22-Oxa-1,25-dihydroxyvitamin D3 (oxacalcitriol, or OCT) is a bioactive analogue of 1alpha,25-dihydroxyvitamin D3 [1,25 (OH)2D3] with lower calcemic activity than the parent compound. We investigated the ability of OCT to stimulate 1) genomic pathways mediated by nuclear receptors for 1,25(OH)2D3 versus 2) nongenomic pathways mediated by voltage-sensitive Ca2+ channels in growth phase rat osteosarcoma cells (ROS 17/2.8) and in chick intestine. Effects on nuclear receptor-mediated pathways were evaluated by measuring the ability of OCT to compete with [H-3]1,25(OH)2D3 for soluble receptors. We also measured the ability of OCT to increase mRNA encoding osteoblast marker proteins osteopontin (OPN) and osteocalcin (OCN), which are both increased by 1,25(OH)2D3. Effects on Ca2+ entry into osteoblasts were measured using Ca-45(2+) influx assays. The rapid stimulation of calcium absorption (transcaltachia) in chick intestine treated with OCT also was measured. We found that OCT bound to the nuclear receptor with lower binding affinity [relative competitive index (RCI) = 48.1 for ROS 17/2.8; RCI = 14.8 for chick intestine] than 1,25(OH)2D, (RCI = 100). Like 1,25(OH)2D3, OCT increased mRNA levels of OPN and OCN in ROS 17/2.8 cells over a 48-h period. In contrast, OCT had no effect on transmembrane influx of Ca-45(2+) across ROS cell membranes, whereas uptake was stimulated within 1 min by 1 nM 1,25(OH)2D3. In transcaltachia assays in perfused duodenum, OCT stimulated absorption with a maximum response at 6.5 nM. These results indicate that not only do analogues of 1,25(OH)2D, display differences in their abilities to stimulate genomic vs. nongenomic pathways in target cells but also that tissue-specific differences in the membrane response component exist.
