Pharmacological characterization of A1 adenosine receptors in isolated rat ventricular myocytes

Summary The aim of the present study was the characterization of adenosine receptors in isolated rat ventricular myocytes. The CAMP-levels of rat ventricular myocytes in the presence of 1 mol/l isoprenaline were reduced by up to 48% by adenosine analogues; the rank order of potency was: R-N⁶-phenylisopropyladenosine (IC₅₀ 60 nmol/1), 5-N-ethylcarboxamidoadenosine (IC₅₀ 360 nmol/l) and S-N⁶-phenylisopropyladenosine (IC₅₀ 16 ol/l). The adenosine receptor antagonist XAC (xanthine amine congener) antagonized the effect of R-N⁶-phenylisopropyladenosine in a concentration-dependent manner with a K_i-value of 20 nmol/l. The A₁ receptor-selective radioligand R-N⁶-¹²⁵I-p-hydroxyphenylisopropyladenosine bound to membranes prepared from rat ventricular myocytes in a saturable manner with a B_max of 17.7 fmol/mg protein and a K_D-value of 1.1 nmol/l. Adenosine analogues competed for the binding with the same rank order of potency as for the inhibition of the isoprenaline-induced cAMP-increase. GTP inhibited radioligand binding with an IC₅₀-value of 73 ol/l. These results suggest the presence of A₁ adenosine receptors on rat ventricular myocytes, which mediate an inhibition of adenylate cyclase. The receptors may be responsible for the effects of adenosine and its analogues on the heart.Abbreviations ¹²⁵I-HPIA R-N⁶-¹²⁵I-p-hydroxyphenylisopropyladenosine - PIA N⁶-phenylisopropyladenosine - NECA 5-N-ethyl-carboxamidoadenosine - XAC 8-4-[([(2-aminoethyl)aminocarbonyl]methyl)oxy]phenyl-1,3-dipropylxanthine (xanthine amine congener) - Ro 20-1724 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone - ScAMPTME 2-O-monosuccinyladenosine-3,5-cyclic monophosphate tyrosyl methyl ester - HEPES N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - GTP guanosine-5-tri-phosphateSend offprint requests to D. Martens