Britain Stimulation of 1 , 25 - dihydroxyvitamin D 3 production by 1 , 25 - dihydroxyvitamin D 3 in the hypocalcaemic rat

Serum 1,25-dihydroxyvitamin D3 concentration and renal 25-hydroxyvitamin D 1 a-hydroxylase activity were measured in rats fed various levels of calcium, phosphorus and vitamin D3. Both calcium deprivation and phosphorus deprivation greatly increased circulating levels of 1,25-dihydroxyvitamin D3. The circulating level of 1,25-dihydroxyvitamin D3 in rats on a low-calcium diet increased with increasing doses of vitamin D3, whereas it did not change in rats on a low-phosphorus diet given increasing doses of vitamin D3. In concert with these results, the 25-hydroxyvitamin D la-hydroxylase activity was markedly increased by vitamin D3 administration to rats on a low-calcium diet, whereas the same treatment of rats on a low-phosphorus diet had no effect and actually suppressed the la-hydroxylase in rats fed an adequatecalcium/adequate-phosphorus diet. The administration of 1,25-dihydroxyvitamin D3 to vitamin D-deficient rats on a low-calcium diet also increased the renal 25-hydroxyvitamin D la-hydroxylase activity. These results demonstrate that the regulatory action of 1,25-dihydroxyvitamin D3 on the renal 25-hydroxyvitamin D3 la-hydroxylase is complex and not simply a suppressant of this system.