Effect of age and the X-linked Hyp mutation on renal adaptation to vitamin D and calcium deficiency.

The age (4 weeks vs 5 weeks vs 6.5 weeks) at which dietary restriction of vitamin D and calcium is initiated has a profound effect on the resulting concentration of serum calcium, urinary cAMP and on renal 25-hydroxyvitamin D3-1-hydroxylase (1-hydroxylase) activity in normal (+/Y) mice; no such age relationship is apparent in Hyp/Y littermates. After 40 days on the restrictive diet, it was found that the younger the +/Y mice at the time of diet initiation, the lower the resulting serum calcium (4 weeks less than 5 weeks less than 6.5 weeks) and the higher the urinary cAMP and 1-hydroxylase activity (4 weeks greater than 5 weeks greater than 6.5 weeks). Age on diet has no effect on serum phosphate and fractional excretion index of phosphate in +/Y and Hyp/Y littermates. Renal 1-hydroxylase activity is significantly lower than normal in the younger groups of Hyp/Y mice whereas 24-hydroxylase (25-hydroxyvitamin D3-24-hydroxylase) activity is higher than normal in all groups of Hyp/Y mice.