POLYAMINES BLOCK CA2-ACTIVATED K+ CHANNELS IN PITUITARY-TUMOR CELLS (GH3)()

The effects of the natural polyamines, putrescine, spermidine and sper mine on single calcium-activated potassium channels from clonal rat pi tuitary tumor cells (GH3) were studied. Applied to inside-out patches, polyamines were found to reduce the current amplitude and open probab ility of the channels in a dose- and voltage-dependent manner, indicat ing that polyamines act as fast blockers which sense a fraction of the electrical field in the channel pore. The K-d for spermine was 11.2 m M for the reduction of unitary current amplitude and 0.7 mM for the re duction of the open probability. The order of effectiveness was spermi ne > spermidine > putrescine. From fitting beta-functions to current a mplitude histograms, blocking and unblocking rates were determined as 11.4 x 10(4) sec(-1) and 21.9 x 10(4) sec(-1), respectively. The reduc tion of the channel open probability was relieved by an increase of th e Ca2+ concentration of the internal solution, indicating that polyami nes compete with Ca2+ at the Ca2+ sensor of the channel. Putrescine an tagonized the effect of spermine on the channel current amplitude. The results suggest that polyamines at intracellular millimolar concentra tions suppress ion channel activity and therefore may effect electrica l discharge behavior of excitable cells.