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

Abstract

Approximately 60-70% of the total fiber Ca 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 Ca content of the TC was released, enough to raise total cytoplasmic Ca concentration by .apprx. 1 mM. This is equivalent to the concentration of binding sites on the Ca binding proteins (troponin and parvalbumin) in frog muscle. Ca release was associated with a significant uptake of Mg and K into the TC, but the amount of Ca released exceeded the total measured cation accumulation by 62 mEq/kg dry wt. Most of the charge deficit may be apparent. Charge compensation apparently 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 Na or Cl content of the TC during tetanus. The unchanged distribution of permeant anion, Chloride, argues against the existence of a large and sustained trans SR potential during tetanus, if the chloride permeability of the in situ SR is as high as suggested by measurements on fractionated SR. The Ca content of the longitudinal SR (LSR) during tetanus did not show the LSR to be a major site of Ca storage and delayed return to the TC. The K concentration in the LSR did not differ significantly from the adjacent cytoplasmic concentration. Analysis of small areas of l-band and large areas, including several sarcomeres, suggested that Chloride is anisotropically distributed, with some of it probably bound to myosin. The distribution of K in the fiber cytoplasm followed the water distribution. The mitochondrial concentration of Ca was low and did not change significantly during a tetanus. The TC of both tetanized and resting freeze-substituted muscles contained electron-lucent circular areas. The appearance of the TC showed no evidence of major volume changes during tetanus, in agreement with the estimates of unchanged (.apprx. 72%) water content of the TC obtained with electron-probe analysis.