Single-channel characterization of the pharmacological properties of the K(Ca2+) channel of intermediate conductance in bovine aortic endothelial cells

The pharmacological profile of a voltage-independent Ca(2+)-activated potassium channel of intermediate conductance (IK(Ca(2+))) present in bovine aortic endothelial cells (BAEC) was investigated in a series of inside-out and outside-out patch-clamp experiments. Channel inhibition was observed in response to external application of ChTX with a half inhibition concentration of 3.3 +/- 0.3 nM (n = 4). This channel was insensitive to IbTX, but channel block was detected following external application of MgTX and StK leading to the rank order toxin potency ChTX > StK > MgTX >>IbTX. A reduction of the channel unitary current amplitude was also measured in the presence of external TEA, with half reduction occurring at 23 +/- 3 mM TEA (n = 3). The effect of TEA was voltage insensitive, an indication that TEA may bind to a site located on external side of the pore region of this channel. Similarly, the addition of d-TC to the external medium caused a reduction of the channel unitary current amplitude with half reduction at 4.4 +/- 0.3 mM (n = 4). In contrast, application of d-TC to the bathing medium in inside-out experiments led to the appearance of long silent periods, typical of a slow blocking process. Finally, the IK(Ca(2+)) in BAEC was found to be inhibited by NS1619, an activator of the Ca(2+)-activated potassium channel of large conductance (Maxi K(Ca(2+))), with a half inhibition value of 11 +/- 0.8 mu M (n = 4). These results provide evidence for a pharmacological profile distinct from that reported for the Maxi K(Ca(2+)) channel, with some features attributed to the voltage-gated K(v)1.2 potassium channel.