Evidence for regulated coupling of A1 adenosine receptors by phosphorylation in Zucker rats

Leptin-induced lipolysis opposes the tonic inhibition of endogenous adenosine in white adipocytes

The aim of the present study was to gain insight into the signaling pathway used by leptin to stimulate lipolysis. The lipolytic rate of white adipocytes from sex- and age-matched lean (+/+) and fa/fa rats was determined in the absence or presence of leptin together with a number of agents acting at different levels of the signaling cascade. Leptin did not modify FSK-, dbcAMP-, and IBMX-stimulated lipolysis. Lipolysis can also be maximally stimulated by lowering media adenosine levels with adenosine deaminase (ADA), i.e., in the ligand-free state. Although ADA produced near maximal lipolysis in adipocytes of lean animals, only half of the maximal lipolytic rate (50.9+/-3.2%) was achieved in fat cells from fa/fa rats (P=0.0034). In adipocytes from lean animals preincubated with ADA, leptin caused a concentration-related stimulation of lipolysis (P=0.0001). However, leptin had no effect on the lipolytic activity of adipocytes in the ligand-free state from fa/fa rats. The adenosine A1 receptor agonist CPA effectively inhibited basal lipolysis in both lean and obese adipocytes (P=0.0001 and P=0.0090, respectively). Leptin had no effect on the lipolytic rate of adipocytes isolated from fa/fa rats and preincubated with CPA. When adipocytes were incubated with the A1 receptor antagonist DPCPX, a significant increase in glycerol release was observed in fa/fa fat cells (P=0.009), whereas cells isolated from lean rats showed no differences to ADA-stimulated lipolysis. After pretreatment with PTX, which inactivates receptor-mediated Gi function, adipocytes of obese rats became as responsive to the stimulatory actions of ISO as cells from lean rats (P=0.0090 vs. ISO in fa/fa rats; P=0.2416 vs. lean rats, respectively). PTX treatment of lean cells, however, did not alter their response to this lipolytic agent. It can be concluded that the lipolytic effect of leptin is located at the adenylate cyclase/Gi proteins level and that leptin-induced lipolysis opposes the tonic inhibition of endogenous adenosine in white adipocytes.

Tissue-specific effects of in vivo adenosine receptor blockade on glucose uptake in Zucker rats

Previous studies have shown that treatment of obese Zucker rats with the adenosine receptor antagonist 1,3-dipropyl-8-(p-acrylic) phenyl xanthine (BWA1433) improves intraperitoneal glucose tolerance. In this study, a euglycemic hyperinsulinemic clamp was performed on obese (fa/fa) and lean (Fa/fa) Zucker rats that had been treated orally with BWA1433 or vehicle for 1 wk. A constant infusion of [3H]glucose was initiated in fasted animals to measure basal whole body glucose kinetics. No differences in glucose concentration or rates of glucose production/disappearance were observed between lean or obese animals with or without BWA1433. During the euglycemic hyperinsulinemic clamp, whole body glucose disposal in obese Zucker rats was only 22% of that observed in lean animals. BWA1433 treatment increased glucose disposal by 88% in obese Zucker rats. At the end of the clamp, [14C]-2-deoxyglucose was injected to determine tissue-specific differences in glucose uptake. Gastrocnemius, soleus, heart, and liver of untreated obese animals had significantly lower glucose uptake than lean controls under hyperinsulinemic conditions. BWA1433 treatment of obese animals increased glucose uptake in gastrocnemius and soleus muscles by 44 and 47%, respectively. Conversely, BWA1433 treatment decreased glucose uptake in adipose tissue by 54 and 49% in obese and lean Zucker rats, respectively. In summary, BWA1433 improves glucose tolerance by increasing glucose uptake in skeletal muscle while decreasing glucose uptake by adipose tissue. This study suggests that insulin resistance in obese Zucker rats is tissue specific and that signaling from adenosine receptors may be a factor contributing to tissue-specific insulin resistance.