Percent extracellular release

Bone serves as a vast reservoir of calcium in the body. Approximately 1 percent of calcium in bone can rapidly exchange with extracellular calcium ion. PTH stimulates demineralization of bone and release of calcium and phosphate into the blood by stimulating osteoclast formation and activity. This process is synergistically enhanced by vitamin D. 

This was possible to detect because the monoculture of Thalassiosira rotula employed showed partly synchronized cell divisions during exponential growth. Brockmann et al. [137] carried out combined measurements of dissolved amino acids and carbohydrates. Glucose and lysine occurred in highest concentrations. Mague et al., [22] found that extracellular production of free amino acids counted for 7.1% of the of the total extracellular C released in an exponentially growing culture of S. costatum Myklestad et al., [26] measured 10.7% for C. affinis or 3.6% when calculated as percent of total incorporated cell N. In contrast to this Admiraal et al., [139] found that none of three benthic diatoms released more than 0.1 % of the cellular N as free amino acids and concluded that benthic diatoms may act as net consumers of amino acids. Several authors did measure both intracellular and extracellular concentrations of many amino acids [22 140 -142]. The clear difference in relative composition of intracellular and extracellular fractions as pointed out by the first mentioned of these authors, show that the released pool is not just a portion of the intact cells content. 

In order to determine whether the released adenosine could serve as a precursor of human erythrocyte adenine nucleotides, a rabbit liver was labeled by perfusion with Hj -hypoxanthine as described. Washout perfusion with an isotonic balanced salt solution removed residual extracellular label. The labeled liver was then perfused by recirculating a 400ml washed human erythrocyte suspension for one hour. Erythrocytes were then collected and washed, and the liver was excised. Extracts were prepared and the purine nucleotides were assayed for distribution of radioactivity (Table II). Within the liver, the radioactivity was approximately evenly distributed between the adenine nucleotides and the hypoxanthine plus xanthine nucleotides, indicating again that extensive conversion of hypoxanthine to IMP to AMP can occur in the liver cell. Within the human erythrocyte, over 80 percent of the label appeared in the adenine nucleotides. Since IMP is not converted to AMP in that cell, the labeled adenosine formed in the liver from the perfused hypoxanthine must have been taken up by the human erythrocyte and converted to AMP by the adenosine kinase. Since free adenine, the only other possible precursor of human erythrocyte adenine nucleotides, was not detected in hypoxanthine perfused liver or in hepatic venous effluent from hypoxanthine perfused liver, a possible role is unlikely. 

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