Depletion of calcium from the sarcoplasmic reticulum during calcium release in frog skeletal muscle

M. F. Schneider, B. J. Simon, G. Szücs

Research output: Contribution to journalArticle

65 Citations (Scopus)

Abstract

1. Free intracellular calcium transients (Δ[Ca2+] were monitored in cut segments of frog skeletal muscle fibres voltage clamped in a double Vaseline-gap chamber and stretched to sarcomere lengths that eliminated fibre movement. The measured calcium transients were used to calculate the rate of calcium release from the sarcoplasmic reticulum (s.r.) as previously described (Melzer, Rios & Schneider, 1984, 1987). 2. Conditioning pulses were found to suppress the rate of calcium release in test pulses applied after the conditioning pulse. Various combinations of conditioning and test pulses were used to investigate the basis of the suppression of calcium release by the conditioning pulse. 3. Using a constant test pulse applied at varying intervals after a constant conditioning pulse, recovery from suppression of release was found to occur in two phases. During the fast phase of recovery, which was completed within about 1 s, the rate of calcium release was smaller and had a different wave form than the unconditioned control release. The early peak in release that is characteristic of the control release wave form was absent or depressed. During the slow phase of recovery, which required about 1 min for completion, the release wave form was the same as control but was simply scaled down compared to the control. 4. Conditioning pulses also slowed the rate of decay of Δ[Ca2+] after a constant test pulse, probably due to an increased occupancy by calcium of slowly equilibrating myoplasmic sites that bind some of the calcium released by the conditioning pulse. Since calcium binding to these sites contributes to the decay of Δ[Ca2+], their increased occupancy would slow the decay of Δ[Ca2+] following the test pulse. This effect was used to estimate the calcium occupancy of the slowly equilibrating sites. 5. Comparison of the time course of the slow recovery from suppression of release following a constant conditioning pulse with the time course of the loss of calcium from the slowly equilibrating myoplasmic calcium binding sites indicated that the two processes occurred in parallel. 6. Using a set 1 s recovery period and a constant test pulse but varying the amplitude and/or duration of the conditioning pulse, the degree of slowly recovering suppression of release was found to be directly related to the amount of calcium remaining outside of the s.r. at the start of the test pulse. 7. Points 3, 5 and 6 above indicate that the slow recovery from suppression of release may be due to slow recovery from depletion of calcium from the s.r. 8. Records of the rate of calcium release were corrected for the decline in release due to the slowly recovering component of suppression of release. Such corrected records indicated that the slowly recovering component produced only a relatively small and slow decline in release during a pulse. The early peak and sharp decline in release during a pulse must therefore be produced by the component of suppression of release that recovers rapidly following a conditioning pulse.

Original languageEnglish
Pages (from-to)167-192
Number of pages26
JournalJournal of Physiology
Volume392
Publication statusPublished - 1987

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Sarcoplasmic Reticulum
Anura
Skeletal Muscle
Calcium
Pulse
Binding Sites
Petrolatum
Sarcomeres
Skeletal Muscle Fibers

ASJC Scopus subject areas

  • Physiology

Cite this

Depletion of calcium from the sarcoplasmic reticulum during calcium release in frog skeletal muscle. / Schneider, M. F.; Simon, B. J.; Szücs, G.

In: Journal of Physiology, Vol. 392, 1987, p. 167-192.

Research output: Contribution to journalArticle

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N2 - 1. Free intracellular calcium transients (Δ[Ca2+] were monitored in cut segments of frog skeletal muscle fibres voltage clamped in a double Vaseline-gap chamber and stretched to sarcomere lengths that eliminated fibre movement. The measured calcium transients were used to calculate the rate of calcium release from the sarcoplasmic reticulum (s.r.) as previously described (Melzer, Rios & Schneider, 1984, 1987). 2. Conditioning pulses were found to suppress the rate of calcium release in test pulses applied after the conditioning pulse. Various combinations of conditioning and test pulses were used to investigate the basis of the suppression of calcium release by the conditioning pulse. 3. Using a constant test pulse applied at varying intervals after a constant conditioning pulse, recovery from suppression of release was found to occur in two phases. During the fast phase of recovery, which was completed within about 1 s, the rate of calcium release was smaller and had a different wave form than the unconditioned control release. The early peak in release that is characteristic of the control release wave form was absent or depressed. During the slow phase of recovery, which required about 1 min for completion, the release wave form was the same as control but was simply scaled down compared to the control. 4. Conditioning pulses also slowed the rate of decay of Δ[Ca2+] after a constant test pulse, probably due to an increased occupancy by calcium of slowly equilibrating myoplasmic sites that bind some of the calcium released by the conditioning pulse. Since calcium binding to these sites contributes to the decay of Δ[Ca2+], their increased occupancy would slow the decay of Δ[Ca2+] following the test pulse. This effect was used to estimate the calcium occupancy of the slowly equilibrating sites. 5. Comparison of the time course of the slow recovery from suppression of release following a constant conditioning pulse with the time course of the loss of calcium from the slowly equilibrating myoplasmic calcium binding sites indicated that the two processes occurred in parallel. 6. Using a set 1 s recovery period and a constant test pulse but varying the amplitude and/or duration of the conditioning pulse, the degree of slowly recovering suppression of release was found to be directly related to the amount of calcium remaining outside of the s.r. at the start of the test pulse. 7. Points 3, 5 and 6 above indicate that the slow recovery from suppression of release may be due to slow recovery from depletion of calcium from the s.r. 8. Records of the rate of calcium release were corrected for the decline in release due to the slowly recovering component of suppression of release. Such corrected records indicated that the slowly recovering component produced only a relatively small and slow decline in release during a pulse. The early peak and sharp decline in release during a pulse must therefore be produced by the component of suppression of release that recovers rapidly following a conditioning pulse.

AB - 1. Free intracellular calcium transients (Δ[Ca2+] were monitored in cut segments of frog skeletal muscle fibres voltage clamped in a double Vaseline-gap chamber and stretched to sarcomere lengths that eliminated fibre movement. The measured calcium transients were used to calculate the rate of calcium release from the sarcoplasmic reticulum (s.r.) as previously described (Melzer, Rios & Schneider, 1984, 1987). 2. Conditioning pulses were found to suppress the rate of calcium release in test pulses applied after the conditioning pulse. Various combinations of conditioning and test pulses were used to investigate the basis of the suppression of calcium release by the conditioning pulse. 3. Using a constant test pulse applied at varying intervals after a constant conditioning pulse, recovery from suppression of release was found to occur in two phases. During the fast phase of recovery, which was completed within about 1 s, the rate of calcium release was smaller and had a different wave form than the unconditioned control release. The early peak in release that is characteristic of the control release wave form was absent or depressed. During the slow phase of recovery, which required about 1 min for completion, the release wave form was the same as control but was simply scaled down compared to the control. 4. Conditioning pulses also slowed the rate of decay of Δ[Ca2+] after a constant test pulse, probably due to an increased occupancy by calcium of slowly equilibrating myoplasmic sites that bind some of the calcium released by the conditioning pulse. Since calcium binding to these sites contributes to the decay of Δ[Ca2+], their increased occupancy would slow the decay of Δ[Ca2+] following the test pulse. This effect was used to estimate the calcium occupancy of the slowly equilibrating sites. 5. Comparison of the time course of the slow recovery from suppression of release following a constant conditioning pulse with the time course of the loss of calcium from the slowly equilibrating myoplasmic calcium binding sites indicated that the two processes occurred in parallel. 6. Using a set 1 s recovery period and a constant test pulse but varying the amplitude and/or duration of the conditioning pulse, the degree of slowly recovering suppression of release was found to be directly related to the amount of calcium remaining outside of the s.r. at the start of the test pulse. 7. Points 3, 5 and 6 above indicate that the slow recovery from suppression of release may be due to slow recovery from depletion of calcium from the s.r. 8. Records of the rate of calcium release were corrected for the decline in release due to the slowly recovering component of suppression of release. Such corrected records indicated that the slowly recovering component produced only a relatively small and slow decline in release during a pulse. The early peak and sharp decline in release during a pulse must therefore be produced by the component of suppression of release that recovers rapidly following a conditioning pulse.

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