1,25-DIHYDROXYVITAMIN D-3 RECEPTORS IN PERIPHERAL-BLOOD MONONUCLEAR-CELLS FROM PATIENTS WITH POSTMENOPAUSAL OSTEOPOROSIS

A decrease in intestinal calcium absorption, in spite of normal serum calcitriol levels, has been reported in postmenopausal osteoporotic wo men, raising the possibility of an intestinal resistance to the hormon e. The mechanism responsible for it could lie at the receptor or postr eceptor level. Intestinal receptors are difficult to study on clinical settings, but calcitriol receptors have been found in peripheral bloo d mononuclear cells (PBMC). We have studied the PBMC calcitriol recept ors by means of Scatchard analysis in II postmenopausal osteoporotic w omen without,any treatment and in 12 normal postmenopausal women of si milar age. No differences were found in the dissociation constant (K,) or the concentration of binding sites (N-max) (K-d in patients: 0.90 +/- 0.75 x 10(-10) M; K-d in controls: 0.85 +/- 0.40 x 10(-10) M; N-ma x in patients: 2.4 +/- 1.2 fmol/10(7) cells; N-max in controls: 2.1 +/ - 0.6 fmol/10(7) cells), supporting the contention that the disorder r esponsible for the resistance to calcitriol in postmenopausal osteopor otic women is located at the postreceptor level. In addition, our stud y included five postmenopausal osteoporotic women treated with calcitr iol (0.5 mu g/day). The number of calcitriol receptors was increased i n this group (N-max: 3.9 +/- 2.0 fmol/10(7) cells vs. 2.1 +/- 0.6 fmol /10(7) cells; P = 0.02).