Functional Properties of CaV1.3 (α1D) L-type Ca2+ Channel Splice Variants Expressed by Rat Brain and Neuroendocrine GH3 Cells

Ca²⁺ enters pituitary and pancreatic neuroendocrine cells through dihydropyridine-sensitive channels triggering hormone release. Inhibitory metabotropic receptors reduce Ca²⁺ entry through activation of pertussis toxin-sensitive G proteins leading to activation of K⁺ channels and voltage-sensitive inhibition of L-type channel activity. Despite the cloning and functional expression of several Ca ²⁺ channels, those involved in regulating hormone release remain unknown. Using reverse transcription-polymerase chain reaction we identified mRNAs encoding three α₁ (α_1A, α _1C, and α_1D), four β, and one α ₂-δ subunit in rat pituitary GH₃ cells; α _1B and α_1S transcripts were absent. GH₃ cells express multiple alternatively spliced α_1D mRNAs. Many of the α_1D transcript variants encode "short" α_1D (α_1D-S) subunits, which have a QXXER amino acid sequence at their C termini, a motif found in all other α ₁ subunits that couple to opioid receptors. The other splice variants identified terminate with a longer C terminus that lacks the QXXER motif (α_1D-L). We cloned and expressed the predominant α_1D-S transcript variants in rat brain and GH₃ cells and their α_1D-L counterpart in GH₃ cells. Unlike α_1A channels, α_1D channels exhibited current-voltage relationships similar to those of native GH₃ cell Ca²⁺ channels, but lacked voltage-dependent G protein coupling. Our data demonstrate that alternatively spliced α_1D transcripts form functional Ca²⁺ channels that exhibit voltage-dependent, G protein-independent facilitation. Furthermore, the QXXER motif, located on the C terminus of α_1D-S subunit, is not sufficient to confer sensitivity to inhibitory G proteins.