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Calcium currents during contraction and shortening in enzymatically isolated murine skeletal muscle fibres
Author(s) -
Friedrich O.,
Ehmer T.,
Fink R. H. A.
Publication year - 1999
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1999.0757s.x
Subject(s) - chemistry , calcium , tonicity , isotonic solutions , isotonic , contraction (grammar) , pipette , extracellular , biophysics , analytical chemistry (journal) , nuclear magnetic resonance , chromatography , biochemistry , medicine , physics , biology , organic chemistry
1 Calcium currents ( I Ca ) were monitored in enzymatically isolated murine toe muscle fibres using the two‐microelectrode voltage‐clamp technique. I Ca was recorded (i) in hypertonic solution to suppress contraction, and (ii) in actively shortening fibres in isotonic solution. 2 In hypertonic solution the threshold potential for I Ca was about −30 mV for both 2 and 10 mM external Ca 2+ solution. Maximum peak currents measured −12.6 ± 2.3 nA (mean ± s.d.; n = 4) in 2 mM Ca 2+ and −65 ± 15 nA ( n = 7 ) in 10 mM Ca 2+ . The time to peak (TTP) I Ca was 96 ± 22 ms ( n = 4 ) in 2 mM Ca 2+ and 132 ± 13 ms ( n = 7 ) in 10 mM Ca 2+ . The exponential decay of I Ca was similar in 2 and 10 mM Ca 2+ with rate constants (τ −1 ( V )) of 3.7 s −1 (2 mM) and 3.8 s −1 (10 mM) at +10 mV. 3 ICa in isotonic 10 mM Ca 2+ solution was recorded by inserting the micropipettes exactly opposite to each other close to the centre of mass of the fibre where negligible contraction‐induced movement occurs. 4 In isotonic 10 mM Ca 2+ solution I Ca had a smaller peak amplitude (−45 ± 5 nA; n = 7) and faster TTP (82.8 ± 22.1 ms; n = 7) than in hypertonic solution. The exponential decay of I Ca showed a significantly larger τ −1 ( V ) of 6.4 s −1 at +10 mV ( P < 0.03 ). 5 To test for calcium depletion, extracellular Ca 2+ was buffered by malic acid in isotonic solution to 9 mM. The decay of I Ca had a time constant of 348 ± 175 ms ( n = 14 ) vs. 107 ± 24 ms ( n = 12; P < 0.001 ) at 0 mV in unbuffered 10 mM Ca 2+ solution. 6 We conclude that calcium depletion from the transverse tubular system contributes significantly to the decay of calcium currents in murine toe muscle fibres under hypertonic as well as isotonic conditions. In the latter, depletion is even more prominent.