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 α_1C (Ca_V1.2). 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]