Eiichiro Shimazawa

The regulatory role of calcium in 25-hydroxycholecalciferol metabolism in chick kidney in vitro

Abstract The regulatory effect of calcium added in vitro on 25-hydroxycholecalciferol metabolism was studied in kidney mitochondria and in renal tubules from vitamin D-deficient chicks. The addition of calcium (0.05 – 0.2 m m ) to mitochondrial suspensions prepared with calcium-chelating agents caused a marked and dose-related stimulation of 1-hydroxylation. A sharp decline in the activity was induced by higher concentrations of calcium (0.3 – 0.7 m m ). A similar but less striking biphasic effect of calcium on 1-hydroxylation was observed in mitochondria prepared in the absence of calcium chelating agents. The effect of calcium was not a consequence of accelerated mitochondrial translocation of either exogenous NADP or Mg2+ but was related to mitochondrial calcium content. The addition of inhibitors of the calcium uptake, i.e., LaCl3 or ruthenium red, or a calcium ionophore (A 23187) significantly inhibited the calcium-induced stimulation of the 1-hydroxylation reaction. Similar calcium effects were also observed in renal tubules isolated from intact, but not from parathyroidectomized, vitamin D-deficient chicks. These data strongly suggest that mitochondrial calcium plays an important role in the regulation of 1-hydroxylase activity in kidney.

Failure to demonstrate stimulatory effect of prolactin on vitamin D metabolism in vitamin-D-deficient rats.

The effect of administration of bovine prolactin in vivo on renal 1 alpha-hydroxylase (1 alpha-OHase) activity was investigated using thyroparathyroidectomized and sham-operated vitamin-D-deficient male rats. A high dose (1.5 IU/hr) and a low dose (0.3 IU/hr) of bovine prolactin was constantly infused into the cannulated femoral vein for 6 hr or 30 hr. The accumulation of 1 alpha, 25-hydroxyvitamin D3 [1 alpha, 25-(OH)2-D3] in plasma produced from 25-hydroxy-vitamin D3 (25-OH-D3) was calculated from the 6-hr conversion of intravenously injected [3H]-25-OH-D3 to [3H]-1 alpha, 25-(OH)2-D3. Both high dose and low dose of prolactin administration caused no significant stimulatory effect on 1 alpha-OHase activity in TPTX animals. Also, in sham-operated rats the prolactin treatment produced no significant change in the 1 alpha-OHase activity. Plasma calcium concentrations showed a tendency to increase slightly. These results suggest that prolactin has no direct stimulatory role on renal 1 alpha-OHase activity in rats. Its effect on calcium metabolism may be mediated by a different mechanism(s).