title: Effect of sodium deprivation on contraction and charge movement in frog skeletal muscle fibres.
abstract: Measurements of isometric tension were performed in single twitch skeletal muscle fibres and the effect of extracellular Na+ removal on contraction was investigated. Na+ withdrawal brought about an increase in the amplitude of K+ contractures and their time course became faster. The potentiation of K+ contractures depended strongly on extracellular Ca2+ and developed slowly following an exponential time course with a time constant of approximately 8 min. Removal of extracellular Na+ greatly increased the amplitude of caffeine contractures and lowered its threshold: caffeine (0.5 mM) had no effect on resting tension in Ringer's but produced contractures in Na(+)-free solutions. Intramembrane charge movement (charge 1) was monitored in contracting voltage-clamped segments of frog skeletal muscle fibres using the triple-Vaseline-gap technique. Movement of charge 1 did not depend on the presence of extracellular Na+. However, the mechanical threshold decreased by approximately 10 mV at several pulse durations and the charge which produced just detectable contractions decreased by approximately 5 nC microF-1 in the absence of extracellular Na+. Intracellular heparin (40 mg ml-1) increased the mechanical threshold by approximately 20 mV without affecting the movement of charge 1. The effect of Na(+)-free solutions on the mechanical threshold was additive to that of heparin. It is concluded that the effects of Na(+)-withdrawal on contraction take place at a location beyond the voltage sensor of excitation-contraction coupling.
authors.affiliation: Department of Pharmacology, Centro de Investigacion y de Estudios Avanzados del IPN, Mexico, DF.
authors.forename: M C
authors.initials: MC
authors.lastname: Garcia
authors.forename: A F
authors.initials: AF
authors.lastname: Diaz
authors.forename: R
authors.initials: R
authors.lastname: Godinez
authors.forename: J A
authors.initials: JA
authors.lastname: Sanchez
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D006493:Heparin / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects*; D011188:Potassium / Q000494:pharmacology; D011894:Rana pipiens; D011898:Ranidae; D012964:Sodium / Q000494:pharmacology* / Q000502:physiology
chemical_list: D002110:Caffeine; D006493:Heparin; D012964:Sodium; D011188:Potassium; D002118:Calcium