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The activation of a monovalent cation current was studied in rat
megakaryocytes using patch clamp techniques combined with photometric
measurements of intracellular concentrations of Ca
Volume 270,
Number 28,
Issue of July 14, pp. 16638-16644, 1995
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
([Ca
]
) and
Na
. ADP evoked a release of
[Ca
]
and transiently
activated a monovalent cation-selective channel, which, at negative
potentials and under physiological conditions, would be expected to
carry an inward Na
current. The single channel
conductance, estimated by noise analysis from whole cell currents at
-50 to -60 mV was 9 picosiemens. Thapsigargin-induced
[Ca
]
increases failed
to stimulate the monovalent cation current, suggesting that neither
[Ca
]
nor the depletion
of internal Ca
stores were activators of this
conductance. However, buffering of
[Ca
]
changes with
1,2-bis-(2-aminophenoxy)ethane-N,N,N`,N`-tetraacetic acid showed that
both activation and inactivation of the current were accelerated by a
rise in [Ca
]
. The
monovalent cation conductance was activated by internal perfusion with
inositol 1,4,5-trisphosphate, both in the presence and in the absence
of a rise in [Ca
]
.
Internal perfusion with inositol 2,4,5-trisphosphate, the poorly
metabolizable isomer of inositol trisphosphate, similarly activated the
monovalent cation current, whereas 1,3,4,5-tetrakisphosphate neither
activated a current nor modified the ADP-induced monovalent current.
Heparin, added to the pipette, blocked activation of the channel by
ADP. The intracellular concentration of Na
, monitored
by sodium-binding benzofuran isopthalate, increased by 10-20
mM in response to ADP under pseudophysiological conditions. We
conclude the existence of a novel nonselective cation channel in the
plasma membrane of rat megakaryocytes, which is activated by IP
and can lead to increases in cytosolic Na after
stimulation by ADP.
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