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In many types of excitable cell, there are several classes of voltage-dependent calcium channel (VDCC) as determined by the characteristic properties of the single channel activity. There is a clear distinction between low conductance channels, activated by moderate depolarizations (low voltage-activated, LVA) and high conductance channels activated by large depolarizations (high voltage- activated, HVA).2,36 In cardiac tissue these were termed T and L channels, respectively, because they were found to be pharmacologically, as well as biophysically, distinct.2 Because of this they can also be clearly differentiated in whole-cell current recordings.2,12 There is also evidence for a third class of single channel conductance (N type), whose biophysical properties were originally described as being intermediate between T and L, and which appears to be expressed only in cells of neuronal origin.12,38,51 The sensitivity of N-type channels to irreversible block by ω-conotoxin GVIA (ωCgTx),38 and the large number of ω-CgTx binding sites in neuronal tissue, indicates that they are likely to be important for neuronal function.31 High threshold Ca2+ currents insensitive to both ω-CgTx and dihydropyridines have been reported,27,38 and a selective blocker for at least part of this current is the peptide toxin from Agelenopsis Aperta, ω-agatoxin IVA (ω-aga IVA). |
