The Effects of Colchicine and Vinblastine on Parathyroid Hormone Secretion in the Rat

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Abstract
Colchicine and vinblastine interfere with the secretory process of several hormones. This inhibitory effect is apparently related to the microtubule-disrupting effects of these 2 drugs. The development of a sensitive radioimmunoassay for rat PTH in the laboratory permitted assessing the effects of colchicine and vinblastine on PTH release in vivo. As demonstrated by others, rats injected with these drugs become hypocalcemic. The administration of colchicine (2.5 mg/kg i.v.) produced a decrease in ionized calcium (Ca2+) from 4.51 .+-. 0.08 to 3.61 .+-. 0.24 mg/100 ml; concomitantly, immunoreactive plasma PTH increased from 80 .+-. 10 to 287 .+-. 23 pg/ml. When vinblastine was administered (10 mg/kg i.v.). Ca2+ decreased to 4.2 .+-. 0.07 mg/100 ml and PTH rose to 257 .+-. 50 pg/ml. Greater degrees of hypocalcemia produced by EDTA administration in rats pretreated with colchicine of hypocalcemia produced by EDTA administration in rats pretreated with colchicine or vinblastine resulted in higher levels of circulating PTH. When synthetic, biologically active, bovine PTH 1-34 was injected into control and colchicine-treated rats, the disappearance of immunoreactivity from peripheral blood was the same in both groups. It appears that the parathyroid gland responded to the hypocalcemic stimulus despite the structural changes induced by colchicine and vinblastine. Colchicine and vinblastine do not inhibit the release of PTH in vivo.

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