Summary
Slow muscle fibers were dissected from cruralis muscles of Rana esculenta and Rana pipiens. Isometric contractures were evoked by application of K+-rich Ringer's containing Ca2+, Ni2+, Co2+, Mn2+ or Mg2+. High (7.2 mmol/liter) external Ca2+ concentration raised, 0 Ca2+ lowered the K+ threshold. Replacing Ca2+ by Ni2+ or Co2+ had an effect similar to that of high Ca2+ Ringer's. In Mg2+ Ringer's the K+ concentration-response curve was flattened. These effects were observed already after short exposure times in both species of slow fibers. When Ca2+ was removed for long periods of time the slow fibers of R. esculenta lost their contractile response to application of high K+ concentrations much more quickly than those of R. pipiens, while the response to caffeine (20 mmol/liter) was maintained. Upon readmission of Ca2+ contractile ability was quickly restored in the slow fibers of both R. esculenta and R. pipiens, but the effects of Ni2+ (or Co2+, Mn2+ and Mg2+) were much larger in R. esculenta than in R. pipiens slow fibers. It is concluded that divalent cations have two different sites of action in slow muscle fibers. K+ threshold seems to be affected through binding to sites at the membrane surface; these sites bind Ni2+ and Co2+ more firmly than Ca2+. The second site is presumably the voltage sensor in the transverse tubular membrane, which controls force production, and where Ca2+ is the most effective species of the divalent cations examined.
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Krippeit-Drews, P., Schmidt, H. Effects of Ca2+ and other divalent cations on K+-evoked force production of slow muscle fibers from Rana esculenta and Rana pipiens . J. Membarin Biol. 129, 211–220 (1992). https://doi.org/10.1007/BF00219516
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DOI: https://doi.org/10.1007/BF00219516