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The Importance of Calcium in the Regulation of EDRF Synthesis in the Pulmonary Vasculature

Authors:
  • Giga Solar Materials

Abstract

The vascular endothelium and smooth muscle exist in close apposition yet they have opposing responses to stimuli which increase free cytosolic calcium concentration, [Ca2+]i. Stimuli which increase [Ca2+]i in the endothelium initiate synthesis of vasodilators, such as prostacyclin and nitric oxide; increases in [Ca2+]i elicits vasoconstriction of smooth muscle. The net response to a stimulus which elevates [Ca2+]i in both the endothelium and smooth muscle is the sum of these divergent responses. This chapter focuses on the central role of the Ca2+ ion in regulating the synthesis of endothelial derived relaxing factor (EDRF), which is nitric oxide (NO) or a related substance. The importance of both intra- and extra-cellular Ca2+ to EDRF/NO synthesis is supported by the observation that agents which cause EDRF synthesis increase endothelial [Ca2+]i1–5. Although elevation of intracellular [Ca2+]i activates NO synthase, a continued influx of Ca2+ is necessary to sustain endothelium-dependent vasodilatation 1. The chapter begins with a description of the NO hypothesis emphasizing the role of Ca2+ in the initiation and sustenance of NO synthesis. While endothelial [Ca2+]i is an important signal for NO synthesis, changes in cytosolic calcium also initiate synthesis of other endogenous vasodilators (e.g. endothelium-derived hyperpolarizing factor (EDHF) and prostacyclin). The importance of age, species and vessel type in determining the diverse means by which the endothelium modulates vascular tone is considered, using the differences between EDRF in the pulmonary and systemic circuits as a paradigm. This is followed by a discussion of the regulation of Ca2+ influx into endothelial cells, emphasizing the importance of endothelial membrane potential as a determinant of the electrochemical gradient for Ca2+ influx. The chapter concludes with a discussion of the techniques for, and value of, measuring endothelial [Ca2+]i using fluorescent fura microscopy. Examples of the value of this technique in detennining mechanisms of endothelium dependent vasodilatation in the pulmonary circulation are provided.
... For these in vitro measurements, Rmin and Rmax was R of 10 mM EGTA and 10 mM Ca 2+ standards, respectively. The measured Ca 2+ concentrations were fairly close to the calculated values for all standards [30], confirming that the equipment was properly calibrated and the Kd appropriate for our experimental setting. Pulmonary artery endothelial [Ca 2+ ]i was measured during 4–5 min of normoxic perfusion, and the cells were then challenged with hypoxia for 3–5 min. ...
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