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Volume 270, Number 28, Issue of July 14, pp. 16638-16644, 1995
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.

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Baggi Somasundaram , Martyn P. Mahaut-Smith

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 ([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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