| Abstrakt: |
The plasma distribution and cellular uptake of [3H]vitamin D3was studied in vitrousing cultured human fibroblasts. Incubation of [3H]vitamin D3(cholecalciferol) with plasma followed by sequential ultracentrifugal fractionation of the lipoproteins indicated that 2–4% of the radioactivity associated with the very low density lipoprotein (VLDL), 12% with low density lipoprotien (LDL), and approximately 60% with the high density lipoprotein (HDL). The remaining radioactivity, 25%, was associated with the sedimented plasma fractions. By comparison, an average of 86% of the radioactivity from [3H] 1,25-dihydroxycholecalciferol associated with the sedimented plasma fractions. The uptake of [3H]vitamin D3from plasma, LDL, or HDL was studied in cultured human cells; uptake by normal fibroblasts was greatest from LDL and least from plasma. The cellular association of vitamin D3was time, concentration, and temperature dependent. At a concentration of 50 μg LDL/ml of medium, the uptake of [3H]vitamin D3from LDL at 37°C was rapid and reached a maximum at approximately 4 hr; it was slower from HDL but continued to increase slowly up to 24 hr. The significance of these in vitrofindings is uncertain since much of the vitamin D3absorbed from the intestine reportedly associates with chylomicrons and is rapidly taken up by the liver. |
