Incubation of metanephroi with vitamin D₃ increases numbers of glomeruli

To characterize actions of vitamin D₃ on metanephroi transplanted from rat embryos to adult recipients, we incubated metanephroi with or without 0.01, 0.1 or 1 μg/ml vitamin D3, 25-hydroxyvitamin D₃ [25(OH)D₃] or 1, 25-hydroxyvitamin D₃ [1, 25(OH) ₂D₃] prior to implantation. The number of glomeruli in developed metanephroi three weeks post-transplantation that had been incubated with 1.0 μg/ml vitamin D₃ was increased relative to the number in metanephroi that were not incubated with vitamin D₃ (control), an effect that was not recapitulated by administration of vitamin D₃ directly to hosts at the time of transplantation. Incubation of metanephroi with 1.0 μg/ml vitamin D₃ also enhanced inulin clearances of metanephroi measured at 12 weeks post-transplantation. The hydroxylated derivative of vitamin D₃, 25(OH)D₃, increased glomerulus number when applied at 0.01 μg/ml but not at higher concentrations, while the twice-hydroxylated derivative 1, 25(OH)₂D₃, failed to increase glomerulus number at any concentration tested. We conclude that incubation with vitamin D₃ prior to implantation enhances inulin clearance possibly by increasing the number of glomeruli that develop post-transplantation. Our findings suggest the vitamin D₃ effect is mediated locally.