Table 1

Affinities and potencies of various opioid agonists and antagonists for the μ-opioid receptor expressed in HEK293 cells

Opioid Ligand Diprenorphine	K_d = 1.03 ± 0.1 nM IC₅₀	B_max = 14.4 ± 2.9 pmol/mg protein K_i
	nM
Etorphine	0.077 ± 0.043 (87.5 ± 2.0%)	1.6 ± 0.5
Morphine	9.8 ± 3.3 (87.1 ± 4.5)	2.6 ± 1.6 (41%), 160 ± 80)
Met⁵-Enkephalin	15.9 ± 7.5 (83.4 ± 4.7)	4.1 ± 1.8 (40%), 320 ± 70
DAMGO	2.0 ± 0.7 (88.6 ± 4.3)	0.18 ± 0.33 (22%), 130 ± 370
Naloxone	6.8 ± 2.2^*	4.9 ± 4.4

Competition and saturation-binding studies with the membranes isolated from HEK293 cells expressing the μ-opioid receptor were carried out as described in Experimental Procedures. The abilities of various concentrations of agonists to inhibit forskolin-stimulated adenylyl cyclase activity also were determined as described. TheK _d value of naloxone^* was determined by the ability of different concentrations of naloxone to shift the etorphine dose-response curves. The values in parentheses in the IC₅₀values are the level of maximal inhibition. The values in parentheses in the K _i values are the percentage of receptor determined in the high-affinity state as calculated from the best-fitted curves derived from the competition-binding studies.