TABLE 1

Stimulation of [³⁵S]GTPγS binding by DAMGO in membranes from C6μ cells expressing PTX-insensitive and RGS-sensitive (PTXi) or PTX-insensitive and RGS-insensitive (RGS/PTXi) Gα_o following pretreatment with 1 μM DAMGO PTX-treated C6μ cells expressing Gα_o that was PTXi or RGS/PTXi were treated for varying times with 1 μM DAMGO. Cells were washed, membranes prepared, and DAMGO stimulation of [³⁵S]GTPγS binding was measured as described under Materials and Methods. Shown are the combined data from four to five assays each measured in duplicate. E_max is given as the percentage of stimulation compared with cells that did not receive DAMGO pretreatment (time 0).

Pretreatment Time	DAMGO Stimulation
	PTXi		RGS/PTXi
	E_max	EC₅₀	E_max	EC₅₀
h	%	NM	%	nM
0	100^b	330 ± 40	100^b	440 ± 40
0.5	79 ± 6	420 ± 50	65 ± 6^a	400 ± 40
1.0	78 ± 4^a	560 ± 60^a	48 ± 5^a^,^c	360 ± 50
18	61 ± 9^a	370 ± 150	26 ± 5^a^,^c	230 ± 70

↵ a p < 0.05, compared with no pretreatment (time 0)
↵ b E_max values at time 0 were 333 ± 45% (PTXi) and 368 ± 44% (RGS/PTXi) of basal binding
↵ c p < 0.05, compared with cells expressing PTXi Gα_o