Andreas Simm

Insulin stimulates the L-type Ca2+ current in rat cardiac myocytes

OBJECTIVE The aim was to study the L-type calcium current (ICa,L) in cardiac myocytes as a possible target of insulin in the regulation of cardiac function. METHOD Using the whole-cell configuration of the patch-clamp technique, we investigated the stimulation of ICa,L by insulin in isolated rat ventricular myocytes. RESULTS The stimulation of ICa,L by insulin was dose-dependent (EC50 = 33 nM) and reversible. Maximum stimulation of ICa,L over basal ICa,L was 86 +/- 11% (n = 25) at 1 microM insulin. Insulin (1 microM) shifted the current-voltage relationship and potential-dependent availability of ICa,L to more negative potentials by about 3.5 and 1.5 mV, respectively. The maximum conductance of ICa,L was increased by 1 microM insulin, from 26 +/- 4 to 39 +/- 5 nS (n = 11). Isoproterenol (100 nM), which stimulated ICa,L by 156 +/- 23% (n = 10) over basal ICa,L, acted faster than insulin. The half-maximum stimulation of ICa,L by isoproterenol and insulin was reached after 44 +/- 5 and 80 +/- 9 s, respectively. Insulin and isoproterenol responses were not additive. Insulin (1 microM) and isoproterenol (100 nM) stimulation of ICa,L was inhibited by Rp-cAMPS (1 mM) to 12 +/- 3 and 32 +/- 4%, respectively. Insulin (1 microM) increased cAMP content in rat cardiomyocytes by about two-fold. Insulin-like growth factor-1 (IGF-1; 5 microM) increased ICa,L by only 5.9 +/- 0.9% (n = 6). CONCLUSIONS Our data show that insulin stimulates the L-type calcium current in isolated rat ventricular myocytes in a dose-dependent and reversible manner and suggest that this effect is mediated by insulin receptors and the cAMP-dependent protein kinase.

Stimulation of L-type Ca2+ current in human atrial myocytes by insulin

OBJECTIVE The L-type calcium current (ICa,L) in isolated human atrial myocytes was investigated as a possible target of insulin in the regulation of cardiac function. METHODS Atrial myocytes were obtained from patients undergoing cardiac surgery. Using the whole-cell configuration of the patch-clamp technique, we investigated the stimulation of ICa,L by insulin in single human atrial myocytes. RESULTS We found a dose-dependent stimulation of ICa,L by insulin at concentrations of 100 nM, 1 microM and 10 microM. Maximum stimulation of ICa,L over basal ICa,L was 140 +/- 12% (n = 11) at 10 microM insulin. The maximum conductance of ICa,L was increased by 10 microM insulin from 4.0 +/- 0.3 nS to 8.3 +/- 1.0 nS (n = 6). The stimulation of ICa,L by insulin was dose-dependent and reversible. Isoproterenol (10 nM) that stimulates ICa,L by 271 +/- 48% (n = 10) over basal ICa,L acted faster than insulin. The half-maximum stimulation of ICa,L by isoproterenol and insulin (10 microM) was reached after 31 +/- 2 s and 52 +/- 5 s, respectively. The insulin effect shown was totally reversed by acetylcholine (3 microM) which is known to inhibit adenylyl cyclase activity/cAMP-production via Gi-proteins. Also, the selective insulin receptor tyrosine kinase inhibitor (hydroxy-2-naphthanelyl-methyl)phosphonic acid completely inhibited the insulin induced effect. CONCLUSION Our data show that insulin stimulates the L-type calcium current in isolated human atrial myocytes in a dose-dependent and reversible manner which appears to involve the insulin receptor tyrosine kinase. Insulin regulation of ICa,L in human atrial myocytes may be an interesting system for the analysis of the metabolic syndrome in man.

VASP-dependent regulation of actin cytoskeleton rigidity, cell adhesion, and detachment

Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) proteins are established regulators of actin-based motility, platelet aggregation, and growth cone guidance. However, the molecular mechanisms involved essentially remain elusive. Here we report on a novel mechanism of VASP action, namely the regulation of tensile strength, contractility, and rigidity of the actin cytoskeleton. Compared to wild-type cells fibroblasts derived from VASP-deficient mice have thicker and more stable actin stress fibres. Furthermore focal adhesions are enlarged, myosin light chain phosphorylation is increased, and the rigidity of the filament-supported plasma membrane is elevated about three- to fourfold, as is evident from atomic force microscopy. Moreover, fibronectin-coated beads adhere stronger to the surface of VASP-deficient cells. The resistance of these beads to mechanical displacement by laser tweezers is dramatically increased in an F-actin-dependent mode. Cytoskeletal stabilization coincides with slower cell adhesion and detachment, while overall adhesion is increased. Interestingly, many of these effects observed in VASP (−/−) cells are recapitulated in VASP-overexpressing cells, hinting towards a balanced stoichiometry necessary for appropriate VASP function. Taken together, our results suggest that VASP regulates surface protrusion formation and cell adhesion through modulation of the mechanical properties of the actin cytoskeleton.

Isolation of full-size mRNA from cells sorted by flow cytometry.

Gene expression is one key mechanism to regulate cell growth and differentiation. It is usually determined by Northern blotting or RT-PCR. However, studies with primary cell cultures are frequently hampered due to contaminating cells such as fibroblasts. We have developed a method to isolate intact full-size mRNA from sorted cells. In many cell types, e.g. cardiac myocytes, cell sorting without prior fixation revealed complete RNA breakdown. Based on a murine fibroblast cell line (AKR-2B), ethanol and formaldehyde at various concentrations and pre-treatment with ribonuclease inactivating DEPC were compared with each other. Fixation with 75% ice-cold DEPC-pre-treated ethanol for 5 min yielded mostly intact RNA. In contrast, antibody staining prior to sorting required 15 min fixation. Addition of RNAse-free BSA (0.5%) and 2 mM CaCl2 optimised the cell recovery ratio and thus a better RNA yield (60% compared to control) after sorting than former studies. Northern blotting and RT-PCR show the intact mRNA species beta-actin. Furthermore, dependent on the cellular PCNA content, we have demonstrated the cell cycle dependent cdk2 and cyclin A expression. This fast and reliable method allows to isolate intact full-size mRNA species appropriate for Northern blotting and RT-PCR to monitor gene expression.

Density dependent expression of PDGF-A modulates the angiotensin II dependent proliferation of rat cardiac fibroblasts

Abstract The proliferative effect of angiotensin II (Ang-II) on primary cardiac fibroblasts is not well understood and controversially discussed. Results described here show that fibroblasts from adult rat hearts exhibit a cell density dependent Ang-II induced cell proliferation.Whereas we could not detect a proliferative effect of Ang-II on confluent cells, which are still able to divide as shown by stimulation with platelet derived growth factor (PDGF), we observed an Ang-II induced cell division of approximately 20% in non-confluent cells. At both densities, signal transduction molecules such as the mitogen activated protein kinases (MAPK) are activated. There has been substantial evidence that Ang-II may induce the expression and secretion of several growth factors. We demonstrated an approximately fivefold increase in platelet derived growth factor-A (PDGF-A) chain and transforming growth factor β 1 (TGF-β1) mRNA-expression within non-confluent cardiac fibroblasts by semiquantitative reverse polymerase chain reaction, TR-PCR. In contrast, the mRNA-expression of basic fibroblast growth factor (bFGF), insulin-like growth factor 1 (IGF-1), PDGF-A/-B chains, and TGF-β1 remained unchanged within confluent cardiac fibroblasts. Experiments with neutralizing antibodies showed that PDGF-AA is an important growth factor regulating cell proliferation, whereas TGF-β1 interferes with cell size regulation. In summary, we could show that the Ang-II induced cell proliferation in adult cardiac fibroblasts is mainly due to cell density dependent expression of growth factors like PDGF-AA.