AMINOPYRIDINE BLOCK OF POTASSIUM CHANNELS IN MOUSE NEUROBLASTOMA-CELLS

Although 4-aminopyridine (4-AP) is known to block a variety of voltage -dependent K channels, details as to the site of action and the mechan ism of block are known for relatively few. Single channel analysis has not been extensively used to answer these questions. The actions of 4 -AP on whole cell K currents and single voltage-dependent K channels t hat exhibit fast activation and inactivation were therefore examined i n N1E-115 neuroblastoma cells. The concentration for half block (K0.5) of the whole cell K current for externally applied compounds was foun d to be 56 mum for 4-AP and 0.3 mM for 3,4-diaminopyridine. 4-AP slowe d the rate of development of outward K current, and the rate of decay after repolarization. These effects were consistent with the idea that 4-AP preferentially blocked a type of K channel generating a transien t current. Block of this component of current was time- and use-depend ent. 4-AP blocked the channel responsible for the transient outward cu rrent by decreasing the probability of an open channel in inside-out p atches. 4-AP reduced the open time, indicating that 4-AP can interact with the open channel. The first latency to opening was also increased . 4-Aminopyridine methiodide (4-APMI), a permanently charged derivativ e, blocked the whole cell current with a K0.5 = 0.19 mM. Block by 4-AP MI was found to be by a different mechanism at a different site compar ed to 4-AP. Single channel analysis revealed that 4-APMI reduced the c urrent through an open channel when applied externally to outside-out patches, in contrast to the action of 4-AP. When applied to the bath i n the intact patch recording mode, 4-AP blocked K current, suggesting an internal site of action, but 4-APMI could not, indicating that bloc k occurs at an external site. Block of the whole cell current by 4-AP did not vary with voltage, but block of K current by 4-APMI was reduce d with depolarization.