DIFFERENTIAL DEPOLARIZATION-ACTIVATED CALCIUM RESPONSES IN FETAL AND NEONATAL RAT OSTEOBLAST-LIKE CELLS

The present study evaluates differential occurrence of voltage-depende nt calcium channels (VDCC) in the membranes of fetal (FROB) and neonat al (NROB) calvarian rat osteoblastic cells in primary culture. The int racellular calcium concentration ([Ca2+]i) was monitored upon depolari zation of the cell membrane with the use of high K+ containing extrace llular 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 technique s. Increasing the extracellular K+ concentration resulted in an instan taneous depolarization of E(m) of both FROB and NROB. This depolarizat ion of E(m) did not significantly affect [Ca2+]i of populations of FRO B and neonatal osteoblast precursors (NpROB). In contrast to FROB and NpROB, NROB populations responded to depolarization with significant t ransient [Ca2+]i increases that could be blocked by the calcium antago nist verapamil and were absent if extracellular Na+ was replaced for c holine instead of K+. In individual cell measurements, response freque ncies as well as the magnitude of [Ca2+]i responses upon depolarizatio n of NROB were much higher than those of FROB, suggesting that more NR OB than FROB possess VDCC. This phenomenon might point to a developmen t-related expression of VDCC in the membranes of osteoblast-like cells .