title: The onic dependence of the strength and spontaneous relations of the potassium contracture induced in the heart of the frog Rana pipiens.
abstract: 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.
mesh_terms: D000818:Animals; D001001:Anura; D001285:Atropine / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D002712:Chlorides / Q000494:pharmacology; D004837:Epinephrine / Q000494:pharmacology; D004983:Ethanolamines / Q000494:pharmacology; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D009994:Osmolar Concentration; D011188:Potassium / Q000494:pharmacology*; D011894:Rana pipiens; D012964:Sodium / Q000494:pharmacology
chemical_list: D002712:Chlorides; D004983:Ethanolamines; D002110:Caffeine; D001285:Atropine; D012964:Sodium; D011188:Potassium; D002118:Calcium; D004837:Epinephrine