© 1996 American Heart Association, Inc.
Articles |
Spontaneous Ca2+ Oscillations and Waves in Pulmonary Vascular Endothelial Cells
the Department of Physiology and Department of Medicine, Division of Cardiology, University of Maryland School of Medicine, Baltimore, Md.
Correspondence to C. William Balke, MD, University of Maryland School of Medicine, Department of Physiology, Howard Hall, Room 560, 660 W Redwood St, Baltimore, MD 21201. E-mail bbalke@heart/ab/umd/edu.
Key Words: Ca2+ • Ca2+ oscillations • Ca2+ wave • pulmonary endothelial cell • fura 2
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Calcium is a ubiquitous second messenger that regulates a number of cellular processes in both excitable and nonexcitable cells.1 2 3 Transient elevations in [Ca2+]i (Ca2+ transients) are important for the regulation of intracellular processes, and when these Ca2+ transients are propagated as Ca2+ waves, they may influence or regulate signaling between cells and through tissues. Intracellular and intercellular Ca2+ waves have been observed in a variety of cell types and are initiated principally by two types of mechanisms: (1) stimulus-mediated mechanisms (eg, chemical agonist, electrical, or mechanical stimuli) and (2) spontaneously occurring mechanisms. In general, spontaneous Ca2+ waves have been observed predominantly in excitable cells1 (eg, skeletal, cardiac, and vascular smooth muscle cells). Nonexcitable cells, such as endothelial cells, which have been studied principally in isolation or in cell culture preparations, do not show typically spontaneous Ca2+ waves, although they do show occasional sporadic Ca2+ oscillations.4 5 6 Therefore, the prevailing view of Ca2+ regulation in pulmonary endothelial cells is that [Ca2+]i is relatively uniform in the absence of chemical agonists or mechanical stimulation. However, the article by Ying et al7 in this issue of Circulation Research challenges these rather simple notions of Ca2+ homeostasis in pulmonary endothelium and suggests a degree of complexity in Ca2+ signaling similar to that found in excitable cells.
The novel findings of these investigators are due, in large part, to their ability to measure intracellular Ca2+ transients in individual endothelial cells in an in situ model of endothelial cell function (ie, venular capillaries of the isolated blood-perfused rat lung). Using intravital microscopy and the Ca2+-sensitive fluorescent indicator, fura 2, these investigators were able to quantify changes in [Ca2+]i using standard ratiometric techniques. In contrast to isolated endothelial cells or endothelial cell cultures, at least two populations of the endothelial cells could be identified in the intact capillary wall segments. Distinguished by spontaneous Ca2+ oscillations, a subtype of endothelial cells (denoted "pacemaker" cells by the authors) was found at the branch points of most capillary segments. The Ca2+ oscillations of these pacemaker cells appeared to depend on the periodic release of Ca2+ from the endoplasmic reticulum (ER), since their amplitude and frequency were not dependent on the presence of external Ca2+ and since they were abolished completely in the presence of a specific inhibitor of the ER Ca2+-ATPase (eg, thapsigargin). Importantly, the Ca2+ oscillations of the pacemaker cells triggered the propagation of spontaneous Ca2+ waves through the remaining subtype of nonpacemaker endothelial cells. These Ca2+ waves propagated down the short distances of the venular capillaries with a relatively slow velocity of 5±2 µm·s-1. In contrast to the spontaneous Ca2+ oscillations of the pacemaker cells, the propagated Ca2+ waves may depend on the integrity of gap junctional communication, since Ca2+ waves were totally occluded by pharmacological uncoupling of gap junctions (eg, heptanol). Pacemaker and nonpacemaker cells were also distinguished by their different responses to the pharmacological agonist histamine. In pacemaker cells, the histamine-induced increases in [Ca2+]i were due to the combination of Ca2+ entry and ER Ca2+-release. In the nonpacemaker cells, histamine induced increases in [Ca2+]i were due also to the combination of Ca2+ entry and ER Ca2+ release, but the component attributable to ER Ca2+ release was completely occluded in the presence of heptanol. Overall, the results of Ying et al7 show that Ca2+ signaling in quiescent intact pulmonary endothelial cells is a much more complex and dynamic process than predicted from numerous previous studies involving isolated endothelial cells or endothelial cell cultures.
Although Ca2+ waves have been observed in diverse cell types, the cellular mechanisms that underlie these waves are not identical, and Ca2+ waves cannot be considered a single phenomenon.8 The article by Ying et al7 adds another type of Ca2+ wave to the list of Ca2+ waves that have been observed in a variety of cells. Although beyond the scope of their article, important issues such as the identity and function of the intercellular messenger(s), among others, will be the focus of future investigations.
In closing, the results reported by Ying et al7 have shown us that Ca2+ signaling in intact quiescent endothelial cells involves spontaneous Ca2+ oscillations in pacemaker cells, which are subsequently propagated as Ca2+ waves along the length of the capillary segment. These observations are important for several reasons: (1) They demonstrate the usefulness of examining preparations that preserve the architecture and function of intact tissues. (2) They show the existence of at least two subpopulations of cells in the pulmonary vascular endothelium that can be distinguished on the basis of different patterns of intracellular Ca2+ homeostasis. (3) This is the first demonstration in pulmonary endothelial cells of the presence of a "pacemaker" cell that coordinates and triggers spontaneous Ca2+ waves in the adjacent capillary segment. Although the study of Ying et al does not establish the physiological usefulness of these Ca2+ waves, it does suggest that pulmonary endothelial cells may use the complex spatial and temporal information carried in these waves to coordinate and regulate their normal physiological functions as well as their response to agonist and mechanical stimuli. For example, coordinated elevations in [Ca2+]i along the capillary segment may be important for resting endothelial functions, such as the production of endothelial relaxation factors and the maintenance of the vascular permeability barrier. To the extent that this model applies to larger vessels that contain vascular smooth muscle cells, spontaneous Ca2+ waves may regulate NO release and smooth muscle relaxation and may be important in the regulation of vascular tone and regional blood flow.
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References |
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