Abstract
Adenosine has been shown to have negative inotropic, chronotropic
and dromotropic effects on the heart. The pharmacological profiles
of these effects suggest that they are mediated via Ri (A1) adenosine
receptors, but a direct demonstration of these receptors is still
missing. In the present study we report direct labelling of these
receptors with (-)N6-125I-p-hydroxyphenylisopropyladenosine (
125IHPIA)1. The radioligand bound in a saturable and reversible
manner to a crude membrane preparation, the Bmax-value was 30.5 fmol/mg
protein and the KD-value 1.1 nmol/l. A similar affinity of the ligand
was obtained in kinetic and competition experiments. Competition
experiments with a variety of adenosine analogues gave a pharmacological
profile characteristic of Ri adenosine receptors with high affinities
of N6-substituted derivatives and a marked stereospecificity for
N6-phenylisopropyladenosine (PIA). Purification of the membrane preparation
by density gradient centrifugation resulted in a 30-fold increase
in the number of binding sites which was paralleled by a similar
increase in the number of binding sites for 3Houabain. Guanine
nucleotides decreased binding of 125IHPIA in a dose-dependent manner,
but the IC50-values were considerably higher than those reported
in other tissues. Finally, binding of 125IHPIA appeared to be entropy-driven
which has been shown to be characteristic of agonist binding to Ri
adenosine receptors. These results suggest the presence of Ri adenosine
receptors in ventricular myocardium which may be responsible for
the mediation of the effects of adenosine and its analogues.
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