Non-insulin dependent diabetes mellitus (NlDDM) is a major cause of morbidity and mortality. Insulin resistance of peripheral tissues, primarily muscle, and of the liver is a prominent feature of NIDDM. The mechanism of this insulin resistance is not clear. The experiments proposed here will address the question of whether in vivo insulin sensitivity and glucose homeostasis can be altered by increasing glucose transport into muscle or by increasing glucokinase activity in the liver. Glucose transport in muscle and hepatic glucokinase in liver will be increased by insertion of transgenes in mice. Work already in progress has shown that increasing glucose transport into muscle by overexpressing Glut 1 or Glut4 glucose transporters in the skeletal muscle of transgenic mice lowers the circulating blood glucose and increases transport of 2- deoxyglucose into isolated muscle. Preliminary data using hyperinsulinemic, euglycemic clamps suggests that overexpressing Glut4 increases insulin sensitivity in muscle in vivo while overexpressing Glut1 decreases insulin sensitivity under the same conditions. The studies proposed here will develop and validate the hyperinsulinemic, euglycemic clamp technique in combination with glucose tracer methods for use in assessing insulin sensitivity of the liver and peripheral tissues in the mouse. Normal murine glucose metabolism will be evaluated initially so that baseline data will be available for evaluation of the effects of the proposed transgenic alterations. Then, clamp studies will be carried out in the Glut1 and Glut4 transgenic mice to evaluate their insulin sensitivity. other studies will look for mechanisms of the insulin resistance in the Glut1 transgenic mice. Possible mechanisms of insulin resistance in these mice include failure of translocation of endogenous Glut4, impairment of glucose transport or glycogen synthesis due to a large existing muscle glycogen mass, and adverse effects of glucosamine. To address the issue of hepatic insulin sensitivity, a new line of transgenic mice will be produced which will overexpress glucokinase in liver. These mice will be studied using glucose clamp and tracer techniques to evaluate the impact of an increase in glucokinase on hepatic insulin sensitivity.
