A NOVEL MONOVALENT CATION CHANNEL ACTIVATED BY INOSITOL TRISPHOSPHATE IN THE PLASMA-MEMBRANE OF RAT MEGAKARYOCYTES

The activation of a monovalent cation current was studied in rat megakaryocytes using patch clamp techniques combined with photometric measurements of intracellular concentrations of Ca2+ ([Ca2+](i)) and Na+. ADP evoked a release of [Ca2+](i) 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 [Ca2+](i) increases failed to stimulate the monovalent cation current, suggesting that neither [Ca2+](i) nor the depletion of internal Ca2+ stores were activators of this conductance. However, buffering of [Ca2+](i) 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 [Ca2+](i). 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 [Ca2+](i). 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 IP3 and can lead to increases in cytosolic Na+ after stimulation by ADP.