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The effects of very low external calcium and sodium concentrations on cardiac contractile strength and calcium‐sodium antagonism.
Author(s) -
Miller D J,
Moisescu D G
Publication year - 1976
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1976.sp011466
Subject(s) - chemistry , calcium , sodium , egta , antagonism , extracellular , biophysics , biochemistry , receptor , organic chemistry , biology
1. The tension levels of the contractures induced in frog atrial trabeculae by reduction of the bathing Na concentration have been investigated over a wide range of [Na+]o and [Ca2+]o, making use of EGTA to buffer [Ca2+]o under 10(‐5)M. 2. The relationship between tension and the quotient [Ca2+]o/[Na+]2o is only an apparent one, and holds approximately for Na concentrations as low as 1‐5 mM. 3. However, for low Na concentrations tension depends on the absolute values of both [Ca2+]o and [Na+]o. 4. Extracellular Ca is the essential factor in tension production. The results indicate an apparent threshold for tension near 5 X 10(‐8) M‐[Ca2+]o. 5. It can be shown that, under the present experimental conditions, the Na concentration around the cells must be lower than about 0‐1 mM in 'zero' Na+ solutions. 6. It is shown that the over‐all process leading to tension production can be explained in terms of a reaction involving two Ca ions. 7. To account for the results, a modification of the reaction scheme put forward by Lüttgau & Niedergerke (1958) is proposed, involving a competitive action between Ca and Na ions at a free receptor site. 8. To account for the scheme, an example of a mechanism involving both internal and external Na and Ca concentrations is considered.