Table 2.

Parameter values of the antagonist SCH 23390 binding to dopamine D1 receptors obtained by fitting data to different models.

Model	Parameters	Values
One-affinity statea	R (pmol/mg protein)	0.86 ± 0.01
	KD (nM)	0.5 ± 0.4
Two state dimer	RT (pmol/mg protein)	0.61 ± 0.09
	KD1 (nM)	0.6 ± 0.3
	KD2 (nM)	2.4 ± 1.2
	D C	0
	B50 (nM)	1.2

Competition experiments were performed using variable concentrations of nonr-adiolabeled SCH 23390 and 2.7 nM [³H]SCH 23390. Data for non-linear regression were taken from Fig. 2. R is the maximum specific binding and K_D is the equilibrium dissociation constants of the competing ligand (SCH 23390). R_T is the total amount of receptor dimers, K_D1 and K_D2 are, respectively, the equilibrium dissociation constants ofvthe first and second binding of SCH 23390 to the dimer. D_C is the “dimer cooperativity” index for the binding of SCH 23390 and B₅₀ is the concentration of the competitor (SCH 23390) providing half saturation. Parameter values are mean ± s.e.m.

^aF test indicated that no significant improvement is obtained by assuming two-affinity states.