Targeted GLUT-4 deficiency in the heart induces cardiomyocyte hypertrophy and impaired contractility linked with Ca(2+) and proton flux dysregulation.
There is clinical evidence to suggest that impaired myocardial glucose uptake contributes to the pathogenesis of hypertrophic, insulin-resistant cardiomyopathy. The goal of this study was to determine whether cardiac deficiency of the insulin-sensitive glucose transporter, GLUT4, has deleterious effect on cardiomyocyte excitation-contraction coupling. Cre-Lox mouse models of cardiac GLUT4 knockdown (KD, 85% reduction) and knockout (KO, >95% reduction), which exhibit similar systemic hyperinsulinemic and hyperglycemic states, were investigated. The Ca(2+) current (I(Ca)) and Na(+)-Ca(2+) exchanger (NCX) fluxes, Na(+)-H(+) exchanger (NHE) activity, and contractile performance of GLUT4-deficient myocytes was examined using whole-cell patch-clamp, epifluorescence, and imaging techniques. GLUT4-KO exhibited significant cardiac enlargement characterized by cardiomyocyte hypertrophy (40% increase in cell area) and fibrosis. GLUT4-KO myocyte contractility was significantly diminished, with reduced mean maximum shortening (5.0+/-0.4% vs. 6.2+/-0.6%, 5 Hz). Maximal rates of shortening and relaxation were also reduced (20-25%), and latency was delayed. In GLUT4-KO myocytes, the I(Ca) density was decreased (-2.80+/-0.29 vs. -5.30+/-0.70 pA/pF), and mean I(NCX) was significantly increased in both outward (by 60%) and inward (by 100%) directions. GLUT4-KO expression levels of SERCA2 and RyR2 were reduced by approximately 50%. NHE-mediated H(+) flux in response to NH(4)Cl acid loading was markedly elevated GLUT4-KO myocytes, associated with doubled expression of NHE1. These findings demonstrate that, independent of systemic endocrinological disturbance, cardiac GLUT4 deficiency per se provides a lesion sufficient to induce profound alterations in cardiomyocyte Ca(2+) and pH homeostasis. Our investigation identifies the cardiac GLUT4 as a potential primary molecular therapeutic target in ameliorating the functional deficits associated with insulin-resistant cardiomyopathy.
Domenighetti
Andrea A
AA
Department of Physiology, University of Melbourne, Victoria 3010, Australia.
Danes
Vennetia R
VR
Curl
Claire L
CL
Favaloro
Jennifer M
JM
Proietto
Joseph
J
Delbridge
Lea M D
LM
D000818:Animals; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism; D051275:Glucose Transporter Type 4 / Q000235:genetics* / Q000378:metabolism / Q000502:physiology*; D006321:Heart; D006863:Hydrogen-Ion Concentration; D006984:Hypertrophy / Q000473:pathology*; D007328:Insulin / Q000378:metabolism; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D008856:Microscopy, Fluorescence / Q000379:methods; D009206:Myocardium / Q000473:pathology*; D032383:Myocytes, Cardiac / Q000378:metabolism / Q000473:pathology*; D018408:Patch-Clamp Techniques
D051275:Glucose Transporter Type 4; D007328:Insulin; C495693:Slc2a4 protein, mouse; D002118:Calcium