Molecular mechanisms of contraction-induced translocation of GLUT4 in isolated cardiomyocytes

Isolated adult rot ventricular cardiomyocytes were used to investigate the effects of contractile activity on 3-O-methylglucose transport on the translocation of the insulin-responsive glucose transporter GLUT4, land the possible activation of intermediates of the insulin signaling cascade, When elicited by field stimulation, contraction at 1 Hz did not significantly affect the adenosine triphosphate (ATP) content of cardiac cells, even after 60 min, At 5 Hz, a stable ATP level was observed until 15 minutes with a rapid decline at later time points, Stimulation of cardiomyocytes at 5 Hz for 5 minutes induced a 2-3 fold increase of 3-O-methylglucose transport with no additional stimulation in the presence of insulin (10(-7)M), Subcellular fractionation and immunoblotting analysis of GLUT4 distribution indicated that both contraction and insulin induced an identical increase (8-9-fold) of GLUT4 in the plasma membrane with a concomitant decrease (one third) in the microsomal fraction, Treatment of cardiomyocytes with wortmannin produced a complete inhibition of insulin-and contraction-induced glucose uptake, However, immunoprecipitation of insulin receptor substrate-1 (IRS-1) showed that the p85 regulatory subunit of phosphatidylinositol-3 kinase did not-associate with IRS-1 upon contraction but with a marked stimulated association in response to insulin, These data suggest the existence of identical insulin-and contraction-recruitable GLUT4 pool, Contraction-induced signaling may use a limited part of the insulin-signaling cascade, possibly involving IRS-2, We further suggest that insulin resistance at the level of IRS-1 will not affect contraction-regulated glucose uptake by the heart. (C) 1997 by Excerpta Medico, Inc.