Differential A(1) adenosine receptor reserve for two actions of adenosine on guinea pig atrial myocytes

Adenosine activates adenosine-induced inwardly rectifying K+ current (I-KAdo) and inhibits isoproterenol (100 nM)-stimulated L-type Ca2+ current (beta-I-Ca,I-L) of guinea pig atrial myocytes with EC50 values of 2.17 and 0.20 mu M, respectively. We determined whether this 11-fold difference in potency of adenosine is due to the existence of a greater A(1) adenosine receptor reserve for the inhibition of beta-I-Ca,I-L than for the activation of I-KAdo. Atrial myocytes were pretreated with vehicle (control) or the irreversible A(1) adenosine 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 beta-I-Ca,I-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 beta-I-Ca,I-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 beta-I-Ca,I-L, estimated according to the method of Furchgott, were 2.7 and 5.6 mu M, respectively. These values were used to determine the relationship between adenosine receptor occupancy and response. Half-maximal and maximal activations of I-KAdo required occupancies of 40% and 98% of A(1) adenosine receptors, respectively. In contrast, occupancies of only 4% and 70%, respectively, of A(1) adenosine receptors were sufficient to cause half-maximal and maximal inhibitions of beta-I-Ca,I-L. Consistent with this result, a partial agonist of the A(1) adenosine receptor SHA040 inhibited beta-I-Ca,I-L, by 60 +/- 3.5% but activated I-KAdo by only 18.1 +/- 2.5%. The results indicate that the A(1) adenosine receptor is coupled more efficiently to an inhibition of beta-I-Ca,I-L than to an activation of I-KAdo.