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Biophysical Journal 55: 793-797 (1989)
© 1989 the Biophysical Society

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Caffeine slows turn-off of calcium release in voltage clamped skeletal muscle fibers.

B J Simon, M G Klein and M F Schneider

Department of Biological Chemistry, University of Maryland School of Medicine, Baltimore 21201.

ABSTRACT

Myoplasmic free calcium transients delta [Ca2+] were monitored with the calcium indicators antipyrylazo III and fura-2 in voltage clamped cut frog skeletal muscle fibers, in the presence and absence of 0.5 mM caffeine. Without caffeine delta [Ca2+] began to decline within a few milliseconds of fiber repolarization for pulses of all durations. In caffeine delta [Ca2+] continued to rise for 10-60 ms after 10 or 20 ms depolarizing pulses, indicating that the release of calcium from the sarcoplasmic reticulum (SR) continued well after repolarization of transverse tubular (TT) membranes in the presence of caffeine. Caffeine also increased the peak amplitude of delta [Ca2+] for all pulses and slowed the decline of delta [Ca2+] after pulses of all durations. The rate of calcium release from the SR calculated from delta [Ca2+] showed that for 10 ms pulses in caffeine release did not turn off abruptly on repolarization but instead declined to zero with a time constant essentially the same as the time constant for inactivation of SR calcium release during depolarizing pulses in the presence or absence of caffeine. The observed loss of TT membrane potential control of SR calcium release in the presence of caffeine suggests the appearance of a significant component of cytosolic Ca2+-induced calcium release in caffeine.




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E Niggli and W. Lederer
Voltage-independent calcium release in heart muscle
Science, October 26, 1990; 250(4980): 565 - 568.
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Copyright © 1989 by the Biophysical Society.