The onic dependence of the strength and spontaneous relations of the potassium contracture induced in the heart of the frog Rana pipiens.

1. The tension generated by isolated frog atrial trabecules, during exposure to solutions containing a high potassium concentration, is not maintained but spontaneously relaxes. The final part of this relaxation can be fitted by a single exponential function.2. The recovery of the tension generating mechanisms following the spontaneous relaxation of a potassium contracture depends on the preceding membrane potential and the time since the last contracture.3. The rate of the exponential phase of the spontaneous relaxation is independent of the [K](o) and hence the membrane potential, the [Ca](o); and when the [Ca](o)/[Na](o) (2) ratio is maintained it is also independent of the [Na](o). This relaxation is not influenced by atropine or pronethalol.4. When sodium is totally excluded from the bathing medium the rate of relaxation of a later potassium contracture is much increased. It is argued that this change is due to a fall in the intracellular sodium concentration.5. The consequences of these results are discussed, and the hypothesis that is favoured would require that contraction is induced by a transient release of calcium into the sarcoplasm, probably triggered by a potential dependent, and probably also transient, influx of calcium through the cell membrane. Relaxation is supposed to occur when this activator-calcium is then removed by an intracellular relaxing system that resembles the sarcoplasmic reticulum of other muscles. What this intracellular structure might be, is also discussed.