L-type calcium channels in growth plate chondrocytes participate in endochondral ossification

Abstract

Longitudinal bone growth occurs by a process called endochondral ossification that includes chondrocyte proliferation, differentiation, and apoptosis. Recent studies have suggested a regulatory role for intracellular Ca-2 divided by (Ca-i(2 divided by)) in this process. Indirect studies, using Ca2+ channel blockers and measurement of Ca-i(2+), have provided evidence for the existence of Ca2+ channels in growth plate chondrocytes. Furthermore, voltage-gated Ca2+ channels (VGCC), and specifically L-and T-typeVGCCs, have been recently described in murine embryonic growth plates. Our aim was to assess the effect of L-type Ca2+ channel blockers on endochondral ossification in an organ culture. We used cultures of fetal rat metatarsal rudiments at 20 days post gestational age, with the addition of the L-type Ca2+ channel blockers verapamil (10-100 mu M) or diltiazem (10-200 mu M) to the culture medium. Longitudinal bone growth, chondrocyte differentiation (number of hypertrophic chondrocytes), and cell proliferation (incorporation of tritiated thymidine) were measured. Verapamil dose-dependently decreased growth, the number of hypertrophic chondrocytes, and cell proliferation, at concentrations of 10-100 mu M. Growth and the number of hypertrophic chondrocytes decreased significantly with diltiazem at 50-100 mu M and proliferation decreased significantly at concentrations of 10-200 mu M. Additionally, there was no increase in apoptosis over physiological levels with either drug. We confirmed the presence of L-type VGCCs in rat rudiments using immunohistochemistry, and showed that the antagonists did not alter the pattern of VGCC expression. In conclusion, our data suggest that L-type Ca2+ channel activity in growth plate chondrocytes is necessary for normal longitudinal growth, participating in chondrocyte proliferation and differentiation.