title: Effects of Ca2+ and other divalent cations on K(+)-evoked force production of slow muscle fibers from Rana esculenta and Rana pipiens.
abstract: 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 read-mission 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.
authors.affiliation: Physiologisches Institut, Universität des Saarlandes, Homburg/Saar, Federal Republic of Germany.
authors.forename: P
authors.initials: P
authors.lastname: Krippeit-Drews
authors.forename: H
authors.initials: H
authors.lastname: Schmidt
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D003035:Cobalt / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D008274:Magnesium / Q000494:pharmacology*; D008345:Manganese / Q000494:pharmacology*; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D009132:Muscles / Q000187:drug effects / Q000502:physiology*; D009532:Nickel / Q000494:pharmacology*; D011188:Potassium / Q000494:pharmacology*; D011893:Rana esculenta / Q000502:physiology*; D011894:Rana pipiens / Q000502:physiology*; D013997:Time Factors
chemical_list: D003035:Cobalt; D002110:Caffeine; D008345:Manganese; D009532:Nickel; D008274:Magnesium; D011188:Potassium; D002118:Calcium