cDNA cloning of a dihydropyridine-sensitive calcium channel from rat aorta. Evidence for the existence of alternatively spliced forms

Abstract

Several complementary DNAs have been isolated from rat aorta cDNA libraries that encode the alpha 1 subunit of the L-type dihydropyridine-sensitive voltage-dependent calcium channel (VDCC). The clones were isolated using previously cloned rabbit skeletal and cardiac VDCC cDNAs as well as newly isolated rat aorta cDNA clones as probes. The complete nucleotide sequence of the 8305-base pair aortic alpha 1 cDNA codes for a protein of 2169 amino acids, which corresponds to an Mr of 243,615. The deduced amino acid sequence exhibits 93 and 65% amino acid identity with previously cloned rabbit cardiac and rabbit skeletal muscle dihydropyridine-sensitive VDCCs, respectively. The close identity of the aortic alpha 1 cDNA with cardiac alpha 1 cDNAs suggests that they arise from the same gene. Specific divergent areas of the aortic cDNA suggest that this cDNA represents an alternatively spliced form. We report the existence of two forms of a specific region within this aortic cDNA which also exists in cardiac tissue. This evidence supports the existence of alternative splicing for calcium channel cDNAs, a mechanism which provides diversity and potential tissue-specific isoforms of the L-type VDCC. The rat aorta alpha 1 isoform has a message size of 8.6 kilobases (kb). Rat aorta also contains additional transcripts of 12.0 and 6.5 kb. Northern blot analysis of poly(A)+ RNA from several rat tissues including heart, brain, and several smooth muscles reveals the co-existence of the 8.6- and 12.0-kb mRNA species but not the 6.5-kb species. Southern analysis of rat genomic DNA reveals a low copy number of the gene encoding this type of calcium channel.