Adenosine, via interaction with A1 adenosine receptors, increases insulin sensitivity and inhibits lipolysis in adipocytes. To investigate regulation of this system, adipocytes were incubated for up to 72 h with the nonmetabolizable adenosine receptor agonist, N6-phenylisopropyl adenosine (PIA). Adenosine receptors were measured by the binding of 125I-hydroxyphenylisopropyl adenosine to membranes. PIA down-regulated adenosine receptors, decreasing the number of binding sites with no change in affinity. Adipocytes were incubated for 48 h without or with 100 nM PIA to down-regulate the A1 receptors by approximately 60%. The cells were washed, and lipolysis and glucose transport were assessed. The ability of PIA to inhibit lipolysis was markedly attenuated in the down-regulated cells. Furthermore, the EC50 of insulin was increased approximately 3-fold in the PIA-treated cells. 125I-Insulin binding to the PIA-treated cells was unchanged, demonstrating that the decreased insulin sensitivity is not due to decreased insulin receptor binding. Pertussis toxin catalyzed ADP-ribosylation of a 41-kDa protein thought to be the alpha-subunit of Gi. This 41-kDa protein was decreased in membranes from cells treated with PIA, with a maximal 50% loss. This suggests that Gi is down-regulated and that loss of both the A1 adenosine receptor and Gi are involved in the metabolic changes observed after PIA treatment.