Abstract

BACKGROUND
The functional and expressional changes of Gi proteins in diabetes have been investigated extensively, no agreement has been reached in the results. Moreover, studies using rats with different diabetic duration, and using subunits (Gialpha) of Gi proteins are lacking in literatures. Thus, we assessed the changes according to the duration of diabetes and examined the expressional changes of Gialphaand functional changes of Gi proteins in hepatocytes from streptozotocin-induced diabetic rats. METHODS: Male Sprague-Dawley rats were injected with streptozotocin to induce diabetes ; 1, 2, 3 and 5 weeks after the onset of diabetes, livers from the control and diabetic rats were fractionated into homogenate, interface, and plasma membrane. The levels of Gialpha1&2, Gialpha3 were quantified with western blots in each fraction. The functional changes of Gi proteins were evaluated by performing pertussis toxin-catalyzed ADP-ribosylation and measuring GTP S binding activity. RESULTS: 1) Gialpha2 and Gialpha3 were present mainly in the plasma membrane of hepatocytes in the diabetic and control rats, but the levels of these subunits were significantly higher in the diabetic rates than in the control rats (p<0.01). The levels of these subunits were not affected by the duration of diabetes. 2) In streptozotocin-induced diabetic rats, the levels of ADP-ribosylation of Gi proteins in liver plasma membranes decreased when pertussis toxin-catalyzed ADP-ribosylation was performed with liver tissues. However, the levels of these proteins were not affected by the duration of diabetes. 3) For the GTP S binding activity of Gi proteins in liver plasma membranes, the diabetic rats showed significantly less activity than the control rats (p<0.01). However, the activity was not affected by the duration of diabetes. The activity was somewhat restored by the insulin treatment of liver plasma membranes in diabetic rats. CONCLUSION: These results suggest that the insulin-deficient diabetic state induces the quantitative and functional changes in Gi proteins of hepatocytes regardless of the duration of diabetes. Therefore, these changes in Gi proteins may be the important compensatory reactions for the insulin resistance occurring in the insulin deficient state.

• Keywords: Gi protein;alpha-subunit;Diabetes mellitus;Hepatocyte