Do tissues other than the kidney produce 1,25-dihydroxyvitamin D3 in vivo? A reexamination.

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Abstract
Recent experiments have shown that 1,25-dihydroxyvitamin D3-like material is produced in cultured nonrenal cells and may be present in the sera of anephric patients. The question of whether 1,25-dihydroxyvitamin D3 can be synthesized extrarenally was reexamined in the rat in vivo. To intact, sham-operated, ureter-ligated or acutely nephrectomized vitamin D-deficient rats raised on a diet normal in Ca and P, a physiologic dose of high-specific-activity 25-hydroxy[3H]vitamin D3 (3.6-3.8 .mu.Ci; .apprxeq. 25 pmol/rat) were examined. Twenty-four hours later the tissues and plasma were examined for the presence of radiolabeled 1,25-dihydroxyvitamin D3. Large amounts of radioactivity that behaved chromatographically as identical with authentic 1,25-dihyroxyvitamin D3 were present in the plasma, bone and intestine of the intact, sham-operated or ureter-ligated rats. However, no radioactivity eluting in a manner similar to 1,25-dihydroxyvitamin D3 was found in plasma, bone or intestine of acutely nephrectomized rats. In the acutely nephrectomized living rat, 1,25-dihydroxyvitamin D3 is not present in plasma, bone or intestine in quantities detectable by the sensitive techniques used. No conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 was observed during a 24-h period after nephrectomy of vitamin D-deprived rats. This fact casts doubt on the significance of the in vitro production of 1,25-dihydroxyvitamin D3 by nonrenal cells as an in vivo phenomenon.