Table 1

Functional properties of wild-type and mutant μ-opioid receptors after expression in HEK 293 cells and X. laevis oocytes

Construct	B_max	K_D	Reduction in Forskolin-Stimulated cAMP Levels	Current Ratio	Specific GTPγS Binding
Construct	B_max	K_D	Reduction in Forskolin-Stimulated cAMP Levels	Current Ratio	Maximal activation	Activation
	fmol/mg	nM	%		dpm	%
rMOR1	758	2.5	65 ± 2	2.0 ± 0.4	5002 ± 1500	205
rS266P	570	3.2	39 ± 5	0.2 ± 0.1	1897 ± 600	133
hMOR	643	2.1	66 ± 6	2.2 ± 0.3	5428 ± 950	229
hS268P	340	5.0	31 ± 4	0.4 ± 0.2	1846 ± 300	132

The B _max and K _D values for the binding of [³H]DAMGO for the wild-type and mutant receptors were determined with membrane preparations from stable transfected HEK 293 cells and calculated by Scatchard analyses. The effect of 1 μM DAMGO on forskolin-stimulated cAMP accumulation in HEK 293 cells and KIR3.1/KIR3.4-mediated potassium current ratio (I_{K16/GIRK.activated}/I_{K16/GIRK-basal}) in X. laevis oocytes was determined as described in Materials and Methods. Agonist potency and efficacy at the rat and human μ-opioid wild-type and mutant receptors was determined using the [³⁵S]GTPγS binding assay. Maximal activation in dpm is obtained from the difference of specific [³⁵S]GTPγS binding in the presence and absence of 1 μM agonist. Maximal activation is also displayed as a percentage with basal [³⁵S]GTPγS binding defined as 100%. Values shown for the reduction of cAMP level and [³⁵S]GTPγS experiments are mean ± S.E. from at least three independent experiments performed in triplicate, and values shown for the DAMGO-induced current ratios are mean ± S.E. from four separate oocytes.