S100A1 deficiency results in prolonged ventricular repolarization in response to sympathetic activation.

S100A1 is a Ca(2+)-binding protein and predominantly expressed in the heart. We have generated a mouse line of S100A1 deficiency by gene trap mutagenesis to investigate the impact of S100A1 ablation on heart function. Electrocardiogram recordings revealed that after beta-adrenergic stimulation S100A1-deficient mice had prolonged QT, QTc and ST intervals and intraventricular conduction disturbances reminiscent of 2 : 1 bundle branch block. In order to identify genes affected by the loss of S100A1, we profiled the mutant and wild type cardiac transcriptomes by gene array analysis. The expression of several genes functioning to the electrical activity of the heart were found to be significantly altered. Although the default prediction would be that mRNA and protein levels are highly correlated, comprehensive immunoblot analyses of salient up- or down-regulated candidate genes of any cellular network revealed no significant changes on protein level. Taken together, we found that S100A1 deficiency results in cardiac repolarization delay and alternating ventricular conduction defects in response to sympathetic activation accompanied by a significantly different transcriptional regulation.

Functional and metabolic remodelling in GLUT4-deficient hearts confers hyper-responsiveness to substrate intervention.

Impaired glucose uptake is associated with both cardiac hypertrophy and contractile dysfunction, but whether there are common underlying mechanisms linking these conditions is yet to be determined. Using a 'gene dose' Cre-Lox GLUT4-deficient murine model, we examined the effect of suppressed glucose availability on global myocardial gene expression and glycolysis substrate bypass on the function of isolated perfused hearts. Performance of hearts from 22- to 60-week-old male GLUT4 knockout (KO, >95% reduction in GLUT4), GLUT4 knockdown (KD, 85% reduction in cardiac GLUT4) and C57Bl/6 wild-type (WT) controls was measured ex vivo in Langendorff mode perfusion. DNA microarray was used to profile mRNA expression differences between GLUT4-KO and GLUT4-KD hearts. At 22 weeks, GLUT4-KO hearts exhibited cardiac hypertrophy and impaired contractile function ex vivo, characterized by a 40% decrease in developed pressure. At 60 weeks, dysfunction was accentuated in GLUT4-KO hearts and evident in GLUT4-KD hearts. Exogenous pyruvate (5 mM) restored systolic pressure to a level equivalent to WT (GLUT4-KO, 176.8+/-13.2 mmHg vs. WT, 146.4+/-9.56 mmHg) in 22-week-old GLUT4-KO hearts but not in 60-week-old GLUT4-KO hearts. In GLUT4-KO, DNA microarray analysis detected downregulation of a number of genes centrally involved in mitochondrial oxidation and upregulation of other genes indicative of a shift to cytosolic beta-oxidation of long chain fatty acids. A direct link between cardiomyocyte GLUT4 deficiency, hypertrophy and contractile dysfunction is demonstrated. These data provide mechanistic insight into the myocardial metabolic adaptations associated with short and long-term insulin resistance and indicate a window of opportunity for substrate intervention and functional 'rescue'.

Threshold effects of glucose transporter-4 (GLUT4) deficiency on cardiac glucose uptake and development of hypertrophy.

The aim of this study was to investigate the metabolic and structural consequences of a decrease in glucose transporter-4 (GLUT4) levels on the heart. The CreLoxP system was utilised to delete GLUT4 in muscle tIssue including heart. The presence of the PGK-neoR cassette in the GLUT4-Lox mice resulted in reduced expression in all tIssues to levels 15-30% of wild-type control mice. In mice expressing Cre recombinase, there was a further reduction of GLUT4 in cardiac tIssue to almost undetectable levels. Cardiac glucose uptake was measured basally and during a euglycaemic/hyperinsulinaemic clamp using 2-deoxy-[1-(14)C]glucose. Insulin-stimulated glucose uptake was normal in hearts expressing 15% of normal GLUT4 levels but markedly reduced in mice with more profound reduction in GLUT4. Cardiac enlargement occurred only when GLUT4 levels were less than 5% of normal values. In heart there is a threshold level of GLUT4 above which insulin-stimulated glucose uptake is maintained. As little as 5% of normal GLUT4 levels expressed in heart is sufficient to prevent the development of cardiac hypertrophy.

An intercellular regenerative calcium wave in porcine coronary artery endothelial cells in primary culture.

1. A regenerative calcium wave is an increase in cytosolic free calcium concentration ([Ca2+]i) which extends beyond the stimulated cells without decrement of amplitude, kinetics of [Ca2+]i increase and speed of propagation. 2. The aim of the present study was to test the hypothesis that such a wave could be evoked by bradykinin stimulation and by scraping cultured endothelial cells from porcine coronary arteries. 3. Calcium imaging was performed using the calcium-sensitive dye fura-2. A wound or a delivery of bradykinin to two to three cells on growing clusters of approximately 300 cells caused an increase in [Ca2+]i which was propagated throughout the cluster in a regenerative manner over distances up to 400 micrometer. This wave spread through gap junctions since it was inhibited by the cell uncoupler palmitoleic acid. 4. The same experiments performed in confluent cultures caused a rise in [Ca2+]i which failed to propagate in a regenerative way. The wave propagation probably failed because the confluent cells were less dye coupled than the growing cells. This was confirmed by immunohistology which detected a dramatic decrease in the number of connexin 40 gap junctions in the confluent cultures. 5. The regenerative propagation of the wave was blocked by inhibitors of calcium-induced calcium release (CICR) and phospholipase C (PLC), and by suppression of extracellular calcium, but not by clamping the membrane potential with high-potassium solution. 6. We conclude that regenerative intercellular calcium waves exist in cultured islets but not in confluent cultures of endothelial cells. An increase in [Ca2+]i is not sufficient to trigger a regenerative propagation. The PLC pathway, CICR and extracellular calcium are all necessary for a fully regenerated propagation.