The vitamin D analog, (23S)-25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone (TEI-9647), is an antagonist of the 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] nuclear receptor (VDR)-mediated differentiation of human leukemia (HL-60) cells. To clarify whether TEI-9647 could function as an antagonist of 1alpha,25(OH)(2)D(3) in vivo, we investigated in vitamin D-deficient (-D) rats the effects of single doses of TEI-9647 on several parameters of calcium metabolism modulated by 1alpha,25(OH)(2)D(3). TEI-9647 (50 microgram/kg iv) acting alone slightly, but significantly, stimulated intestinal calcium transport (ICA) and bone calcium mobilization (BCM) only at 8 h, but not at 24 h. In contrast, TEI-9647 dose-dependently inhibited ICA and BCM stimulated by an iv dose of 0.25 microgram/kg 1alpha,25(OH)(2)D(3) after 24 h, but not after 8 h. With respect to serum PTH levels, the administration of either TEI-9647, 50 microgram/kg, or 1alpha,25(OH)(2)D(3), 0.25 microgram/kg, began to decrease the circulating levels by 4 h, which reached a nadir 24 h after administration. But, when TEI-9647 and 1alpha,25(OH)(2)D(3) were simultaneously administered to -D rats, the TEI-9647 dose-dependently reversed the inhibition of PTH secretion caused by 1alpha,25(OH)(2)D(3), 0.25 microgram/kg, at 8 and 24 h after the treatment. In separate experiments, the daily iv administration of 20 microgram/kg of TEI-9647 alone to +D rats for 2 weeks resulted in no significant changes in the prevailing serum Ca(2+) concentration. But doses of 1-20 microgram/kg of TEI-9647 in combination with 0.5 microgram/kg of 1alpha,25(OH)(2)D(3), for 2 weeks, dose-dependently and significantly suppressed the serum calcium concentration increase caused by the 1alpha,25(OH)(2)D(3). Collectively, these results show that TEI-9647 acting alone displays in vivo weak agonistic actions, but when administered in combination with 1alpha,25(OH)(2)D(3), is a potent antagonist of three genomic-mediated calcium metabolism parameters. We conclude that TEI-9647 can also function as an antagonist of 1alpha,25(OH)(2)D(3) in vivo in the rat.