L-type channel proteins from other cells

Studies with polyclonal antibodies against the 0.2 d protein have identified neuronal 0.2 polypeptides [58] and other studies with anti- xi antibodies have identified a similar protein in brain [72]. Future studies will be aimed at further clarifying the similarities and differences in the structures of L channel proteins from various sources. 

The DNAs for each of the subunits of the skeletal muscle channel have been cloned. From these studies there is no doubt that the subunit is the main channel forming unit of the DHP-sensitive channels (see below). 

Key findings that demonstrated that the 0 subunit is the essential component of L-type channels have come from studies of the channel activity of the expressed protein. Expression studies performed in mammalian liver fibroblasts have demonstrated that the oti subunit alone can form a channel [77] and contains the receptors for the DHPs, PAAs and diltiazem [64]. In very elegant studies using a mouse model of muscular dysgenesis it has been demonstrated that the ] subunit DNA can restore Ca currents and the charge movement that arises from the voltagesensing function of the channels to the mutant cells that normally lack these activities [21,78,79]. The restoration of these activities restores excitation-contraction coupling. Thus it is clear that the aj subunit is the major functional unit of L-type Ca channels. 

Isoforms of the oi, subunit have now also been cloned from cardiac [80] and smooth muscle [81,82] cDNA libraries, and partial clones resembling this structure have been obtained from brain libraries [20]. In addition, a cDNA encoding a high 

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