Evidence for Impaired Coupling of Receptors to G_i Protein in Adipocytes From Streptozocin-Induced Diabetic Rats

Adenosine and prostaglandins of the E series inhibit lipolysis in adipocytes by binding to cell surface receptors. This inhibition is mediated via G_i. It has been reported that G_i is almost absent in livers from diabetic rats. Therefore, we have evaluated the sensitivity of adipocytes from diabetic rats to the adenosine analogue N⁶-phenylisopropyl adenosine (PIA) and to prostaglandin E₁ (PGE₁). Diabetes was induced with streptozocin (65 mg/kg i.v.), and after 7 days, adipocytes were isolated. Lipolysis (measured in the presence of adenosine deaminase) was inhibited by PIA and PGE, in both control and diabetic cells. However, the dose-response curves were markedly shifted to the right in the cells from diabetic rats. The IC₅₀ for PIA was 0.30 ± 0.02 nM in controls and 0.83 ± 0.08 in diabetic rats (P < 0.001), and the IC₅₀ for PGE, was 3.16 ± 0.18 nM in controls and 5.26 ± 0.57 nM in diabetic rats (P < 0.02). These findings indicate decreased sensitivity to both adenosine and PGE₁. Adipocyte membranes were isolated from control and diabetic rats. Adenosine receptors (measured by binding of ¹²⁵I-labeled hydroxy-PIA) were not altered in cells from diabetic rats. However, the ability of Gpp(NH)p (a nonhydrolyzable GTP analogue) to inhibit adenosine-receptor binding was markedly decreased in membranes from diabetic rats, suggesting a change at the level of G₁. The α-subunits of G₁₁, G₁₂, G₁₃, and G_s were quantitated on Western blots with a series of recently characterized anti-peptide antisera. This revealed that the amounts of each of these G proteins were normal in membranes from the diabetic rats. These findings suggest that there is a functional but not quantitative abnormality in G₁ in adipocytes from diabetic rats that results in abnormal coupling of both A₁-adenosine receptors and prostaglandin receptors and would explain the decreased sensitivity to PIA and PGE₁.