Species comparison of adenosine and β-adrenoceptors in mammalian atrial and ventricular myocardium

Abstract The antagonist radioligand 1,3-[ 3 H]dipropyl-8-cyclopentylxanthine ([ 3 H]DPCPX) was used to characterize adenosine A 1 receptors in membrane preparations from atrial and ventricular myocardium of rat, rabbit, guinea pig and pig. K d values in crude membranes from guinea pig atria and ventricles (3.3 and 3.0 nM) were higher than those in the other species (ranges, 1.5–1.8 and 1.5–1.9 nM). B max values were greater in atria than in ventricles in all four species, and in atria and ventricles of guinea pig (76 and 34 fmol/mg), than in the other species (ranges, 15–17 and undetectable to 12 fmol/mg). In contrast, guinea pig K d and B max values for β-adrenoceptors, which were labelled with (−)3-[ 125 I]iodocyanopindolol, fell within the range of values for the other three mammalian species. In semipurified membrane preparations from pig, [ 3 H]DPCPX and the agonist radioligand [ 125 I]-N 6 -4-aminobenzyladenosine appeared to label a similar population of receptors and gave comparable K d values in atria (0.73 and 0.66 nM) and in ventricles (0.57 and 0.70 nM). In semipurified preparations from pig, the agonist R-(−)-N 6 -2(-phenylisopropyl)adenosine (R-PIA) displaced [ 3 H]DPCPX in a manner consistent with the presence of both high- and low-affinity adenosine A 1 receptors. The data from this study indicate that the density of adenosine A 1 receptors in atria is greater than in ventricles, but similar K d values suggest that the A 1 receptor population is the same in the two cardiac tissues. Also, the data demonstrate that the [ 3 H]DPCPX antagonist binding characteristics of guinea pig myocardium differ from those in rat, rabbit and pig.