We used isolated islets of lean and obese Zucker rats to determine whether inhibitory pathways mediated by pertussis toxin-sensitive guanyl nucleotide-binding (Gi) proteins contribute to hyperinsulinemia in obese rats. Epinephrine (10(-4) M) and somatostatin (10(-7) M) inhibited insulin secretion by +/- 75% in lean and fa/fa rats. Overnight culture of islets with pertussis toxin (300 ng/ml) enhanced insulin release more in lean (+/- 120%) than obese (+/- 60%) rats. In lean rats incubation of pertussis toxin-treated islets with epinephrine resulted in lower immunoreactive insulin release (p = 0.0005) than pertussis toxin-treated islets without epinephrine. However, in obese rats pertussis toxin treatment reversed this inhibition. Pertussis toxin completely reversed inhibition by somatostatin in both phenotypes. Galanin had no effect on insulin secretion. Cellular cAMP content was similar in lean and obese rats. Inhibitory hormones had no effect on cAMP production. We conclude that islets of obese rats respond normally to inhibitors of insulin release. Reversal of somatostatin-induced inhibition by pertussis toxin indicates normal function of Gi in obese rats. A subtle difference in sensitivity to pertussis toxin between lean and obese islets was noted.