HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Cota, G Articles by Stefani, E Search for Related Content PubMed PubMed Citation Articles by Cota, G Articles by Stefani, E

1. The amplitude and time course of K contractures (Cl- constant) of single twitch muscle fibres of the frog have been analysed in three external Ca2+ concentrations. 2. The resting potential, effective resistance, threshold for the Na current and K-induced depolarizations were not modified by replacing 1.8 mM-Ca2+ by 3 mM-Mg2+ in absence (low-Ca saline: 3-6 micro M-Ca2+) or in the presence of 5 mM-EGTA (Ca-free saline: less than or equal to 10(-9) M-Ca2+). 3. The tension development during the initial phase of K contractures was independent of external Ca2+ while the amplitude, the duration and the time constant of spontaneous relaxation decreased progressively as Ca2+ concentration was diminished. 4. When the concentration of Mg2+ was increased to 5 mM in Ca-free saline K contractures were slower and smaller than those in 3 mM-Mg2+. 5. In Ca-free saline the activation curve (peak tension vs. logarithm of external K+ concentration) shifted by 3-5 mV towards more positive potentials while the inactivation curve (peak tension of the test contracture vs. logarithm of external K+ concentration during the conditioning period) shifted by 16-18 mV towards more negative potentials. Both curves became steeper in Ca-free saline. 6. The effects of external Ca2+ reduction were not modified by replacing all chloride for methanesulphonate. 7. Direct effects of external Ca2+ on excitation-contraction coupling during K contractures could involve the inward Ca current and/or specific interactions between external Ca2+ ions and the coupling mechanism.