In contrast to the large volume of data supporting the dependence of cardiac excitability and phasic contractility on external Na, Van der Kloot and Rubin (1962) and Singh (1962) have reported the persistence of both electrical and phasic mechanical activity in frog atrial and ventricular preparations soaked in isotonic sucrose solutions. The acute ionic dependence of excitability and contractility in small frog atrial trabeculae has been investigated with the conclusion that excitability and phasic contractions may continue for extended periods of time in sucrose media if the extracellular ionic concentrations remain above 2% of normal. This behavior is attributed to the slow exchange properties of the cell surfaces of the frog cardiac trabeculae and the antagonistic effects of Na, K, and Ca ions on both membrane excitability and fiber contractility.

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