1. The electrophysiological properties of the cell membrane of guinea‐pig megakaryocytes were studied using the whole‐cell patch‐clamp technique. The megakaryocytes (diameter, 17‐42 microns) were dissociated mechanically from the bone marrow of adult guinea‐pigs. 2. In a proportion of cells, spike‐like action potentials were generated in response to depolarization when the cells were immersed in standard saline containing 10 mM‐Ca2+. Under voltage clamping, a transient inward current followed by a slowly Ca2+. Under voltage clamping, a transient inward current followed by a slowly developing outward current was produced when the membrane potential was made more positive than ‐55 mV. 3. The inward currents were identified as Ca2(+)‐carried current, since the amplitude depended distinctly on external Ca2+ concentration and since replacement of external Ca2+ with Mn2+ reversibly diminished the current. The Ca2+ channels involved are most probably of the transient type (T‐type). 4. The reversal potential of the outward current changed from ‐87 to ‐46 and ‐7 mV when the external K+ concentration was raised from 5 to 25 and 125 mM. 5. The outward current was insensitive to chelation of internal Ca2+ but was blocked by external application of quinine, 4‐aminopyridine and tetraethylammonium, and was thus very probably a membrane potential‐dependent K+ current. The dependence of the current activation and inactivation on the membrane potential was consistent with that of a delayed K+ rectifier. 6. The amplitudes of the Ca2+ currents and K+ currents showed considerable intercell variation. However, the density of the Ca2+ current showed a tendency to increase with megakaryocyte size, presumably accompanying maturation. The roles of these currents in cellular function remain to be elucidated.
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