Molecular characterization of the α₁ subunit of the L type voltage calcium channel expressed in rat calvarial osteoblasts

Voltage-activated calcium channels (VACCs) regulate extracellular calcium influx in many cells. VACCs are composed of five subunits. The α₁ subunit is considered the most important in regulating channel function. Three isoforms of this subunit have been described: skeletal, cardiac, and neuroendocrine. It was the purpose of the present study to determine the molecular identity of the α₁ subunit of the VACCs in rat calvarial osteoblasts and to study the nature of the regulation of these channels as a function of cellular growth. We also attempted to identify which isoform of the α₁ subunit of the VACCs mediates the effects of epidermal growth factor (EGF) on osteoblastic cell proliferation. Reverse transcription-polymerase chain reaction was used to detect the isoforms of the VACCs that are expressed in osteoblastic cells. These analyses showed that the proliferative state of the cell and the time in culture influence RNA expression. The only α₁ subunit detected in osteoblasts corresponds to the cardiac isoform. In additional experiments, the effects of EGF on cytosolic calcium and osteoblast proliferation were determined. For these experiments, the synthesis of the different isoforms of the VACCs was selectively blocked by antisense oligonucleotides prior to EGF stimulation. These studies showed that the cardiac isoform mediates the effects of EGF on cytosolic calcium and cellular proliferation in rat calvarial osteoblasts.