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Department of Physiology and Biophysics, University of California at Irvine, Irvine, California 92697
Extracellular ATP (ATPo) elicits a robust
change in the concentration of intracellular Ca2+
([Ca2+]i) in fura-2-loaded mouse thymocytes. Most thymocytes (60%) exposed to ATPo exhibited a biphasic
rise in [Ca2+]i; [Ca2+]i rose slowly at first to a mean
value of 260 nM after 163 s and then increased rapidly
to a peak level of 735 nM. In many cells, a declining plateau, which lasted for more than 10 min, followed the
crest in [Ca2+]i. Experiments performed in the absence
of extracellular [Ca2+]o abolished the rise in thymocyte
[Ca2+]i, indicating that Ca2+ influx, rather than the release of stored Ca2+, is stimulated by ATPo. ATPo-
mediated Ca2+ influx was potentiated as the [Mg2+]o was
reduced, confirming that ATP4
is the active agonist
form. In the absence of Mg2+o, 3
-O-(4-benzoyl)benzoyl-ATP (BzATP) proved to be the most effective agonist of
those tested. The rank order of potency for adenine nucleotides was BzATP4>ATP4>MgATP2>ADP3,
suggesting purinoreceptors of the P2X7/P2Z class mediate the ATPo response. Phenotyping experiments illustrate that both immature (CD4CD8, CD4+CD8+)
and mature (CD4+CD8, CD4CD8+) thymocyte populations respond to ATP. Further separation of the
double-positive population by size revealed that the
ATPo-mediated [Ca2+]i response was much more pronounced in large (actively dividing) than in small (terminally differentiated) CD4+CD8+ thymocytes. We
conclude that thymocytes vary in sensitivity to ATPo
depending upon the degree of maturation and suggest
that ATPo may be involved in processes that control
cellular differentiation within the thymus.
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