Smooth muscle uses another promoter to express primarily a form of human Ca_V1.2 L-type calcium channel different from the principal heart form

Several different first exons and amino termini have been reported for the cardiac Ca channel known as α_1C or Ca_V1.2. The aim of this study was to investigate whether the expression of this channel is regulated by different promoters in smooth muscle cells and in heart in humans. Ribonuclease protection assay (RPA) indicates that the longer first exon 1a is found in certain human smooth muscle-containing tissues, notably bladder and fetal aorta, but that it is not expressed to any significant degree in lung or intestine. On the other hand, all four smooth muscle-containing tissues examined strongly express transcripts containing exon 1b, first reported cloned from human fibroblast cells [1]. In addition, primary cultures of human colonic myocytes and coronary artery smooth muscle cells express predominantly transcripts containing exon 1b. The promoter immediately upstream of exon 1b was cloned, and it displays functional promoter activity when luciferase-expressing constructs were transfected into three different cultured smooth muscle cells: primary human coronary artery smooth muscles cells, primary human colonocytes, and the fetal rat aorta-derived A7r5 cell line. These results indicate that expression in smooth muscle is primarily driven by a promoter different from that which drives expression in cardiac myocytes.