Insulin-sensitive phosphodiesterase and insulin receptor binding in fat cells from spontaneously obese rats

The effects of insulin on insulin-sensitive phosphodiesterase were investigated in fat cells from rats aged 4, 8 and 16 weeks. The enzyme activities in rats aged 4 and 8 weeks higher at 0.1-30 nmol/l insulin concentrations than in rats aged 16 weeks, and half-maximum stimulations were obtained at 0.08 nmol/l in rats aged 4 weeks, at 0.15 nmol/l in rats aged 8 weeks and at 0.22 nmol/l in rats aged 16 weeks. Specific binding of insulin in fat cells from rats aged 4, 8 and 16 weeks was 3.3%, 5.0% and 11.6%/2 X 10(5) cells, respectively. Scatchard analysis indicated that increased insulin binding in fat cells from rats aged 16 weeks was due mainly to an increase of binding affinity. These results suggest that impairment of the phosphodiesterase activation system in fat cells from spontaneously obese rats is predominantly due to post-receptor defects.