Characterization of voltage-sensitive calcium channels in growth plate chondrocytes

Growth plate chondrocytes (GPCs), cells integrally involved with the process of endochondral bone formation, facilitate Ca2+ infux to provide a source of ion for processes such as Ca2+ signaling and matrix vesicle loading. We hypothesize that this Ca2+ entry into GPCs is achieved through the action of voltage-sensitive Ca2+ channels. This hypothesis was tested by measuring intracellular [Ca2+] changes in fura 2-loaded GPCs that were depolarized by challenge with a K(+)-containing medium. KCl doses between 55 and 95 mM evoked significant Ca2+ responses that were blocked by addition of extracellular EGTA. The Ca2+ response evoked by 95 mM K+ was insensitive to 100 microM doses of nifedipine or nitrendipine, ruling out L-type channel involvement. This finding was corroborated by the observation that 10 microM BAY K 8644 did not activate a Ca2+ response of its own. However, 10 microM Cd2+ significantly inhibited the 95 mM Ks(+)-evoked effects, suggesting N-type channel activity. Use of 1 microM Ni+ in an attempt to block possible T-type channel activity caused nonspecific cellular effects, precluding pharmacological assessment of a possible T-type channel activity. These data (i) provide the first direct evidence for voltage-sensitive Ca2+ channel activity in GPCs and (ii) suggest at least partial facilitation of that activity through N type channels.