Differential A₁ Adenosine Receptor Reserve for Two Actions of Adenosine on Guinea Pig Atrial Myocytes

Abstract

Adenosine activates adenosine-induced inwardly rectifying K⁺ current (I_KAdo) and inhibits isoproterenol (100 nm)-stimulated L-type Ca²⁺ current (β-I_Ca,L) of guinea pig atrial myocytes with EC₅₀ values of 2.17 and 0.20 μm, respectively. We determined whether this 11-fold difference in potency of adenosine is due to the existence of a greater A₁adenosine receptor reserve for the inhibition of β-I_Ca,Lthan for the activation of I_KAdo. Atrial myocytes were pretreated with vehicle (control) or the irreversible A₁adenosine receptor antagonist 8-cyclopentyl-3-[3-[[4-(fluorosulfonyl)benzoyl]oxy]propyl]-1-propylxanthine (FSCPX) (10 and 50 nm) for 30 min, and after a 60-min washout period, concentration-response curves were determined for the adenosine-induced activation of I_KAdo and inhibition of β-I_Ca,L. Pretreatment of atrial myocytes with 10 nm FSCPX reduced the maximal activation of I_KAdo by 60% (7.9 ± 0.2 to 3.2 ± 0.1 pA/pF). In contrast, a higher concentration of FSCPX (50 nm) was required to reduce the maximal inhibition of β-I_Ca,L by 39% (95 ± 4% to 58.7 ± 5.6%) and caused a 15-fold increase in the EC₅₀ value of adenosine. Values of the equilibrium dissociation constant (K_A) for adenosine to activate I_KAdo and inhibit β-I_Ca,L, estimated according to the method of Furchgott, were 2.7 and 5.6 μm, respectively. These values were used to determine the relationship between adenosine receptor occupancy and response. Half-maximal and maximal activations of I_KAdorequired occupancies of 40% and 98% of A₁ adenosine receptors, respectively. In contrast, occupancies of only 4% and 70%, respectively, of A₁ adenosine receptors were sufficient to cause half-maximal and maximal inhibitions of β-I_Ca,L. Consistent with this result, a partial agonist of the A₁adenosine receptor SHA040 inhibited β-I_Ca,L by 60 ± 3.5% but activated I_KAdo by only 18.1 ± 2.5%. The results indicate that the A₁ adenosine receptor is coupled more efficiently to an inhibition of β-I_Ca,L than to an activation of I_KAdo.