NORMAL HEPATIC GLUCOSE-PRODUCTION IN THE ABSENCE OF GLUT2 REVEALS AN ALTERNATIVE PATHWAY FOR GLUCOSE-RELEASE FROM HEPATOCYTES

Glucose production by liver is a major physiological function, which i s required to prevent development of hypoglycemia in the postprandial and fasted states. The mechanism of glucose release from hepatocytes h as not been studied in detail but was assumed instead to depend on fac ilitated diffusion through the glucose transporter GLUT2, Here, we dem onstrate that in the absence of GLUT2 no other transporter isoforms we re overexpressed in liver and only marginally significant facilitated diffusion across the hepatocyte plasma membrane was detectable. Howeve r, the rate of hepatic glucose output was normal. This was evidenced b y (i) the hyperglycemic response to i.p. glucagon injection; (ii) the in vivo measurement of glucose turnover rate; and (iii) the rate of re lease of neosynthesized glucose from isolated hepatocytes. These obser vations therefore indicated the existence of an alternative pathway fo r hepatic glucose output. Using a [C-14]-pyruvate pulse-labeling proto col to quantitate neosynthesis and release of [C-14]glucose, we demons trated that this pathway was sensitive to low temperature (12 degrees C), It was not inhibited by cytochalasin B nor by the intracellular tr affic inhibitors brefeldin A and monensin but was blocked by progester one, an inhibitor of cholesterol and caveolae traffic from the endopla smic reticulum to the plasma membrane. Our observations thus demonstra te that hepatic glucose release does not require the presence of GLUT2 nor of any plasma membrane glucose facilitative diffusion mechanism. This implies the existence of an as yet unsuspected pathway for glucos e release that may be based on a membrane traffic mechanism.