22-OXACALCITRIOL - DISSECTION OF 1,25(OH)2D3 RECEPTOR-MEDIATED AND CA2-STIMULATING PATHWAYS( ENTRY)

22-Oxa-1,25-dihydroxyvitamin D3 (oxacalcitriol, or OCT) is a bioactive analogue of 1alpha,25-dihydroxyvitamin D3 [1,25 (OH)2D3] with lower c alcemic 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-sen sitive Ca2+ channels in growth phase rat osteosarcoma cells (ROS 17/2. 8) and in chick intestine. Effects on nuclear receptor-mediated pathwa ys 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 OC T to increase mRNA encoding osteoblast marker proteins osteopontin (OP N) and osteocalcin (OCN), which are both increased by 1,25(OH)2D3. Eff ects on Ca2+ entry into osteoblasts were measured using Ca-45(2+) infl ux 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 [relati ve 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 increa sed 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+) a cross ROS cell membranes, whereas uptake was stimulated within 1 min b y 1 nM 1,25(OH)2D3. In transcaltachia assays in perfused duodenum, OCT stimulated absorption with a maximum response at 6.5 nM. These result s indicate that not only do analogues of 1,25(OH)2D, display differenc es in their abilities to stimulate genomic vs. nongenomic pathways in target cells but also that tissue-specific differences in the membrane response component exist.