POLYMYXIN-B HAS MULTIPLE BLOCKING ACTIONS ON THE ATP-SENSITIVE POTASSIUM CHANNEL IN INSULIN-SECRETING CELLS

The action of polymyxin B (0.1 mu M) on ATP-sensitive K+ (K-ATP(+)) ch annels in RINm5F insulin-secreting cells was investigated by patch-cla mp techniques. Using inside-out patches, open-cells and outside-out pa tches, polymyxin B was found to block K-ATP(+) channels by, on average , approximately 90-95% of the initial control level of channel activit y. The effects were rapid in onset, sustained and readily reversible. Similar effects were found in patches excised from cells pretreated ov ernight with 1 mu M of the phorbol ester phorbol myristate acetate (PM A). External block of channels was associated with a marked decrease i n single-channel current amplitude, whereas these effects were not see n when polymyxin B was added to the inside face of the membrane. In pa tches bathed with internally applied ATP (0.5 mM) and ADP (0.5 mM), po lymyxin B inhibited channels but its actions were not reversible upon removal of the compound. However, when the same protocol was undertake n upon cells pre-treated with PMA, the effects of polymyxin B were rea dily reversed. Our data suggests that polymyxin B is a novel modulator of K-ATP(+) channels, exhibiting multiple blocking actions that may p ossibly involve a direct effect upon the channel and indirect effects mediated through the inhibition of endogenous protein kinase(s).