Radioactive calcium-45 uptake

The study shown in Figure 9.53 illustrates the role of 1,25-dihydroxyvitamin D3 in stimulating the uptake of calcium ions by the intestines. Chick duodena were Incubated in a salt solution containing radioactive calcium ions and vitamin D at the indicated levels Each point in the figure represents the uptake of calcium occurring during a separate incubation period. The forms of the vitamin used included 1,25-(OH)2D3 ( ), l-fOHJD (A), 25-(OH)D3 (A), 24,25-(OH)jDj (O), and... 

Hsiao, S. C., and H. Boroughs. 1958. The uptake of radioactive calcium by sea urchin eggs. I. Entrance of Ca into unfertilized egg cytoplasm. Biol. Bull. 114(2) 196-204. 

The respiratory activity of the brain tissue was determined by measuring the rate of oxygen uptake with a Clark oxygen electrode (7). The sample of tissue (a brain half) was treated exactly as if used in a calcium efflux experiment except no radioactivity or RF power was used. Following this procedure which required about 55 minutes, the tissue was placed in the oxygen electrode cell containing 1.6 ml of the standard medium (pH 7.8) at 37°C and the rate of oxygen uptake was recorded. 

Figure 10. Experiment demonstrating consecutive dissociation of two calcium ions from the uptake sites of the SR Ca2+-ATPase in the unphosphorylated E, state. 45Ca2+ bound from the cytoplasmic side at the two high-affinity sites on the enzyme can be seen to dissociate back to the medium on the cytoplasmic side in two distinct phases upon dilution of the radioactivity (A). The first phase is rapid and independent of the concentration of Ca2+ in the medium. The second phase is also rapid in the absence of Ca2+ (presence of EGTA) in the buffer medium (open triangles) but slow in the presence of nonradioactive Ca2+ in the buffer medium (closed triangles, 10 pM Ca2+ open circles, 30 pM Ca2+ closed circles, 100 pM Ca2+ open squares, 300 pM Ca2+ closed squares 1 mM Ca2+). The inset shows the rate constant for dissociation of the second Ca2+ as function of the Ca2+ concentration in the medium. The lower panel (B) explains these observations in terms of steric hindrance of the dissociation of the deeper ion by the presence of a more superficial ion at its binding site (closed circles, radioactive 45Ca2+ open circles, nonradioactive 40Ca2+ from the medium). Reproduced from Orlowski and Champeil, 1991a with permission from The American Chemical Society.

Calcium absorption can be measured by the double-isotope technique. In this technique a meal containing caldum-45 is consumed and the radioactivity in the urine measured. The measurement of urinary Ca alone cannot provide the fractional absorption, because some of the Ca absorbed is taken up by cells, deposited in bone, or excreted in the bile. A second isotope of calcium, Ca, is used to correct for the fates of absorbed calcium, other than excretion in the urine. The use of the Ca is intended to eliminate cell uptake, bone deposit, and biliary losses as variables in the study of the absorption of the dose of cdcium-45. 

The strontium uptake of plants parallels that of calcium and may be of some practical significance in view of the long half-life of certain radioactive isotopes of strontium (Mitchell 1957, Schilling 1960). Uptake by... 

Measured in weight the total amount of radionuclides do not represent a huge amount compared to the presence of nonradioactive components in seawater. The radioisotopes of cesium and strontium are both important in a radioecological context since they have chemical behavior resembling potassium and calcium, respectively. Cesium follows potassium in and out of the soft tissue cells whereas strontium follows calcium into bone cells and stays. Since uptake and release in organisms is due to the chemical characteristics and rarely if the element is radioactive or not, radionuclides such as Cs and °Sr have to compete with the nonradioactive isotopes of cesium and strontium. 

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