Calcium flux across chick duodenal brush border membrane vesicles: regulation by 1,25-dihydroxyvitamin D.

The vitamin D metabolite 1,25-dihydroxyvitamin D [1,25-(OH)2D] given in vivo stimulates calcium accumulation by subsequently isolated duodenal brush border membrane vesicles (BBMV). Stimulation is rapid (within 2 h), reaching a maximum between 2-4 h. This effect occurs well before stimulation of in vivo calcium transport (2-4 h), cytosolic calcium-binding protein production (4-8 h, or alkaline phosphatase activity (8 h). No cytosolic calcium-binding protein was found in the BBMV at any time. The extent of calcium accumulation by BBMV exceeds by severalfold the predicted value based on the equilibrium distribution of glucose, indicating a substantial amount of binding. The ability of the calcium ionophore A23187 to increase the rate of accumulation suggests that this binding is intravesicular. The Eadie Hofstee analysis of the rate of calcium accumulation as a function of calcium concentration is nonlinear. At submillimolar calcium concentrations, the difference in the apparent Km for calcium accumulation by BBMV from vitamin D-deficient and 1,25-(OH)2D-treated chicks is nearly 2-fold (1.9 X 10(-4) vs. 1.1 X 10(-4)M, respectively), a difference that is not observed at higher calcium concentrations. Release of calcium from preloaded BBMV with the addition of EGTA is rapid but not complete (20-30% of the initial value after 60 min). The rapidity, but not the extent, of release is increased with A23187. BBMV from vitamin D-replete and vitamin D-deficient duodena do not differ in their rate or extent of calcium release, in contrast to their different rates of calcium accumulation. We conclude that the stimulation by 1,25-(OH)2D of calcium accumulation by BBMV is one of the earliest actions of 1,25-(OH)2D on the intestine, that this process does not involve alkaline phosphatase or cytosolic calcium-binding protein, and that influx, but not efflux, of calcium is regulated.