A novel irreversible antagonist of the A₁-adenosine receptor

We determined the effects of 8-cyclopentyl-3-[3-[[4- (fluorosulfonyl)benzoyl]oxy]propyl]-1-propylxanthine (FSCPX), a putative irreversible antagonist of the A₁-adenosine receptor, on cardiac A₁- adenosine receptor-mediated responses and on the specific binding of [³H]- 8-cyclopentyl-1,3-dipropylxanthine ([³H]CPX) to guinea pig cardiac and brain membranes. FSCPX (5 μM) completely reversed the increase in K⁺ current of guinea pig atrial myocytes caused by 100 μM adenosine (259 ± 30 to 20 ± 7 pA) but had no significant effect on K⁺ currents caused by either 0.5 μM carbachol or 100 μM GTPγS. The attenuation of K⁺ current by FSCPX was both time and concentration dependent and persisted after washout of the antagonist. Pretreatment of atrial myocytes with FSCPX (50 nM) markedly attenuated the activation of K⁺ current and the inhibition of isoproterenol- stimulated I(Ca,L) caused by adenosine by 90.1% and 84.2%, respectively, but did not alter the responses of atrial myocytes to carbachol. FSCPX (1 μM) irreversibly antagonized the A₁-adenosine receptor-mediated increase in atrioventricular nodal conduction time of isolated perfused guinea pig hearts from 10.5 ± 0.5 to 0.7 ± 0.6 msec but did not significantly alter the A₂- adenosine receptor-mediated decrease in coronary resistance. Preincubation of guinea pig cardiac membranes with 0.1, 1.0, or 3.0 μM FSCPX for 30 min reduced the B(max) of [³H]CPX binding by 41 ± 10%, 67 ± 6%, and 80 ± 1% (mean ± standard error, three experiments), respectively, with no significant change in the K(d). Similarly, 0.1 and 1.0 μM FSCPX irreversibly reduced the binding of [³H]CPX to guinea pig forebrain membranes by 65 ± 5% and 83 ± 2% (four experiments), respectively, but did not reduce the binding of [³H]CGS 21680, an A(2a)-adenosine receptor agonist, to striatal membranes. FSCPX did not affect the potency of 5'-guanylylimidodiphosphate to inhibit the binding of [³H]CCPA, an A₁-adenosine receptor agonist, to brain membranes. The results indicate that FSCPX is a specific, irreversible, A₁- adenosine subtype-selective receptor antagonist.