Repriming and activation alter the frequency of stereotyped discrete Ca2+ release events in frog skeletal muscle.
1. Brief localized elevations in myoplasmic [Ca2+] (Ca2+ sparks) in individual sarcomeres of voltage-clamped frog skeletal muscle fibres were examined by laser scanning confocal microscopy. 2. Fibres held at 0 mV were briefly repolarized to -90 mV (repriming pulse) to restore only a small fraction of sarcoplasmic reticulum (SR) calcium release. Subsequent depolarization to 0 mV (test pulse) caused the appearance of small numbers of Ca2+ sparks at different sarcomeres from pulse to pulse. Increasing the repriming time resulted in an increase in the frequency of occurrence of the Ca2+ sparks. 3. The amplitude and spatio-temporal extent of the Ca2+ sparks were independent of the repriming time and test pulse voltage. Ca2+ sparks recorded during small depolarizations of fibres held at -90 mV had a similar amplitude and spatio-temporal extent as those recorded after brief repriming of the same fibre held at 0 mV. 4. We conclude that stereotyped Ca2+ sparks underlie calcium release at all voltages and all extents of repriming. The amplitude of Ca2+ release is thus graded by the frequency but not by the amplitude or spatio-temporal extent of the individual SR Ca2+ release events.
Lacampagne
A
A
Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore 21201, USA.
Lederer
W J
WJ
Schneider
M F
MF
Klein
M G
MG
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004594:Electrophysiology; D018613:Microscopy, Confocal; D018482:Muscle, Skeletal / Q000378:metabolism*; D011894:Rana pipiens; D012518:Sarcomeres / Q000378:metabolism
D002118:Calcium