SPECIES COMPARISON OF ADENOSINE AND BETA-ADRENOCEPTORS IN MAMMALIAN ATRIAL AND VENTRICULAR MYOCARDIUM

The antagonist radioligand 1,3-H-3!dipropyl-8-cyclopentylxanthine (H -3!DPCPX) was used to characterize adenosine A1 receptors in membrane preparations from atrial and ventricular myocardium of rat, rabbit, gu inea 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 s pecies (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 ventri cles of guinea pig (76 and 34 fmol/mg), than in the other species (ran ges, 15 17 and undetectable to t2 fmol/mg). In contrast, guinea pig K( d) and B(max) values for beta-adrenoceptors, which were labelled with (-)3-I-125!iodocyanopindolol, fell within the range of values for the other three mammalian species. In semipurified membrane preparations from pig, H-3!DPCPX and the agonist radioligand I-125!-N6-4-aminoben zyladenosine appeared to label a similar population of receptors and g ave comparable K(d) values in atria (0.73 and 0.66 nM) and in ventricl es (0.57 and 0.70 nM). In semipurified preparations from pig, the agon ist R-(-)-N6-(2-phenylisopropyl)adenosine (R-PIA) displaced H-3!DPCPX in a manner consistent with the presence of both high- and low-affini ty adenosine A1 receptors. The data from this study indicate that the density of adenosine A1 receptors in atria is greater than in ventricl es, but similar K(d) values suggest that the A1 receptor population is the same in the two cardiac tissues. Also, the data demonstrate that the H-3!DPCPX antagonist binding characteristics of guinea pig myocar dium differ from those in rat, rabbit and pig.