The effects of very low external calcium and sodium concentrations on cardiac contractile strength and calcium‐sodium antagonism.

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.