Calcium currents during contraction and shortening in enzymatically isolated murine skeletal muscle fibres

1. Calcium currents (I-Ca) were monitored in enzymatically isolated murine toe muscle fibres using the two-microelectrode voltage-clamp technique. I-C a 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 Ca2+ solution. Maximum peak currents measure d -12.6 +/- 2.3 nA (mean +/- S.D.; n = 4) in 2 mM Ca2+ and -65 +/- 15 nA (n = 7) in 10 mM Ca2+. The time to peak (TTP) I-Ca was 96 +/- 22 ms (n = 4) i n 2 mar Ca2+ and 132 +/- 13 ms (n = 7) in 10 mM Ca2+. The exponential decay of I-Ca was similar in 2 and 10 mM Ca2+ with rate constants (tau(-1)(V)) o f 3.7 s(-1) (2 mM) and 3.8 s(-1) (10 mM) at +10 mV. 3. I-Ca in isotonic 10 mM Ca2+ solution was recorded by inserting the micro pipettes 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 Ca2+ 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 tau( -1)(V) of 6.4 s(-1) at +10 mV (P < 0.03). 5. To test for calcium depletion, extracellular Ca2+ 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 u nbuffered 10 mM Ca2+ solution. 6. We conclude that calcium depletion from the transverse tubular system co ntributes significantly to the decay of calcium currents in murine toe musc le fibres under hypertonic as well as isotonic conditions. In the latter, d epletion is even more prominent.