Figure 3.

Pretreatment with a modest concentration of EPI produces α_2A-AR desensitization in SH-SY5Y cells only when the β₂-AR is present. Wild-type SH-SY5Y cells (A), cells expressing recombinant β₂-AR (SHβ₂AR4, B and D), or SH-SY5Y cells expressing the vector alone (C) were pretreated 16–24 hr with any or all of the following: EPI (300 nM), NE (1 μM or 30 μM), EPI + Prop (30 nM), Prop (30 nM) alone or vehicle (0.1 mM ascorbate). Following pretreatment, the ability of UK14,304 to inhibit forskolin-stimulated cAMP accumulation was evaluated. A) Neither chronic EPI nor 1 μM NE pretreatments were sufficient to alter the α_2A-AR signal (n = 6) in native SH-SY5Y cells. The α_2A-AR signal in these cells desensitized only when exposed to higher agonist concentrations (30 μM NE, n = 3; 100 μM EPI, n = 3, data not shown). B) Unlike native SH-SY5Y cells, pretreatment with 300 nM EPI is sufficient to desensitize the α_2A-AR signal in SHβ₂AR4 cells (n = 6; p < 0.05). NE (1 μM), acting predominantly at α_2A-AR with little affinity for the β₂-AR, does not produce α_2A-AR desensitization. C) In SH-SY5Y cells transfected with the vector alone, neither EPI nor NE pretreatments altered α_2A-AR signal (n = 4).D) Addition of propranolol (30 nM) prevents EPI-induced α_2A-AR desensitization, suggesting a β₂-AR-dependent process (# p < 0.05 as compared to EPI treatment).