Effects of continuous administration of 1,25-dihydroxyvitamin D3 on plasma minerals and unoccupied colon mucosal 1,25-dihydroxyvitamin D3 receptor concentrations.

Six mature nonlactating, nonpregnant Jersey cows were implanted with Alzet mini-osmotic pumps, which delivered 50 micrograms of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] each day for 7 d in an effort to mimic plasma concentrations of 1,25-dihydroxyvitamin D [1,25-(OH)2D] observed in cows at parturition. Plasma samples were obtained daily beginning 6 d prior to implantation and ending 8 d after removal of the implants. Six biopsies of the descending colon mucosa were obtained per rectum before and after implantation and assayed for unoccupied 1,25-(OH)2D3 receptor concentration. Plasma concentration of 1,25-(OH)2D increased from 37 pg/ml pretreatment to 294 pg/ml with the pumps implanted. Plasma Ca concentration increased within 2 d after implantation and remained elevated for 7 d after the pumps were removed. Unoccupied colon mucosa 1,25-(OH)2D3 receptor mean concentration prior to treatment was 14.6 fmol/mg protein and increased within 2 d following implantation to 30.5 fmol/mg protein. These data suggest that 1,25-(OH)2D3 upregulates its own receptor in the intestine of the cow.