Sodium-Calcium Exchange Across the Nerve Cell Membrane
If the intracellular concentration of free calcium were determined solely by the Donnan ratio, then calcium should be accumulated in most cells. In quantitative terms, with an internal potential 50–60 mV negative to the outside and external calcium concentrations of 1–10 mm, the predicted intracellular concentration of free calcium should be 0·1–1·0 M. These values are very different from those found by experiment. In the few large nerve and muscle cells where direct measurement has proved possible, the free intracellular calcium concentration is at most 10−5 m and may be as low as 10−7 m (Hodgkin & Keynes, 1955; Portzehl, Caldwell & Rüegg, 1964; Luxoro & Yanez, 1968). A discrepancy in this direction must mean that in these cells Ca ions are actively excluded from the cytoplasm. Although comparable measurements have not been possible, it seems likely that most other cells behave similarly.
KeywordsFree Calcium Cardiac Glycoside Donnan Ratio Squid Axon Free Intracellular Calcium Concentration
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