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Vitamin D-regulated, ATP-dependent calcium transport by intestinal Golgi vesicles during maturation in the rat
Insights
Intestinal Golgi vesicles actively transport calcium in rats, with uptake increasing with age and regulated by Vitamin D. This ATP-dependent process is crucial for calcium absorption throughout development.
Area of Science:
- Cell Biology
- Developmental Biology
- Nutritional Science
Background:
- Intestinal calcium absorption is vital for bone health and overall development.
- The role of Golgi vesicles in intestinal calcium transport and its developmental regulation is not fully understood.
Purpose of the Study:
- To investigate the developmental changes in calcium uptake by intestinal Golgi vesicles in rats.
- To elucidate the mechanism of calcium transport and its regulation by ATP and Vitamin D.
Main Methods:
- Purified Golgi vesicles were isolated from rat enterocytes at different developmental stages (suckling, weanling, adolescent).
- Calcium uptake was measured using radiolabeled calcium, assessing temperature dependency, ionophore-induced efflux, and ATP requirement.
- Kinetic parameters (Km and Vmax) were determined, and the effect of Vitamin D and calmodulin was evaluated.
Main Results:
- Calcium uptake by Golgi vesicles is an ATP-dependent, intravesicular transport process, evident across all age groups.
- Maximal calcium transport capacity (Vmax) and affinity (Km) showed significant developmental variations, increasing with age.
- Vitamin D deficiency reduced ATP-dependent calcium uptake, while 1,25-(OH)2 vitamin D3 administration enhanced it by increasing Vmax.
Conclusions:
- Intestinal Golgi vesicles exhibit an ATP-driven calcium uptake system with distinct developmental patterns in kinetic parameters.
- This transport system is regulated by Vitamin D, playing a crucial role in intestinal calcium absorption from the suckling period onwards.
Abstract:
The developmental aspects of calcium uptake by intestinal Golgi vesicles was determined using highly purified Golgi vesicles from enterocytes of suckling (2 wk old), weanling (3 wk old), and adolescent (6 wk old) rats. Calcium uptake by Golgi vesicles at all age groups represented transport into the intravesicular space as evident by temperature dependency and by calcium ionophore A23187-induced calcium efflux studies. Calcium uptake was driven by ATP at all age groups, however, maximal uptake at 15 min was significantly greater in Golgi vesicles of adolescent rats compared to mean values in Golgi vesicles of suckling rats (p less than 0.01). Calcium uptake in the absence of ATP was minimal. The requirement for the adenine base and the hydrolysis of the beta-gamma-phosphodiester was tested by replacement of ATP in the incubation media by CTP and the nonhydrolyzable ATP analogue, adenylyl-(beta-gamma-methylendiphosphonate). Both agents had no stimulatory effect on calcium uptake. Calcium uptake was linear up to 40 s. Kinetic parameters of calcium uptake at free calcium concentrations of 0.04 to 1.0 microM showed a maximal transport capacity of 0.99 +/- 0.05, 0.55 +/- 0.04, and 0.29 +/- 0.03 nmol/mg protein/15 s for adolescent, weanling, and suckling rats, respectively. Km values were 0.16 +/- 0.02, 0.12 +/- 0.03, and 0.07 +/- 0.02 microM for adolescent, weanling and suckling rats, respectively. Km and Vmax values were significantly different between adolescent and suckling rats (p less than 0.01). The calcium regulatory protein calmodulin has no effect on calcium uptake by Golgi vesicles. Vitamin D deficiency in all age groups decreased ATP-dependent calcium uptake. Administration of 1,25-(OH)2 vitamin D3 8 h before death enhanced ATP-dependent calcium uptake in all age groups studied. This enhancement was the result of increase in maximal transport capacity of ATP-dependent calcium uptake. This study demonstrates a vitamin D-regulated ATP-driven calcium uptake by intestinal Golgi vesicles at all age groups including the suckling period. This transport system shows developmental patterns in regard to its kinetic parameters.