CHARACTERIZATION OF ADENOSINE RECEPTORS IN INTACT CULTURED HEART-CELLS

Adenosine receptors were studied on heart cells grown in cultures by t he radioligand binding technique. We used the hydrophilic A(1) adenosi ne receptor radioligand [H-3]-8-cyclopentyl-1,3-dipropylxanthine ([H-3 ]CPX), to monitor the level of the receptors on intact cardiocytes. Th e binding showed high affinity (K-d = 0.13 nM) and the number of [H-3] CPX binding sites (B-max) was 23.1 fmol/dish (21 fmol/mg protein). The K-i of the agonists R-N-6-(2-phenylisopropyl)-adenosine (R-PIA) and S -N-6-(2-phenylisopropyl)-adenosine (S-PIA), and of the antagonists CPX and theophylline were 3.57, 49.0, 1.63 and 4880 nM, respectively. The number of adenosine receptors was very low during the first days in c ultures (5 fmol/dish) and increased gradually until it reached a plate au on days 8-10. Treatment with norepinephrine or isoproterenol which accelerated the rate of contractions, induced up regulation of the rec eptors. B-max increased 2-3 fold by application of norepinephrine for 4 days, while receptor affinity to the radioligand was unaffected. Lac tate dehydrogenase (LDH) and creatine kinase (CK) activity increased o nly by 22 and 38%, respectively. Similarly, 3 days treatment with trii odothyronine (T-3, 10(-8) M), which also accelerated heart rate, incre ased the number of adenosine receptors by 56% without a significant ch ange in the affinity of the receptors to [H-3]CPX. Carbamylcholine (5 x 10(-6) M), which reduced the rate of heart contractions, caused 26% down regulation while the affinity of the receptors remained unchanged . It is concluded that there is a linkage between the rate of cardiac contractions and the level of adenosine receptors. Thus, the level of adenosine receptors may respond to drug-induced chronic changes in car diac contractile activity so as to restore conditions to normal (basal ) contractions.