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Persistent Activation by and Receptor Reserve for an Irreversible A 1 -Adenosine Receptor Agonist in DDT 1 MF-2 Cells and in Guinea Pig Heart

1997; American Society for Pharmacology and Experimental Therapeutics; Volume: 52; Issue: 3 Linguagem: Inglês

10.1124/mol.52.3.491

1521-0111

Jiahui Zhang, Luiz Belardinelli, Kenneth A. Jacobson, Deborah H. Otero, Stephen P. Baker,

Pharmacological Receptor Mechanisms and Effects

The p - and m -isothiocyanate adenosine derivatives N 6 -[4-[[[4-[[[[2-[[[( p -( m )-isothiocyanatophenyl)amino]thiocarbonyl]amino]ethyl]amino]carbonyl]methyl]anilino]carbonyl]methyl]phenyl]adenosine ( p - and m -DITC-ADAC) were examined for irreversible agonist effects at the A 1 -adenosine receptor (A 1 -AdoR) in DDT 1 MF-2 (DDT) cells and a functional A 1 -AdoR response in the guinea pig isolated heart. The p - and m -DITC-ADAC inhibited (−)-isoproterenol stimulated cAMP accumulation in DDT cells in the low nanomolar range, and the maximal responses elicited by both compounds were similar to that for N 6 -cyclopentyladenosine. Once established, the p -DITC-ADAC-mediated inhibition of cAMP accumulation in DDT cells was not affected by the addition of the AdoR antagonist 8-cyclopentyl-1,3-dipropylxanthine (CPX). Pretreatment of DDT cells with p -DITC-ADAC (1 μm), followed by washing, reduced [ 3 H]CPX binding to the A 1 -AdoR by 44% without altering the K d value for the radioligand to the remaining receptors. The relationship between irreversible A 1 -AdoR occupancy by p -DITC-ADAC and inhibition of cAMP accumulation revealed a relatively large receptor reserve (64%) for the maximal response. In guinea pig isolated hearts, m -DITC-ADAC (5 μm) prolonged the stimulus to His bundle (SH) interval by 2.1-fold; this response could be prevented by the antagonist 8-cyclopentyltheophylline (5 μm). However, after the SH interval prolongation was established, extensive washout or the addition of 8-cyclopentyltheophylline had little reversal effect on the m -DITC-ADAC response. Binding of [ 3 H]CPX to the guinea pig ventricular membranes after m -DITC-ADAC treatment and washing was reduced by 35%. The A 1 -AdoR occupancy response relationship for m -DITC-ADAC to prolong the SH interval indicated a small (10–20%) receptor reserve. Both p -and m -DITC-ADAC seem to be irreversible full agonists at the A 1 -AdoR and may prove to be useful probes to further investigate A 1 -AdoR structure-function relationships.