Calcium release and ionic changes in the sarcoplasmic reticulum of tetanized muscle: an electron-probe study.

Abstract

Approximately 60-70% of the total fiber calcium was localized in the terminal cisternae (TC) in resting frog muscle as determined by electron-probe analysis of ultrathin cryosections . During a 1 .2 s tetanus, 59% (69 mmol/kg dry TC) of the calcium content of the TC was released, enough to raise total cytoplasmic calcium concentration by -1 mM . This is equivalent to the concentration of binding sites on the calcium-binding proteins (troponin and parvalbumin) in frog muscle . Calcium release was associated with a significant uptake of magnesium and potassium into the TC, but the amount of calcium released exceeded the total measured cation accumulation by 62 mEq/kg dry weight . It is suggested that most of the charge deficit is apparent, and charge compensation is achieved by movement of protons into the sarcoplasmic reticulum (SR) and/or by the movement of organic co-or counterions not measured by energy dispersive electron-probe analysis . There was no significant change in the sodium or chlorine content of the TIC during tetanus. The unchanged distribution of a permeant anion, chloride, argues against the existence of a large and sustained transSR potential during tetanus, if the chloride permeability of the in situ SR is as high as suggested by measurements on fractionated SR .The calcium content of the longitudinal SR (LSR) during tetanus did not show the LSR to be a major site of calcium storage and delayed return to the TC . The potassium concentration in the LSR was not significantly different from the adjacent cytoplasmic concentration. Analysis of small areas of I-band and large areas, including several sarcomeres, suggested that chloride is anisotropically distributed, with some of it probably bound to myosin . In contrast, the distribution of potassium in the fiber cytoplasm followed the water distribution . The mitochondrial concentration of calcium was low and did not change significantly during a tetanus. The TIC of both tetanized and resting freeze-substituted muscles contained electron-lucent circular areas . The appearance of the TIC showed no evidence of major volume changes during tetanus, in agreement with the estimates of unchanged (-72%) water content of the TIC obtained with electron-probe analysis .The release of Ca from and its subsequent return to the triadic portion of the sarcoplasmic reticulum (SR) (28,80,82) are the major determinants of the contractile cycle of striated muscle (for review, see reference 24) . Since the demonstration of the SR as the ATP-dependent relaxing factor (57), a wealth of information has been accumulated about the kinetics and THE JOURNAL OF CELL BIOLOGY " VOLUME 90 SEPTEMBER 1981 577-594 © The Rockefeller University Press -0021-9525/81/09/0577/18 $1 .00 mechanisms of calcium uptake by the SR (e.g., 41,48,63,104,106, and for review, see references 62, 102). In contrast, comparatively little is known about the mechanism of release and associated ion movements, largely because isolated SR preparations do not lend themselves to reproduction of the ph...

Simultaneous monitoring of changes in magnesium and calcium concentrations in frog cut twitch fibers containing antipyrylazo III.

“…It seems unlikely that the sarcoplasmic reticulum participates in this increase since it appears to gain, not lose, Mg during activity; after a 1.2-s tetanus, the Mg content of the sarcoplasmic reticulum is slightly increased (Somlyo et al, 1981). Mitochondria are also an unlikely candidate since they occupy only 1% of the fiber volume (Mobley and Eisenberg, 1975) and their Mg content is unchanged after a 1.2-s tetanus (Somlyo et al, 1981).…”

“…4 A shows a transient increase in pH, possibly due to protons entering the sarcoplasmic reticulum accompanying Ca release (Somlyo et al, 1981;Baylor et al, 1982a;. The rise time and duration of the transients are different for the two indicators.…”

“…Second, some of the parvalbumin sites may not have released Mg during large depolarizations, either because Mg did not occupy the site at rest or because Mg was not replaced by Ca during the period when free [Ca] was elevated. Third, there may be sources or sinks for Mg other than parvalbumin and phosphocreatine; for example, some Mg appears to enter the sarcoplasmic reticulum during a 1.2-s tetanus (Somlyo et al, 1981). Fourth, an average fraction 0.68 of the antipyrylazo III inside a cut fiber appears to be bound or sequestered or, for some other reason, unable to diffuse freely (Maylie et al, 1987c); this may be related to the finding that the indicator's absorbance spectrum in a resting fiber, in the wavelength range 570-670 nm, is red shifted compared with that in a cuvette (Figs.…”

BisG10, a K+ channel blocker, affects the calcium release channel from skeletal muscle sarcoplasmic reticulum

“…Several studies support the hypothesis that K* ions are the counter-ions for Ca:* fluxes. (i) Electron probe analysis showed that Ca 2* release from the SR during a t,nanus was accompanied by an increase in the concentration of K* ions in the tem linal cisternae [9]. (it) Rapid filtration studies on isolated SR vesicles indicated that the Ca2÷-induced Ca a* release was dependent upon the presence of K + in the release medium and influenced by the K* gradient created across the SR membrane [10].…”

THE INHIBITORY EFFECT OF SHAKUYAKUKANZOTO ON K+ CURRENT IN H9c2 CELLS

“…Suppression of IKur would reduce K + efflux from muscle fibers and cramped myofibers are tetanized 21) . 22) reported that paeoniflorin (a major component of Shakuyaku) and glycyrrhizin depolarized and blocked the neuromuscular junctions.…”