Abstract
Various types of evidence suggest that some changes in cellular in cellular calcium may well signal the initiation of a chain of events leading to the physiological effects of the bone resorbing agents. The effects of 1,25-dihydorxycholecalciferol, $1.25\textrm{(OH)}_2\textrm{D}_3$, Ca ionophore A23187 and calcium antagonist, diltiazem on bone resprption and the cellular transport of Ca were investigated. Bone $^{45}\textrm{Ca}$ desaturation experiment was realized in isolated heterogenous rat bone cells after equilibrating the cells with $^{45}\textrm{Ca}$. Results of $^{45}\textrm{Ca}$ desaturation experiments were analysed by fitting the $^{45}\textrm{Ca}$ desaturation curve to a model of 2 exponential terms which indicated the presence of 2 exchangeable cellular calcium pools. $1.25\textrm{(OH)}_2\textrm{D}_3$ (0.5ng/$m\ell$) induced significantly bone resorption which was decreased by the physiological dose of diltiazeme(above 5nmol/$m\ell$) although it was ineffective alone. Ionophore A23187 (0.2$\mu\textrm{g}$/$m\ell$) decreased Ca release from bone but no additivity of effect with diltiazem(20nmol/$m\ell$) was observed. $1.25\textrm{(OH)}_2\textrm{D}_3$ (0.5ng/$10^{6}$ cells) had a moderate effect on the two kinetic phases of $^{45}\textrm{Ca}$ desaturation curve and these values were normalized when diltiazeme (20nmol/$10^{6}$ cells) was added along with $1.25\textrm{(OH)}_2\textrm{D}_3$. Ionophore($0.05\mu\textrm{g}$/$10^{6}$ cells) alone increased specifically the value of the slow turnover rate which was not affected by addition of diltiazem. The hypothesis concerning the involvement of calcium in bone resorption seems in fact to be verified in case of $1.25\textrm{(OH)}_2\textrm{D}_3$ but more unsettled for Ca inophore A23187.