Differential A1 adenosine receptor reserve for two actions of adenosine on guinea pig atrial myocytes.
Adenosine activates adenosine-induced inwardly rectifying K+ current (IKAdo) and inhibits isoproterenol (100 nM)-stimulated L-type Ca2+ current (beta-ICa,L) of guinea pig atrial myocytes with EC50 values of 2.17 and 0.20 microM, respectively. We determined whether this 11-fold difference in potency of adenosine is due to the existence of a greater A1 adenosine receptor reserve for the inhibition of beta-ICa,L than for the activation of IKAdo. Atrial myocytes were pretreated with vehicle (control) or the irreversible A1 adenosine receptor antagonist 8-cyclopentyl-3-[3-[[4-(fluorosulfonyl)benzoyl]oxy]propyl]-1-propylxa nthine (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 IKAdo and inhibition of beta-ICa,L. Pretreatment of atrial myocytes with 10 nM FSCPX reduced the maximal activation of IKAdo 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-ICa,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 (KA) for adenosine to activate IKAdo and inhibit beta-ICa,L, estimated according to the method of Furchgott, were 2.7 and 5.6 microM, respectively. These values were used to determine the relationship between adenosine receptor occupancy and response. Half-maximal and maximal activations of IKAdo required 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 beta-ICa, L. Consistent with this result, a partial agonist of the A1 adenosine receptor SHA040 inhibited beta-ICa,L by 60 +/- 3.5% but activated IKAdo by only 18.1 +/- 2.5%. The results indicate that the A1 adenosine receptor is coupled more efficiently to an inhibition of beta-ICa,L than to an activation of IKAdo.
Srinivas
M
M
Department of Pharmacology, University of Florida, Gainesville, Florida 32610, USA.
Shryock
J C
JC
Dennis
D M
DM
Baker
S P
SP
Belardinelli
L
L
D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D006168:Guinea Pigs; D006325:Heart Atria / Q000187:drug effects* / Q000378:metabolism; D015221:Potassium Channels / Q000187:drug effects; D058906:Purinergic P1 Receptor Agonists; D018047:Receptors, Purinergic P1 / Q000378:metabolism*; D014970:Xanthines / Q000494:pharmacology
D015221:Potassium Channels; D058906:Purinergic P1 Receptor Agonists; D018047:Receptors, Purinergic P1; D014970:Xanthines; C100640:8-cyclopentyl-3-(3-((4-(fluorosulfonyl)benzoyl)oxy)propyl)-1-propylxanthine; D000241:Adenosine