Evidence of spare A1-adenosine receptors in guinea pig atrioventricular node

In normoxic, isolated perfused guinea pig hearts instrumented for measurement of atrioventricular nodal conduction time (AVCT), an analysis utilizing the irreversible A1-adenosine (Ado) antagonist, meta-1,3-phenylene diisothiocyanate xanthine amine cogener (m-DITC-XAC), a novel isothiocyanate derivative of 1,3-dialkylxanthine, was used to investigate whether spare A1-Ado receptors exist in the guinea pig atrioventricular (AV) node and the degree of amplification (reserve) between A1-Ado receptor occupancy and dromotropic response (e.g., AVCT slowing). The potency, dose dependency, and kinetic profile (time dependence of washout and washin) of m-DITC-XAC was determined and compared with those of known competitive (reversible) A1-Ado receptor antagonists. In the presence of m-DITC-XAC, Ado and N6-cyclopentyladenosine (CPA) produced submaximal dromotropic responses. In a series of 19 hearts, m-DITC-XAC caused 100% apparent antagonism of the effect of Ado on AVCT even after 60 min of washout. In contrast, greater than 90% of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and XAC-induced antagonism of the effect of Ado on AVCT dissipated within 35 min. Unlike XAC, which caused maximal attenuation of Ado's AVCT effect within 5 min and remained constant thereafter, m-DITC-XAC showed marked time- and concentration-dependent behavior. It was found that 5 min of 0.5 microM m-DITC-XAC pretreatment irreversibly inactivated 72% of the A1-Ado receptors mediating the dromotropic effect, and the estimated agonist equilibrium dissociation constant for CPA was 84 +/- 4 nM. The percent of spare A1-Ado receptors at the EC50 and extrapolated maximal S-H interval prolongation levels was 20 and 54%, respectively, and the reserve (coupling amplification) varied from 1 to 2.3 within the 0-50% maximal response range. In summary, m-DITC-XAC appears to specifically and irreversibly antagonize the negative dromotropic effect of Ado and CPA, and guinea pig AV nodal tissue possesses spare A1-Ado receptors.