COUNTERREGULATION OF INSULIN'S ACTION BY CATECHOLAMINES

Project: Research project

Project Details

Description

Insulin-stimulated glucose transport activity in rat adipocytes is
inhibited by isoproterenol and enhanced by adenosine. Both of these
effects occur without corresponding changes in the subcellular
distribution of the GLUT4 glucose transporter isoform. In this report, we
have utilized the impermeant, exofacial bis-mannose glucose transporter-
specific photolabel, 2-N-4- (1-azi-2,2,2-trifluoroethyl)benzoyl-1, 3-bis
(D-mannos-4-yloxy) -2-propylamine (ATB-BMPA), to examine the cell surface
accessibility of GLUT4 glucose transporters under these conditions.
compared to cells treated with insulin alone, adenosine in the presence of
insulin increases the accessibility of GLUT4 to the extracellular
photolabel by approximately 25% consistent with its enhancement of
insulin-stimulated glucose transport activity; the plasma membrane
concentration of GLUT4 as assessed by Western blotting is unchanged.
Conversely, isoproterenol, in the absence of adenosine, promotes a time-
dependent (t[1/2] equals approximately 2 min) decrease in the
accessibility of insulin-stimulated cell surface GLUT4 by >50%, which
directly correlates with the observed inhibition of transport activity;
the plasma membrane concentration of GLUT4 decreases by 0-15%.
Photolabeling the corresponding plasma membranes revealed that these
alterations in the ability of the photolabel to bind to GLUT4 are
transient as the levels of both photolabel incorporation and plasma
membrane glucose transport activity were consistent with the observed
GLUT4 concentration. These data suggest that insulin-stimulated GLUT4
glucose transporters can exist in two distinct states within the adipocyte
plasma membrane, one functional and accessible to extracellular substrate
and one non-functional and unable to bind extracellular substrate. These
effects are only observed in the intact adipocyte and are not retained in
isolated plasma membranes isolated from these cells when analyzed for the
ability to transport glucose or bind photolabel.
StatusNot started