STIMULATION OF GLUCOSE-TRANSPORT IN CLONE-9 CELLS BY INSULIN AND THYROID-HORMONE - ROLE OF GLUT-1 ACTIVATION

Thyroid hormone (T-3) and insulin are both shown ro stimulate glucose transport in Clone 9 cells, a rat liver sell line in which the utiliza tion of glucose is limited by transport rate and in which only tile GL UT-1 transporter isoform is expressed. Pre-treatment of these cells wi th T-3 moreover substantially enhances the stimulatory effect of insul in such that at maximally effective hormone concentrations the effects of T-3 and insulin on glucose transport are more than aciditive and i ndeed nearly multiplicative suggesting that the mechanisms mediating t he enhancement of glucose transport differ between the two hormones. t ell surface biotinylation followed by Western-blot analysis of plasma membrane fractions showed that the stimulatory effects of T-3 and insu lin on glucose transporter, whether acting singly or in combination, e xceed the attendant increases in the abundance of GLUT-I is the plasma membrane. Ir is suggested that activation of GLUT-I molecules pre-exi sting in the plasma membrane plays a major role ill mediating the stim ulatory effects of T-3 and insulin on glucose transport in this cell l ine.