Binding properties of duodenal 1,25-dihydroxyvitamin D3 receptors as affected by phosphorus depletion in lactating goats.

1. Capacity and affinity of duodenal 1,25(OH)2D3 receptors were measured in P depleted goats and in control animals kept on an adequate P supply. Plasma concentrations of Pi, Ca and vitamin D3 metabolites and activity of plasma alkaline phosphatase were measured to characterize the effects of P depletion. 2. During P depletion plasma Pi concentrations decreased significantly whereas plasma Ca and alkaline phosphatase activity increased. No changes were recorded for plasma vitamin D3, 25OHD3 and 1,25(OH)2D3 concentrations. 3. P depletion resulted in a significant decrease of the equilibrium dissociation constant Kd of duodenal 1,25(OH)2D3 receptors without affecting the maximum binding capacity.