Biphasic recovery of vitamin D-dependent Ca2+ uptake by rat intestinal Golgi membranes

Abstract

A previous study had demonstrated that rat intestinal epithelial cell Golgi, lateral-basal, and microvillus membrane vesicles translocated Ca2+, and that this process was markedly decreased in vitamin D-deficient rats. In the present study the kinetics of recovery of Ca2+ uptake by intestinal Golgi vesicles was examined in vitamin D-deficient rats after a single i.v. injection of 125 ng 1,25-dihydroxycholecalciferol (1,25-(OH)2D3). Calcium uptake was measured by determination of 45Ca2+ associated with Golgi membranes after collection by micropore filtration. Golgi membranes recovered rapidly (66% in 15 min, 83% in 30 min) but then quickly returned to prerepletion levels by 8 hr. This was followed by a second phase of recovery starting at 48 hr, with approximately 80% recovery by 96 hr which was maintained at least to 120 hr. By comparison, partial recovery of calcium uptake by microvillus membrane vesicles was first detected at 15 hr and full recovery did not occur until 72 hr; a biphasic pattern to microvillus membrane recovery was not apparent. The vitamin D-dependent recovery of Ca2+ uptake by Golgi membrane vesicles was also compared to studies with gut sacs which showed 50% recovery by 3 hr and 100% recovery by 6 hr; gut sac recovery was sustained for at least 120 hr. The recovery of Ca2+ uptake by Golgi was shown to be specific for 1,25-(OH)2D3. The first peak of recovery of Ca2+ uptake by intestinal Golgi membrane vesicles is one of the earliest reported effects of 1,25-(OH)2D3. The biphasic nature of recovery demonstrated by Golgi membranes suggests that there may be more than one mechanism of action of 1,25-(OH)2D3 on the intestine.