ADP AND INOSITOL TRISPHOSPHATE EVOKE OSCILLATIONS OF A MONOVALENT CATION CONDUCTANCE IN RAT MEGAKARYOCYTES

1. A combination of conventional whole-cell patch clamp recordings and fura-2 fluorescence photometry was used to study the membrane current s during oscillations of intracellular Ca2+ concentration ([Ca2+](i)) in single rat megakaryocytes. 2. At a holding potential of -60 mV, in NaCl external saline and KCl internal saline with low levels of Ca2+ b uffering, 10 mu M ADP evoked [Ca2+](i) oscillations and simultaneous C a2+-gated K+ currents at a frequency of 3-10 spikes min(-1). A smaller inward current was also activated, with a time course that identified this component as the inositol 1,4,5-trisphosphate (IP3)-activated mo novalent cation current previously demonstrated in rat megakaryocytes. 3. Cs+ replacement of internal K+ combined with 100 nM external chary bdotoxin (CTX) abolished the outward currents and revealed that an inw ard current was also transiently activated during each [Ca2+](i) spike . This underlying conductance was permeable to Na+ and Cs+, but posses sed little or no permeability to Cl- or divalent cations. 4. Intracell ular dialysis with IP3 (5-50 mu M) activated the monovalent cationic c onductance prior to release of Ca2+ from intracellular stores. The [Ca 2+](i) increase was associated with a second phase of cationic current , implying that both IP3 and Ca2+ can activate this conductance. Buffe ring of [Ca2+](i) with BAPTA abolished the second phase of current, le aving monophasic spikes of inward current, often occurring at regular intervals. 5. These data demonstrate that a monovalent cation current, which results in Na+ influx under normal ionic conditions, oscillates in response to ADP receptor stimulation due to activation by both IP3 and [Ca2+](i). This provides a route for long-term Na+ entry in the m egakaryocyte following stimulation of receptors coupled to phospholipa se C activation and may play a role in cell shape change.