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THE CONTROL OF THE STRENGTH OF THE CAFFEINE CONTRACTURE IN FROG ATRIAL TRABECULAE: AN ACTIVITY OF THE SODIUM‐CALCIUM EXCHANGE
Author(s) -
Chapman R. A.,
Tunstall J.
Publication year - 1983
Publication title -
quarterly journal of experimental physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.925
H-Index - 101
eISSN - 1469-445X
pISSN - 0144-8757
DOI - 10.1113/expphysiol.1983.sp002733
Subject(s) - caffeine , chemistry , contracture , muscle contracture , calcium , sodium , biophysics , activator (genetics) , medicine , endocrinology , anatomy , biochemistry , surgery , biology , organic chemistry , gene
In frog atrial trabeculae low‐Na contractures and caffeine contractures evoked in Na‐poor solutions are increased by raising the external Ca concentration ([Ca] o ), lowering the external Na concentration ([Na] o ) or by tissue depolarization in K‐rich fluids. The source of activator Ca for the low‐Na and the caffeine contractures is probably different because Na‐withdrawal contractures are inhibited by Mn 2+ but unaffected by local anaesthetics, while the caffeine contractures are potentiated by Mn 2+ and inhibited by local anaesthetics. The spontaneous relaxation of the low‐Na contracture has a rate constant of 0·055 ± 0·009 s -1 , at room temperature for any [Na] o , [Ca] o or external K concentration ([K] o ). The spontaneous relaxation of the caffeine contracture in Na‐free fluid has a similar rate constant. However, the rate of spontaneous relaxation is increased if Na + is present in the bathing medium and analysis of the results of experiments done in solutions with different [Na] o , [Ca] o or [K] o , suggests that two processes are involved in the reduction of the internal Ca concentration ([Ca] i ) and the fall of tension. One, also responsible for the spontaneous relaxation of the low Na contracture, is dependent on metabolic energy and may be intracellular in origin. The other depends upon the activity of the sarcolemmal Na/Ca exchange, persists in the presence of metabolic inhibitors and is also activated during a low Na contracture when [Na] o is raised, or [Ca] o is lowered. The contractile response, of frog cardiac muscle to caffeine, would seem to be largely due to a release of Ca 2+ from an intracellular store, but the strength of the resulting contracture depends upon a competition between the contractile proteins and the Na/Ca exchange for the released Ca 2+ .