Excitation-contraction Coupling In Cardiomyocytes Research Articles

Development of excitation-contraction coupling in cardiomyocytes.

The excitation-contraction (E-C) coupling system during the development of heart can be investigated because of marked progression in electrophysiology and microfluorescence studies. During the developmental period, Ca2+ influx mediating the E-C coupling is mainly through the L-type Ca2+ channels. In the fetal period, T-type Ca2+ channels and the reverse mode of the Na-Ca exchange system also contribute to Ca2+ influx. These contributions probably reduce gradually until adulthood. The contraction of fetal cardiomyocytes has been thought to depend mainly on the Ca2+ influx. However, recent studies reveal that immature sarcoplasmic reticulum (SR) already works in the early fetal period. Ca2+ spark, a local and unitary movement of Ca2+, can be observed in adult cardiomyocytes by the use of a confocal microscope. On the other hand, no Ca2+ spark is observed in fetal cardiomyocytes. The frequency of Ca(2+)-spark evocation increases during the postnatal period. Therefore a close distance between the L-type Ca2+ channel and the SR Ca(2+)-release channel is essential to the establishment of the Ca2+ spark.

Heart-selective expression of the chicken FK506-binding protein (FKBP) 12.6 gene during embryonic development.

FKBP12.6, a member of the family of FK506-binding proteins, selectively associates with the cardiac isoform of the ryanodine receptor and thereby stabilizes this Ca(2+) release channel. A chicken FKBP12.6 (chFKBP12.6) cDNA was cloned and shown to encode a protein of 108 amino acids. The deduced amino acid sequence of chFKBP12.6 is 91-92% identical to those of mammalian FKBP12.6 proteins. Northern blot analysis revealed that chFKBP12.6 mRNA is largely restricted to the heart during embryonic development and that the abundance of this mRNA in the heart decreases, and it becomes restricted to the atrium during cardiogenesis. In situ hybridization revealed that chFKBP12.6 mRNA is localized to the precardiac mesoderm before formation of the primitive heart tube. Expression of the chFKBP12.6 gene was initially apparent throughout the developing multichambered heart but became restricted to the atria before hatching. Reverse transcription and polymerase chain reaction analysis demonstrated that chFKBP12.6 mRNA is present in the embryo from early gastrulation and is most abundant immediately after the onset of the heartbeat. These observations suggest that the chFKBP12.6 gene is expressed before heart morphogenesis to play a role in excitation-contraction coupling in cardiomyocytes and that the function of the encoded protein becomes increasingly restricted to the atrium during embryonic development.

Embryonic stem cells: a model to study structural and functional properties in cardiomyogenesis.

Time for primary review 20 days. In order to study cardiac myocyte development different approaches were established during the last decades. The main purpose of these studies was the differentiation of cardiac precursor cells into specialized, differentiated cell types, as well as the development of functional properties such as Ca2+ handling, rhythm generation and excitation-contraction coupling of cardiomyocytes during development. Although considerable data exist about skeletal myogenesis [1–3], limited knowledge is available with regard to the origin of the commitment and differentiation of cardiac cells. A comprehensive, morphological study on the cytodifferentiation from mesenchymal cells into cardiac myocytes is described in the embryonic murine heart [4]: According to the authors, different stages of myofibrillogenesis are present during embryological myocardial development. Cells with no or only little myofibrillar arrangement develop to myocardial cells with orientated myofibrils [5, 6]. A number of morphological studies have investigated heart development on embryonic, neonatal and adult isolated cardiomyocytes also from different species [7–16]. Although the ultrastructure during cardiac development has been thoroughly investigated [17], still relatively little is known on the development of excitability of the mammalian heart, most importantly: (1): The relation between expression of cardio-specific genes (see review [23]), the formation of cardiac phenotypes and the functional expression of different types of ion channels; (2): The regulation and genetic control of expression of ion channels (e.g. by growth factors, hormones, extracellular matrix); (3): The development of the regulation of ion channels and morphological correlates. The progress in this field is hampered by the inability to study cardiomyocytes from early, embryonal hearts because of their very small size and because of the lack of cardiac cell lines that mimic various stages of cardiac development. The development of ion currents has been studied in cardiomyocytes prepared from mammalian embryos … * Corresponding author. Tel.: (+49-221) 4786960; Fax: (+49-221) 4786965.

Modeling spiral Ca2+ waves in single cardiac cells: role of the spatial heterogeneity created by the nucleus.

Excitation-contraction coupling in cardiomyocytes is known to rely on the Ca(2+)-induced Ca2+ release mechanism. This autoamplification process is also very apparent when voltage-clamped or Ca(2+)-overloaded myocytes exhibit fast-propagating Ca2+ waves. Although most of the fronts are planar, some adopt a spiral shape, revealing additional characteristics about the excitability and structure of the cardiac cell (P. Lipp and E. Niggli, Biophys. J. 65: 2272-2276, 1993: J. Engel, M. Fechner, A. Sowerby, S. Finch, and A. Stier, Biophys. J. 66: 1756-1762, 1994). Using a previously developed model for Ca2+ oscillations and waves (A. Goldbeter, G. Dupont, and M.J. Berridge, Proc. Natl. Acad. Sci. USA 87: 1461-1465, 1990; G. Dupont and A. Goldbeter, Biophys. J. 67: 2191-2204, 1994), we study by numerical simulations different conditions in which spiral Ca2+ waves can occur as a result of the spatial heterogeneity created by the nucleus in a system with geometry resembling that of a myocyte. A region of the cell lacking Ca2+ pools, acting as an obstacle able to break the propagation of planar waves, suffices to initiate a spiral wave; however, this region must be properly placed with respect to the pacemaker. An obstacle behaving as a barrier to diffusion is also able to create the initial bending that can lead to the spiral wave. We study how the occurrence of spiral Ca2+ waves in single cardiomyocytes is influenced by factors such as the stimulus location and the position, shape, and dimensions of the obstacle to planar wave propagation.

High-speed digital imaging of cytosolic Ca2+ and contraction in single cardiomyocytes

A charge-coupled device (CCD) camera, with the capacity for simultaneous spatially resolved photon counting and rapid frame transfer, was utilized for high-speed digital image collection from an inverted epifluorescence microscope. The unique properties of the CCD detector were applied to an analysis of cell shortening and the Ca2+ transient from fluorescence images of fura-2-loaded [corrected] cardiomyocytes. On electrical stimulation of the cell, a series of sequential subimages was collected and used to create images of Ca2+ within the cell during contraction. The high photosensitivity of the camera, combined with a detector-based frame storage technique, permitted collection of fluorescence images 10 ms apart. This rate of image collection was sufficient to resolve the rapid events of contraction, e.g., the upstroke of the Ca2+ transient (less than 40 ms) and the time to peak shortening (less than 80 ms). The technique was used to examine the effects of beta-adrenoceptor activation, fura-2 load, and stimulus frequency on cytosolic Ca2+ transients and contractions of single cardiomyocytes. beta-Adrenoceptor stimulation resulted in pronounced increases in peak Ca2+, maximal rates of rise and decay of Ca2+, extent of shortening, and maximal velocities of shortening and relaxation. Raising the intracellular load of fura-2 had little effect on the rising phase of Ca2+ or the extent of shortening but extended the duration of the Ca2+ transient and contraction. In related experiments utilizing differential-interference contrast microscopy, the same technique was applied to visualize sarcomere dynamics in contracting cells. This newly developed technique is a versatile tool for analyzing the Ca2+ transient and mechanical events in studies of excitation-contraction coupling in cardiomyocytes.