Rodent Osteoblastic Cells Express Voltage-Sensitive Calcium Channels Lacking a γ Subunit.

Voltage-sensitive calcium channels (VSCC) open in response to external stimuli, including calcitropic hormones, that alter plasma membrane calcium (Ca2+) permeability. Ca2+ that enters the cell through these channels serves a second messenger function, eliciting cellular responses that include secretion and changes in gene expression. In osteoblasts, VSCCs serve as key regulators of Ca2+ permeability and are a major class of calcitropic hormone-sensitive Ca2+ channels present in the plasma membrane. The members of the VSCC family exist as a complex of polypeptide subunits that are comprised of a pore-forming α1 subunit, an intracellular β subunit, a dimer of disulfide-linked α2 and δ subunits, and in some tissues, a γ subunit. Previous studies in our laboratory have shown that the major functional α1 subunit present in osteoblasts is the α1C (CaV1.2). To determine the complement of auxiliary subunits present in rodent osteoblastic cells, we employed RT-PCR using a battery of subunit specific primers and appropriate tissue controls. Immunohistochemistry also was performed, using available subunit specific antibodies, to measure protein expression and localization. Cell types examined included MC3T3-E1 at various stages of differentiation, ROS 17/2.8 osteosarcoma, and primary cultures of rat calvarial osteoblasts. The results indicate that all cells expressed multiple β subunit classes and α2δ dimers, but no γ subunits, regardless of differentiation state. We propose a structure for the functional osteoblast VSCC that consists of α1, β, α2δ subunits and is devoid of a γ subunit. [ABSTRACT FROM AUTHOR]