Effect of the inactivating "ball" peptide of Shaker B on intermediate conductance Ca2+-dependent inwardly rectifying K+ channels of HeLa cells

The patch-clamp technique was used to study the effect of intracellularly a dded inactivating "ball" peptide (BP) of the Shaker B K+ channel upon Ca2+- dependent inwardly rectifying K+ channels of the intermediate conductance t ype expressed in HeLa cells. Intracellular BP caused only moderate inhibiti on of outward K+ currents when assayed at an intracellular Ca2+ concentrati on of 100 nmol/l. Increasing intracellular Ca2+ levels led in itself to som e voltage-dependent blockade of K+ currents, which was absent when high ext racellular K+ was used. An additional strong blockade by intracellular BP w as nevertheless observed both in Na+- and K+ rich extracellular solutions. A non-inactivating BP analogue had no effect. At this higher intracellular Ca2+ concentration the inhibition of these intermediate conductance Ca2+-de pendent channels by BP was voltage-dependent, being absent at hyperpolarizi ng potentials, and could be relieved by increasing extracellular K+. These data suggest that BP acts at an internal pore site in Ca2+-dependent interm ediate conductance K+ channels of HeLa cells, and that these might possess a receptor site for the peptide similar to that of other K+ channels such a s Ca2+-activated maxi-K+ channels.