The importance of A(1)-adenosine receptor reserve in the heart

In guinea pig atrial myocytes, the A{dollar}sb1{dollar}-adenosine receptor (A{dollar}sb1{dollar}AdoR) is coupled to an activation of an inwardly-rectifying current, I{dollar}rmsb{lcub}KAdo{rcub}{dollar} and to an inhibition of {dollar}beta{dollar}-adrenergic receptor-mediated increases in I{dollar}rmsb{lcub}Ca,L{rcub}{dollar} ({dollar}beta{dollar}-{dollar}rm Isb{lcub}Ca,L{rcub}).{dollar} The EC{dollar}sb{lcub}50{rcub}{dollar} values of adenosine to activate {dollar}rm Isb{lcub}KAdo{rcub}{dollar} and to inhibit {dollar}beta{dollar}-{dollar}rm Isb{lcub}Ca,L{rcub}{dollar} are 2.1 and 0.2 {dollar}mu{dollar}M, respectively. The objectives of this study are to determine if (1) the 11-fold difference in potency of adenosine is due to a greater receptor reserve for the inhibition of {dollar}beta{dollar}-{dollar}rm Isb{lcub}Ca,L{rcub}{dollar} than for the activation of {dollar}rm Isb{lcub}KAdo{rcub}{dollar} and (2) a partial A{dollar}sb1{dollar}AdoR agonist, SHA040, selectively activates the response with a large receptor reserve. Receptor reserve was estimated by use of a flurosulfonyl derivative (FSCPX) of 8-cyclopentyl-1,3-dipropylxanthine, an irreversible A{dollar}sb1{dollar}AdoR antagonist that caused both an irreversible attenuation of cardiac A{dollar}sb1{dollar}AdoR-mediated responses in a selective and specific manner and a concentration-dependent reduction of A{dollar}sb1{dollar}AdoR density of cardiac membranes. Atrial myocytes were pretreated with either vehicle (control) or FSCPX (10 and 50 nM) for 30 min, and after a 60 min washout period, concentration-response curves for the adenosine-induced activation of {dollar}rm Isb{lcub}KAdo{rcub}{dollar} and inhibition of {dollar}beta{dollar}-{dollar}rm Isb{lcub}Ca,L{rcub}{dollar} were determined. Pretreatment of myocytes with 10 nM FSCPX reduced the maximal activation of {dollar}rm Isb{lcub}KAdo{rcub}{dollar} by 60%. In contrast, a higher concentration of FSCPX (50 nM) was required to reduce the maximal inhibition of {dollar}beta{dollar}-{dollar}rm Isb{lcub}KAdo{rcub}{dollar} by 39%. Values of equilibrium dissociation constant for adenosine to activate {dollar}rm Isb{lcub}KAdo{rcub}{dollar} and to inhibit {dollar}beta{dollar}-{dollar}rm Isb{lcub}Ca,L{rcub},{dollar} estimated using Furchgott's method, were 2.7 {dollar}mu{dollar}M and 5.6 {dollar}mu{dollar}M, respectively. Occupancies of 40% of A{dollar}sb1{dollar}AdoRs was required to cause half-maximal activation of {dollar}rm Isb{lcub}KAdo{rcub}{dollar} whereas occupancies of only 4% A{dollar}sb1{dollar}AdoRs were sufficient to cause half-maximal inhibition of {dollar}beta{dollar}-{dollar}rm Isb{lcub}Ca,L{rcub}.{dollar}; Consistent with this result, SHA040, inhibited {dollar}beta{dollar}-{dollar}rm Isb{lcub}Ca,L{rcub}{dollar} by 60 {dollar}pm{dollar} 3.5% but activated {dollar}rm Isb{lcub}KAdo{rcub}{dollar} by only 18.1 {dollar}pm{dollar} 2.5%. Similarly, reduction of the atrial action potential duration by SHA040 was greater in the presence (69 {dollar}pm{dollar} 2.3%) than in the absence (15.7 {dollar}pm{dollar} 5.6%) of ISO. The results indicate that (1) there is a larger A{dollar}sb1{dollar}AdoR reserve for adenosine to inhibit {dollar}beta{dollar}-{dollar}rm Isb{lcub}Ca,L{rcub}{dollar} than to activate {dollar}rm Isb{lcub}KAdo{rcub}{dollar} (2) a partial agonist of the A{dollar}sb1{dollar}AdoR, SHA040 selectively activates the response with the large receptor reserve.