Study of adenosine receptors in intact rat fat cells by radioligand binding

Binding of (-)N6-phenylisopropyl[3H]adenosine ([3H]PIA) to intact rat fat cells was studied in the presence of the adenosine uptake blocker dipyridamole. Specific binding of 5 nmol/l [3H]PIA at 37 degrees C was rapid, reversible and dependent on cell concentration and the presence of adenosine deaminase. Saturability of specific binding was not achieved at concentrations up to 200 mumol/l [3H]PIA. In competition experiments (-)PIA (IC50 42 nmol/l) was the most potent analogue, followed by 2-chloroadenosine and 5'-N-ethylcarboxamidoadenosine. Binding of [3H]PIA was stereospecific, since (-)PIA was 200 times more potent than (+)PIA. The adenosine antagonist theophylline inhibited binding with an IC50 of 16.9 mumol/l, whereas adenine, inosine and GTP did not affect binding. The results allow us to describe several characteristics of [3H]PIA binding to intact fat cells but a considerable component of nonreceptor binding impedes a detailed study of adenosine receptors under physiological conditions.