In:
American Journal of Physiology-Gastrointestinal and Liver Physiology, American Physiological Society, Vol. 276, No. 1 ( 1999-01-01), p. G164-G172
Abstract:
The effects of extracellular ATP and other nucleotides on the cytosolic free Ca 2+ concentration ([Ca 2+ ] i ) have been studied in single primary human hepatocytes and in human Hep G2 and HuH-7 hepatoma cells. ATP, adenosine 5′- O-(3-thiotriphosphate) (ATPγS), and UTP caused a concentration-dependent biphasic increase in [Ca 2+ ] i with an initial peak followed by a small sustained plateau in most cells. In some cells, however, repetitive Ca 2+ transients were observed. The rank order of potency was ATP ≥ UTP 〉 ATPγS, and complete cross-desensitization of the Ca 2+ responses occurred between ATP and UTP. The initial transient peak in [Ca 2+ ] i was resistant to extracellular Ca 2+ depletion, which demonstrates mobilization of internal Ca 2+ by inositol 1,4,5-trisphosphate whose formation was enhanced by ATP and UTP. In contrast, the sustained plateau phase required influx of external Ca 2+ . Ca 2+ influx occurs most likely through a capacitative Ca 2+ entry mechanism, which was shown to exist in these cells by experiments performed with thapsigargin. On the molecular level, specific mRNA coding for the human P2Y 1 , P2Y 2 , P2Y 4 , and P2Y 6 receptors could be detected by RT-PCR in Hep G2 and HuH-7 cells. However, ADP and UDP, which are agonists for P2Y 1 and P2Y 6 receptors, respectively, caused no changes in [Ca 2+ ] i , demonstrating that these receptors are not expressed at a functional level. Likewise, α,β-methylene-ATP, β,γ-methylene-ATP, AMP, and adenosine were inactive in elevating [Ca 2+ ] i , suggesting that the ATP-induced increase in [Ca 2+ ] i was not caused by activation of P2X or P1 receptors. Thus, on the basis of the pharmacological profile of the nucleotide-induced Ca 2+ -responses, extracellular ATP and UTP increase [Ca 2+ ] i by activating P2Y 2 and possibly P2Y 4 receptors coupled to the Ca 2+ -phosphatidylinositol signaling cascade in human hepatocytes. This suggests that extracellular nucleotides from various sources may contribute to the regulation of human liver cell functions.
Type of Medium:
Online Resource
ISSN:
0193-1857
,
1522-1547
DOI:
10.1152/ajpgi.1999.276.1.G164
Language:
English
Publisher:
American Physiological Society
Publication Date:
1999
detail.hit.zdb_id:
1477329-6
SSG:
12
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