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Abstract

Many observations suggest that all cells possess transport systems for the elimination of ionized calcium from the cytoplasm. Presumably no cell can tolerate high concentrations of free calcium ions intracellularly, but so far the concentration of free calcium ions which can be tolerated is known for only one cell with any degree of accuracy. In the resting giant muscle fibres of Maia (Portzehl, Caldwell & Rüegg, 1964) and Balanus (Hagiwara, 1966) the free calcium concentration has been found not to exceed 5 × 10−7 m. On the other hand, these muscles like any other muscle need relatively large amounts of calcium ions for the activation of their contractile machinery (Weber & Herz, 1963; Jöbsis, 1967; Ashley & Ridgway, 1969). These quantities are presumably liberated in a few milliseconds during excitation. The removal of these ions from the cytoplasm brings activation to a halt, and at room temperature the time required for this process is in the range of 10–100 msec. In skeletal muscles these rapid calcium movements take place mainly across the membranes of the sarcoplasmic reticulum. The sarcoplasmic membranes are especially suitable objects for studying the mechanism of and the structural basis for the active transport of calcium ions.
A Symposium on Calcium and Cellular Function A Symposium on Calcium and Cellular Function Look
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