METHODS

All experiments were conducted with young, male, albino rats obtained as weanlings from the Sprague-Dawley Company, Madison, Wis. The animals were housed individually in hanging wire cages and raised for 4-5 weeks on the vitamin D-deficient diet described by DeLuca et al.(7). Animals weighing between 100 and 140 g after 4 weeks on the diet were used for the experiments. Unless stated otherwise, each animal received 40,000 IU vitamin D3 48 hr before the experiment and was fasted overnight before use. Calcium transport was studied using an everted gutsac technique based on that of Wilson and Wiseman (30) as adapted for calcium transport by Schachter and Rosen (22). The animals were killed by decapitation and the first 10 cm of the intestine distal to the pyloric valve dissected free. The tissue was immediately rinsed with an isotonic solution of either sodium chloride, choline chloride, or mannitol. The choice of the rinse solution depended on the composition of the incubation medium to be used with that particular sac; choline chloride or mannitol was used whenever the same compound was included in the incubation medium. Following the rinse, much of the mesentery was trimmed free and the intestine everted in a manner such that the distal end of the segment remained tied to the everting rod. The intestine was then tied with a ligature just distal to the pyloric valve and rinsed as before. It was then blotted, trimmed to a length of 5.5 cm, and filled with 0.5 ml of incubation medium using a syringe fitted with a blunt needle. The sac was then placed in a 25-ml Erlenmeyer flask containing 10 ml of the same incubation medium and incubated at 37 C for 90 min unless otherwise stated. Oxygen was continuously bubbled through the incubation medium throughout the experiment. In experiments in which the inside fluid of the sac was changed, the sac was removed from the flask and most of the fluid allowed to drip off. After the final drop was carefully blotted away, the sac was cut and drained from the pyloric end, placed on a piece of glass, and rinsed both inside and out with the