Abstract
The effects of insulin on insulin-sensitive phosphodiesterase were investigated in fat cells from control (Eight weeks old) and obese (32 weeks old) rats. Isolated fat cells were incubated at 37° for 10 minutes with and without insulin. A crude microsomal fraction prepared by differential centrifugation was assayed for phosphodiesterase activity. The enzyme activities in obese rats were lower at 0.1-30 nM insulin concentrations than in control rats (p<0.001). The dose-response curve of insulin was biphasic and of the convex type in both groups. In obese rats, the curve shifted tothe right, and half-maximum stimulation was obtained at 0.7 nM in obese rats compared to 0.17 nM in control rats (p<0.001), indicating a decrease of insulin sensitivity. Insulin responsiveness, expressed as percent of basal enzyme activity, was also markedly reduced in obese rats. Specific binding of insulin in fat cells from control and obese rats was 4.8% and 13.0%/2×105 cells, respectively, at 24° for 60 minutes of incubation. Scatchard analysis indicated that overall insulin receptor in fat cells from obese rats was greater than in fat cells from control rats. These results suggest that the insulin effector system related to the phosphodiesterase activation is affected in fat cells from spontaneously obese rats and that the inpairment of the phosphodiesterase activation system is predominantly due to postreceptor defects.
