Molecular diversity of voltage-sensitive calcium channels in smooth muscle cells

Voltage-sensitive calcium channels play an important role in the excitation-contraction coupling of smooth muscle. Several subunits form the oligomeric channel complex and determine its functional properties. Therefore a differential distribution of the various channel subunits and their splice forms could contribute to the functional specialization of smooth muscle cells. To test this hypothesis, specific primers were designed to amplify messenger ribonucleic acid (mRNA) from vascular and gastrointestinal smooth muscle of the rabbit by reverse transcription and polymerase chain reaction (RT-PCR). The presence of high- and low-threshold voltage-dependent calcium channels was also examined in a smooth muscle-derived cell line (A7R5). Consistent with the physiologic data, smooth muscle contains mRNA for the pore-forming subunits of high- and low-threshold voltage-dependent calcium channels, alpha-1C and alpha-1G. Three splice variants of the alpha-1C-subunit were identified in smooth muscle. These may affect dihydropyridine binding and the interaction between the alpha-1C and the beta-subunit. In addition, three of the four cloned beta-subunits (beta-1b, beta-2, and beta-3) could be found in all smooth muscle tissues examined. These data demonstrate that various splice forms of the L-type calcium channel exist in smooth muscle tissue. Moreover, these experiments also show for the first time that smooth muscle cells contain mRNA for low-threshold voltage-sensitive calcium channels. Combinations of the pore-forming subunits with one of the three beta-subunits could account for functional differences between smooth muscle cells from distinct regions. A better understanding of the structure and function of these channels may help in our understanding of diseases affecting smooth muscle and help in the development of novel drugs targeting these molecules.