Differential response to kidney mitochondrial calcium-regulating mixed function oxidases to thyrotoxicosis

Induction of hyperthyroidism in chicks by 6 consecutive daily injections of either L-thyroxine, 40 microgram/100 g of body weight, or 3,3',5'-triiodo-L-thyronine, 4 microgram/100 g of body weight, suppresses kidney mitochondrial 25-hydroxyvitamin D3-1 alpha-hydroxylase activity by 50% but is stimulates severalfold the 24-hydroxylation of the substrate in the same organelle. Plasma calcium concentrations of vitamin D3-deficient or normal birds, 1.68 and 3.47 mM, respectively, do not change significantly in response to the two thyroid hormones. However, plasma phosphorus concentration in the vitamin D3-deficient birds increases markedly from 1.47 to 2.70 mM. A slight increase was noted in the plasma phosphorus level of normal birds receiving the hormones. The present study suggests that alterations in the enzymatic metabolism of vitamin D3 in the kidney are responsible for the disturbances of mineral balance in hyperthyroidism.