The steroid hormone 1alpha,25-dihydroxyvitamin D-3 [1alpha,25(OH)(2)D-3] produces biological responses by interaction with both a well-characterized nuclear receptor (VDRnuc) to regulate gene transcription and with an as-yet uncharacterized membrane-associated protein/receptor (VDRmem) to generate a variety of rapid, non-genotropic responses. We report for the first time that [H-3]1alpha,25(OH)(2)D-3 binds with high affinity to a chick duodenal caveolae-enriched membrane fraction (CMF) isolated without the use of detergents. Caveolae are plasma membrane invaginations implicated in signal transduction and molecular transport processes. Using the CMF fraction as a possible source of VDRmem, we found that the in vitro binding of [H-3]1alpha,25(OH)(2)D-3 was ligand dependent and saturable; the K-D and B-max were 1.3 +/- 0.6 nM and 29 +/- 11 fmol 1,25(OH)(2)D-3/mg protein (n = 17), respectively. Immunoblot analysis of the CMF confirms the presence of caveolin-1, a marker protein for membranes with caveolae. Therefore, chick CMF may represent a good source for isolation and characterization of the putative VDRmem for 1alpha,25(OH)(2)D-3. (C) 2002 Elsevier Science (USA). All rights reserved.