Table 4.

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

		5.0 nM [3H] SCH 23390	0.5 nM [3H] SCH 23390	0.1 nM [3H] SCH 23390
Model	Parameters	Values
Two-affinity statesa	RH (%)	46 ± 1	28b ± 1	0
	KDH (nM)	6 ± 1	11b ± 2	-
	RL (%)	54 ± 1	72b ± 2	100c
	KDL (nM)	460 ± 20	440 ± 10	320c ± 10
Two state dimer	RT (pmol/mg protein)	0.499 ± 0.001	0.499 ± 0.001	0.501 ± 0.001
	KDB1 (nM)	21 ± 5	18 ± 2	22 ± 4
	KDB2 (nM)	390 ± 30	410 ± 30	420 ± 20
	KDAB (nM)	18 ± 1	19 ± 3	23 ± 5
	D C	−0.67	−0.75	−0.68
	B50 (nM)	90	86	96

Competition curves were simulated as indicated in the legend of Fig. 4. Parameter values were obtained by fitting three data point sets simulated considering a random experimental error in the range of ± 2.5%. R_H and R_L are, respectively, the % of maximum specific binding corresponding to high and low affinity sites and K_DH and K_DL are the equilibrium dissociation constants of the competing ligand (SKF 38393) for, respectively, high and low affinity sites. R_T is the total amount of receptor dimers, K_DB1 and K_DB2 are, respectively, the equilibrium dissociation constants of the first and second binding of SKF 38393 to a dimer. K_DAB is the hybrid equilibrium dissociation constant of SKF 38393 binding to a receptor dimer semi-occupied by [³H]SCH 23390. DCB is the “dimer cooperativity” index for the binding of ligand SKF 38393 and B₅₀ is the concentration providing half saturation. Parameter values are mean ± s.e.m.

^aF test indicated that a significant improvement is obtained by assuming two-affinity states (compared to one-affinity state) except at 0.1 nM [³H]SCH 23390.

^bp<0.05 compared with values obtained at 5 nM [3H]SCH 23390 concentration.

^cp<0.05 compared with values obtained at 5 and 0.5 nM [3H]SCH 23390 concentration.