COMPLEXITY OF THE OUTWARD K+ CURRENT OF THE RAT MEGAKARYOCYTE

Megakaryocytes isolated from rat bone marrow express a voltage-depende nt, outward K+ current with complex kinetics of activation and inactiv ation. We found that this current could be separated into at least two components based on differential responses to K+ channel blockers. On e component, which exhibited features of the ''transient'' or ''A-type '' K+ current of excitable cells, was more strongly blocked by 4-amino pyridine (4-AP) than by tetrabutylammonium (TBA). This current, which we designated as ''4-AP-sensitive'' current, activated rapidly at pote ntials more positive than -40 mV and subsequently underwent rapid volt age-dependent inactivation. A separate current that activated slowly w as blocked much more effectively by TBA than by 4-AP. This ''TBA-sensi tive'' component, which resembled a typical delayed rectifier current, was much more resistant to voltage-dependent inactivation. The relati ve contribution of each of these components varied from cell to cell. The effect of charybdotoxin was similar to that of 4-AP. Our data indi cate that the voltage-dependent K+ current of resting megakaryocytes i s more complex than heretofore believed and support the emerging conce pt that megakaryocytes possess intricate electrophysiological properti es.