1,25-Dihydroxyvitamin D3 Analog Structure-function Assessment of the Rapid Stimulation of Intestinal Calcium Absorption (transcaltachia)

The possibility is now emerging that 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] can mediate biologic responses via both genomic and nongenomic pathways. To understand the molecular basis of the nongenomic response of transcaltachia, defined as the 1,25-(OH)2D3-mediated rapid (2-10 minutes) stimulation of calcium transport from the brush border to the basal lateral membrane of the epithelial cell in vitamin D-replete chick intestine, and to address the issue of whether the same receptor for the secosteroid serves as the signal transducer for both genomic and nongenomic pathways, we carried out structure-function studies using seven analogs of 1,25-(OH)2D3 with different affinities for the classic nuclear 1,25-(OH)2D3 receptor as measured by determination in a steroid competition assay of the relative competitive index (RCI). The RCI of 1,25-(OH)2D3 is by definition 100. 1,25-(OH)2D3 initiates transcaltachia within 2-10 minutes of vascular perfusion and yields a biphase response curve. Dose-dependent stimulations of Ca2+ transport by the seven analogs indicates that different structural features are essential for initiating the transcaltachic response as contrasted with binding to the classic nuclear receptor. Vascular perfusion with analogs AT (25-OH-16-ene-23-yne-D3) and Y (25-OH-23-yne-D3), which are known to activate Ca2+ channels but bind very poorly to the classic receptor (RCI less than 0.5), are efficient in stimulating Ca2+ transport. By comparison, compounds BT [1 alpha,24S-(OH)2-22-en-26,27-dehydrovitamin D3] and V (1,25-(OH)2-16-ene-23-yne-D3], which bind very well to the classic nuclear receptor (RCI 75-111) but do not activate Ca2+ channels, are inefficient in stimulating Ca2+ transport. These results indicate that the membrane components that respond to the analogs of 1,25-(OH)2D3 with activation of Ca2+ channels have a different ligand specificity than the classic nuclear receptor.