The effects of colchicine or vinblastine on the blood calcium level in rats

In order to clarify how microtubule inhibitors induce hypocalcemia, rats were injected with intravenous colchicine (1 mg/kg) or vinblastine (2 mg/kg). The blood calcium levels decreased rapidly, and the minimum values, reached 4 h after the injection, were 7.55 +/- 0.70 mg/100 ml (mean +/- S.D., P < 0.001) for colchicine and 7.61 +/- 0.17 mg/100 ml (P < 0.001) for vinblastine. At 24 h, these values returned to the normal range (10.13 +/- 0.42 mg/100 ml). The blood calcium values in rats fed a low calcium diet and in thyroparathyroidectomized rats were also reduced by colchicine. The incorporation of blood 45Ca into bone was reduced by the injection of colchicine. Histologically, the bone cells of rats injected with either drug were severely damaged 8 h after the injection. These results indicate that hypocalcemia may be mediated by interference with the regulatory mechanisms of bone cell calcium homeostasis, and that the destruction of microtubules may be closely related to the development of the hypocalcemia.