A study of the contractures induced in frog atrial trabeculae by a reduction of the bathing sodium concentration.

1. The relationship between the Ca, the Na and the strength of the contracture evoked when the Na is reduced has been investigated in isolated frog atrial trabeculae.2. The strength of the contracture varies by the Ca (2) and by 4 radical(Na) over the lower tension range.3. The contracture induced by reduction of Na is not sustained, but relaxes spontaneously. The rate of this relaxation is only dependent on the Na is the presence of strophanthidin.4. After the spontaneous relaxation of an Na-free contracture, the ability of the trabecula to develop tension upon a second challenge with Na-free solution returns in about 3 min if the muscle is perfused with Na-containing fluid. This recovery process is slowed if the Na is low during the recovery period, but the recovery is hastened by electrical stimulation of the preparation or by perfusion with K-free or strophanthidin containing sodium-Ringer.5. It is suggested that the influx of Ca(2+) which induces the Na-free contracture depends on the presence of Na(+) inside the cells. When the intracellular Na concentration falls, the Ca influx falls, and the muscle relaxes as a result of the activity of an intracellular relaxing structure.