SINGLE-CHANNEL CHARACTERIZATION OF THE PHARMACOLOGICAL PROPERTIES OF THE K(CA2-CELLS() CHANNEL OF INTERMEDIATE CONDUCTANCE IN BOVINE AORTICENDOTHELIAL)

The pharmacological profile of a voltage-independent Ca2+-activated po tassium channel of intermediate conductance (IK(Ca2+)) present in bovi ne aortic endothelial cells (BAEC) was investigated in a series of ins ide-out and outside-out patch-clamp experiments. Channel inhibition wa s observed in response to external application of ChTX with a half inh ibition concentration of 3.3 +/- 0.3 nM (n = 4). This channel was inse nsitive to IbTX, but channel block was detected following external app lication of MgTX and StK leading to the rank order toxin potency ChTX > StK > MgTX >>IbTX. A reduction of the channel unitary current amplit ude was also measured in the presence of external TEA, with half reduc tion occurring at 23 +/- 3 mM TEA (n = 3). The effect of TEA was volta ge insensitive, an indication that TEA may bind to a site located on e xternal side of the pore region of this channel. Similarly, the additi on of d-TC to the external medium caused a reduction of the channel un itary 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(Ca2+) in BAEC was found to be inhibited by NS1619, an activator of the Ca2+-activated potassium chan nel of large conductance (Maxi K(Ca2+)), with a half inhibition value of 11 +/- 0.8 mu M (n = 4). These results provide evidence for a pharm acological profile distinct from that reported for the Maxi K(Ca2+) ch annel, with some features attributed to the voltage-gated K(v)1.2 pota ssium channel.