Functional effects of glucose transporters in human ventricular myocardium

Insulin-dependent positive inotropic effects (PIE) are partially Ca(2+) independent. This mechanism is potentially glucose dependent. In contrast to most animal species, human myocardium expresses high levels of sodium-glucose-transporter-1 (SGLT-1) mRNA besides the common glucose-transporters-1 and -4 (GLUT1, GLUT4). We used ventricular myocardium from 61 end-stage failing human hearts (ischaemic cardiomyopathy, ICM and dilated cardiomyopathy, DCM) and 13 non-failing donor hearts. The effect of insulin on isometric twitch force was examined with or without blocking of PI3-kinase, GLUT4-translocation, or SGLT-1. Substrate-dependent (glucose vs. pyruvate vs. palmitoyl-carnitine) effects were tested in atrial myocardium. mRNA expression of glucose transporters was analysed. Insulin increased developed force by 122 +/- 7.4, 121.7 +/- 2.5, and 134.1 +/- 5.7% in non-failing, DCM, and ICM (P < 0.05 vs. DCM), respectively. Positive inotropic effect was partially blunted by inhibition of PI-3-kinase, GLUT4, or SGLT1. Combined inhibition of PI3-kinase and glucose-transport completely abolished PIE. Positive inotropic effect was significantly stronger in glucose-containing solution compared with pyruvate or palmitoyl-carnitine containing. mRNA expression showed only a tendency towards elevated GLUT4-expression in ICM. Positive inotropic effect of insulin is pronounced in ICM, but underlying mechanisms are unaltered. The Ca(2+)-independent PIE of insulin is mediated via glucose-transporters. Together with the Ca(2+)-dependent PI-3-kinase mediated pathway, it is responsible for the entire PIE. Substrate-dependency affirms a glucose-dependent part of the PIE.