Calcium Channels In Ventricular Myocytes Research Articles

The biological activity of selected cyclic dipeptides.

Cyclic dipeptides are widely used as models for larger peptides because of their simplicity and limited conformational freedom. Some cyclic dipeptides have been shown to be antiviral, antibiotic and anti-tumour. The aim of this study was to determine the biological activity of four cyclic dipeptides synthesized in this laboratory: cyclo(L-phenylalanyl-L-prolyl), cyclo(L-tyrosyl-L-prolyl), cyclo(L-tryptophanyl-L-prolyl) and cyclo(L-tryptophanyl-L-tryptophanyl). The enhancement or inhibition of calcium channels in ventricular myocytes from rats and delayed-rectifier potassium channels in ventricular myocytes from guinea-pigs were determined by use of the whole-cell patch-clamp technique. The induction of differentiation in HT-29 cells was assessed by assaying for an increase in the expression of alkaline phosphatase. Antibiotic properties against Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Staphylococcus aureus, Bacillus subtilus and Streptococcus sp. were determined by use of the Kirby-Bauer disc-diffusion assay. Results from these assays indicate that the cyclic dipeptides have biological activity in both prokaryotes and eukaryotes. Three of the dipeptides block cation channels in ventricular myocytes and all increase the expression of alkaline phosphatase. All the dipeptides have concentration-dependent antibacterial properties. These results suggest that with increased solubility the cyclic dipeptides might have potential as muscle relaxants, anti-tumour compounds and antibiotics.

Role of the T-Type Calcium Channel CaV3.2 in the Chronotropic Action of Corticosteroids in Isolated Rat Ventricular Myocytes

The mineralocorticoid receptor is involved in the development of several cardiac dysfunctions, including lethal ventricular arrhythmias associated with heart failure or hyperaldosteronism, but the molecular mechanisms responsible for these effects remain to be clarified. Reexpression of low voltage-activated T-type calcium channels in ventricular myocytes together with other fetal genes during cardiac pathologies could confer automaticity to these cells and would represent a pro-arrhythmogenic condition if occurring in vivo. In the present study, we demonstrated that in isolated neonatal rat ventricular myocytes, corticosteroids selectively induced the expression of a particular isoform of T channel, Ca(V)3.2/alpha1H. This response was accompanied by an increase of the Ca(V)3.2 T-type current, identified with the patch clamp technique by its sensitivity to nickel, and a concomitant acceleration of the myocyte spontaneous contractions. Silencing Ca(V)3.2 expression markedly reduced the chronotropic response to steroids. Moreover, modulation of the frequency of cell contractions by different redox agents was independent of channel expression but involved a direct regulation of channel activity. Although oxidants increased both Ca(V)3.2 current amplitude and beating frequency, they decreased L-type channel activity. Reducing agents had the opposite effect on these parameters. In conclusion, the acceleration of ventricular myocyte spontaneous contractions induced by corticosteroids in vitro appears dependent on the expression of the Ca(V)3.2 T channel isoform and modulated by the redox potential of the cells. These results provide a molecular model that could explain the high incidence of arrhythmias observed in patients upon combination of inappropriate activation of the mineralocorticoid receptor and oxidative stress.

B-type natriuretic peptide (BNP) attenuates the L-type calcium current and regulates ventricular myocyte function

A fundamental question in physiology is how hormones regulate the functioning of a cell or organ. It was therefore the aim of this study to investigate the effect(s) of BNP-32 on calcium handling by ventricular myocytes obtained from the rat left ventricle. We specifically tested the hypothesis that BNP-32 decreased the L-type calcium current ( I Ca,L). Perforated patch clamp technique was used to record I Ca,L and action potential (AP) in voltage and current clamp mode, respectively. Myocyte shortening was measured using a photodiode array edge-detection system and intracellular calcium transients were measured by fluorescence photometry. Western blotting was used to determine the relative change in the expression of proteins. At the concentrations tested, BNP-32 significantly decreased cell shortening in a dose-dependent manner; increased the phase II slope of the AP by 53.0%; increased the APD 50 by 16.9%; reduced the I Ca,L amplitude with a 22.9% decrease in the peak amplitude and reduced Ca 2+-dependent inactivation; increased the V 1/2 activation of the L-type calcium channel by 51.1% and decreased V 1/2 inactivation by 31.8%; and, intracellular calcium transient amplitude was significantly decreased by 32.0%, whereas the time to peak amplitude and T 1/2 were both significantly increased by 38.7% and 89.4% respectively. Sarcoplasmic reticulum Ca 2+-ATPase (SERCA2a) protein expression was reduced by BNP-32. These data suggest that BNP-32 regulates ventricular myocyte function by attenuating I Ca,L, altering the AP and reducing SERCA2a activity and/or expression. This study suggests a novel constitutive mechanism for the autocrine action of BNP on the L-type calcium channel in ventricular myocytes.

Gα o is necessary for muscarinic regulation of Ca 2+ channels in mouse heart

Heterotrimeric G proteins, composed of G alpha and G betagamma subunits, transmit signals from cell surface receptors to cellular effector enzymes and ion channels. The G alpha(o) protein is the most abundant G alpha subtype in the nervous system, but it is also found in the heart. Its function is not completely known, although it is required for regulation of N-type Ca2+ channels in GH3 cells and also interacts with GAP43, a major protein in growth cones, suggesting a role in neuronal pathfinding. To analyze the function of G alpha(o), we have generated mice lacking both isoforms of G alpha(o) by homologous recombination. Surprisingly, the nervous system is grossly intact, despite the fact that G alpha(o) makes up 0.2-0.5% of brain particulate protein and 10% of the growth cone membrane. The G alpha(o)-/- mice do suffer tremors and occasional seizures, but there is no obvious histologic abnormality in the nervous system. In contrast, G alpha(o)-/- mice have a clear and specific defect in ion channel regulation in the heart. Normal muscarinic regulation of L-type calcium channels in ventricular myocytes is absent in the mutant mice. The L-type calcium channel responds normally to isoproterenol, but there is no evident muscarinic inhibition. Muscarinic regulation of atrial K+ channels is normal, as is the electrocardiogram. The levels of other G alpha subunits (G alpha(s), G alpha(q), and G alpha(i)) are unchanged in the hearts of G alpha(o)-/- mice, but the amount of G betagamma is decreased. Whichever subunit, G alpha(o) or G betagamma, carries the signal forward, these studies show that muscarinic inhibition of L-type Ca2+ channels requires coupling of the muscarinic receptor to G alpha(o). Other cardiac G alpha subunits cannot substitute.