Effects of the Level of mRNA Expression on Biophysical Properties, Sensitivity to Neurotoxins, and Regulation of the Brain Delayed-rectifier K+ Channel Kv1.2

Injection of 0.2 ng of cRNA encoding the brain Kvl.2 channel into Xenopusoocytes leads to the expression of a very slowly inactivating K⁺current. Inactivation is absent in oocytes injected with 20 ng of cRNA although activation remains unchanged. Low cRNA concentrations generate a channel which is sensitive to dendrotoxin I(IC₅₀= 2 nM at 0.2 ng of cRNA/oocyte) and to less potent analogs of this toxin from Dendroaspis polylepisvenom. A good correlation is found between blockade of the K⁺current and binding of the different toxins to rat brain membranes. High cRNA concentrations generate another form of the K⁺channel which is largely insensitive to dendrotoxin I(IC₅₀= 200 nM at 20 ng of cRNA per oocyte). At low cRNA concentrations, the expressed Kv1.2 channel is also blocked by other polypeptide toxins such as MCD peptide (IC₅₀= 20 nM), charybdotoxin (IC₅₀= 50 nM), and β-bungarotoxin(IC₅₀= 50 nM), which bind to distinct and allosterically related sites on the channel protein. The pharmacologically distinct type of K⁺channel expressed at high cRNA concentrations (20 ng of cRNA/oocyte) is nearly totally resistant to 100 nM MCD peptide and hardly altered by charybdotoxin and β-bungarotoxin at concentrations as high as 1µM, Both at low and at high cRNA concentrations, the expressed Kv1.2 channel is blocked by an increase in intracellular Ca²⁺from the inositol trisphosphate sensitive pools and by the phorbol ester PMA that activates protein kinase C.