Differential depolarization-activated calcium responses in fetal and neonatal rat osteoblast-like cells

Abstract

The present study evaluates differential occurrence of voltage-dependent calcium channels (VDCC) in the membranes of fetal (FROB) and neonatal (NROB) calvarian rat osteoblastic cells in primary culture. The intracellular calcium concentration ([Ca²⁺]_i) was monitored upon depolarization of the cell membrane with the use of high K⁺ containing extracellular solutions. [Ca²⁺]_i was measured in populations of cells as well as in individual cells using Fura-2, whereas the membrane potential (E_m) was recorded in parallel experiments using patch-clamp techniques. Increasing the extracellular K⁺ concentration resulted in an instantaneous depolarization of E_m of both FROB and NROB. This depolarization of E_m did not significantly affect [Ca²⁺]_i of populations of FROB and neonatal osteoblast precursors (NpROB). In contrast to FROB and NpROB, NROB populations responded to depolarization with significant transient [Ca²⁺]_i increases that could be blocked by the calcium antagonist verapamil and were absent if extracellular Na⁺ was replaced for choline instead of K⁺. In individual cell measurements, response frequencies as well as the magnitude of [Ca²⁺]_i responses upon depolarization of NROB were much higher than those of FROB, suggesting that more NROB than FROB possess VDCC. This phenomenon might point to a development-related expression of VDCC in the membranes of osteoblast-like cells.