Table 1

Comparison of NECA-mediated effects on ERK1/2 phosphorylation/cAMP production and NECA binding between the four human adenosine receptor subtypes

Receptor Subtype	EC₅₀ Value for ERK1/2 Phosphorylation	K_D orK_i	EC₅₀ Value for cAMP Production	Total Receptor Number per Cell
	nM
A₁	115.4	13.6 nM (K _D high affinity)	26 nM	∼40,000
		(Klotz et al., 1998)	(Bevan et al., 1999)	(seeDiscussion)
A_2A	26.4	20.1 nM	26.1 nM	∼20,000
		(Klotz et al., 1998)	(Kull et al., 1999a)	(Kull et al., 1999a)
A_2B	19.4	N.D.	1.4 μM	<200,000
			(present study)	(seeDiscussion)
A₃	65.4	6.18 nM	129 nM	Similar to A₁
		(Klotz et al., 1998)	(Salvatore et al., 1993)	(Klotz et al., 1998)

Summary of EC₅₀ values for ERK1/2 activation after NECA stimulation in CHO cells stably transfected with the human adenosine A₁, A_2A, A_2B, or A₃ receptor. For a comparison, K _D or K _i values and EC₅₀ values from other sources have been used.

N.D., not determined.