Abstract 15256: Sarcoplasmic Reticulum Calcium Handling Regulates Glucose Transporter -4, but Not -12, Trafficking in the Healthy and Diabetic Myocardium
The heart is one of the main organs to utilize glucose. Contraction/calcium (Ca) dependent processes are major regulators of cardiac glucose transport. Glucose uptake is regulated by a family of membrane proteins called glucose transporters (GLUTs). Although GLUT4 is the major isoform, GLUT12, one of the most recently identified GLUT isoforms, could play a crucial role in the regulation of glucose uptake in the heart. While we previously demonstrated that GLUT2 trafficking is not mediated by insulin, its regulation by Ca-mediated signaling is unknown. We hypothesized that, as for GLUT4, sarcoplasmic reticulum (SR) Ca handling regulates GLUT12 translocation to the myocardial cell surface in healthy and diabetic (Dx) state. Active cell surface GLUT (4 & 12) content was quantified by a biotinylated photolabeled assay in both intact perfused myocardium and isolated cardiac myocytes of healthy and type 1 (streptozotocin-induced) diabetic rodents. GLUT localization was confirmed by immunofluorescent confocal microscopy. Total GLUT protein expression was measured by Western blotting. In healthy myocytes, incubation with insulin or Ca increased translocation of GLUT-4, but not -12, in the healthy myocardium. In addition, incubation with caffeine, which stimulates SR Ca release, increased cell surface GLUT-4, but not -12, content; while incubation with a SR Ca ATPase (SERCA) pump inhibitor (thapsigargin) or with a Ca chelator (BAPTA) decreased cell surface GLUT-4, but not -12, content. We then used 4 groups of mice (n=4-10/group): healthy transgenic (TG) mice overexpressing the SERCA1a pump in the heart and their wildtype (WT), and Dx TG mice and their Dx WT littermates. SERCA1a overexpression, which induced faster SR Ca kinetics, significantly increased cell surface GLUT-4, but not -12, content in the perfused heart of healthy TG vs. WT mice. Diabetes increased cardiac cell surface GLUT12 content in WT mice (as a potential compensatory mechanism for the observed downregulation of active GLUT4), with no further change in Dx TG mice overexpressing the SERCA pump. In conclusion, our data suggest that: 1) Ca signaling pathway mediates myocardial GLUT4, but not GLUT12, trafficking; 2) GLUT12 functions as a basal GLUT in the heart located primarily at the cell surface.
- © 2013 by American Heart Association, Inc.