REPRIMING AND ACTIVATION ALTER THE FREQUENCY OF STEREOTYPED DISCRETE CA2-MUSCLE( RELEASE EVENTS IN FROG SKELETAL)

1. Brief localized elevations in myoplasmic [Ca2+] (Ca2+ sparks) in in dividual sarcomeres of voltage-clamped frog skeletal muscle fibres wer e examined by laser scanning confocal microscopy. 2. Fibres held at 0 mV were briefly repolarized to -90 mV (repriming pulse) to restore onl y a small fraction of sarcoplasmic reticulum (SR) calcium release. Sub sequent depolarization to 0 mV (test pulse) caused the appearance of s mall numbers of Ca2+ sparks at different sarcomeres from pulse to puls e. Increasing the repriming time resulted in an increase in the freque ncy of occurrence of the Ca2+ sparks. 3. The amplitude and spatio-temp oral extent of the Ca2+ sparks were independent of the repriming time and test pulse voltage. Ca2+ sparks recorded during small depolarizati ons of fibres held at -90 mV had a similar amplitude and spatio-tempor al extent as those recorded after brief repriming of the same fibre he ld at 0 mV. 4. We conclude that stereotyped Ca2+ sparks underlie calci um release at all voltages and all extents of repriming. The amplitude of Ca2+ release is thus graded by the frequency but not by the amplit ude or spatio-temporal extent of the individual SR Ca2+ release events .