By far the largest part of the calcium in the body takes part ~in making up the skeletal structure. Metabolism studies have shown that there are many factors which add or subtract from the body's supply of calcium. Thus pregnancy, lactation, a low calcium intake, ingestion of acids, administration of parathormone I and many other influences serve to increase the demand for calcium from the body. The question arises: Is there a special storehouse of calcium, or is the whole body skeleton at the mercy of these various factors? A very significant observation in this connection was made by Hunter and Aub (1). In patients with chronic lead poisoning they observed an increased lead excretion, which accompanied the increased calcium excretion following the administration of parathormone. If these same patients were then put on a high calcium diet so as to store up the amount of calcium lost during the parathormone medication, and were then given a second course of parathormone, they failed to show an increased lead excretion although the calcium excretion was
Ferritin, added to the incubation medium of ascites tumor cells, was used as an electron microscopic marker to study the uptake of large protein molecules by morphologically intact cells. A definite uptake could be detected after I hour of incubation in Tyrode bicarbonate solution containing 0.04 to 13.3 mg ferritin/ml. Ferritin was found in a variety of membrane-surrounded structures, suggesting that pinocytcsis and related membrane movements are occurring under physiological conditions and can account for the penetration of intact macromolecules into isolated tumor cells. Supplementation of the medium with serum albumin (33 mg/ml) increased the average amount of ferritin per cell and per pinocytotic structure. Ferritin was strongly adsorbed by fragments of lysed cells, which were readily taken up by intact cells. Besides its role as carrier, this debris appeared to stimulate membrane movements. Only rare examples were found to suggest the release of ferritin from the pinocytotic structures into the cytoplasm. Thus, the disintegration of such structures cannot be considered an obvious step towards a rapid metabolic utilization of protein by the cell. Particles of colloidal gold presented to the cell under the same conditions were not taken up to any significant extent, thus providing good evidence for a selective ingestion of particles of comparable sizes.
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