO) in inhibiting adenylyl cyclase activity (left panel); in contrast, naltrexone diminished the potency, but not the efficacy, of DAMGO (right panel). Left Panel. Adenylyl cyclase activity was measured in the presence of increasing concentrations of DAMGO in the absence (open circles) or the presence of 1 μM RGS4 (filled circles). The EC50s of DAMGO in the absence and presence of RGS4 were 40.0 nM (95% confidence interval 27–59 nM) and 66.0 nM (95% confidence interval 32–137 nM), respectively (EC50 values were not significantly different). In the absence and presence of 1 μM RGS4, DAMGO maximally inhibited adenylyl cyclase activity by 54% (95% confidence interval 50.3–57.2%) and 28.2% (95% confidence interval 25.3–32.6%), respectively (efficacies were significantly different p < 0.05). Data from four independent experiments were combined, n = 16 for each point. Right Panel. Adenylyl cyclase activity was measured in the presence of increasing concentrations of DAMGO in the absence (open squares) or presence of 0.3 μM naltrexone (filled squares). The EC50s of DAMGO in the absence and presence of naltrexone were 19 nM (95% confidence intervals 10.5–31 nM) and 1.30 μM (95% confidence intervals 0.351–4.74 μM), respectively (p < 0.05). The maximal inhibitions caused by DAMGO in the absence or presence of naltrexone were to 3.28 pmoles cAMP formed in 10 min (95% confidence intervals 2.44–4.01 pmoles) and 3.92 pmoles cAMP formed in 10 min (95% confidence intervals 1.28–6.56 pmoles), respectively (not significantly different). Data shown are from a single experiment (n = 4 for each point).