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 approximately 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 TC 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 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 (approximately
72\%) water content of the TC obtained with electron-probe analysis.
- 6974735
- animals,
- calcium,
- chlorides,
- contraction,
- cytoplasm,
- electron
- gov't,
- magnesium,
- microanalysis,
- mitochondria,
- muscle
- muscle,
- muscles,
- p.h.s.,
- pipiens,
- potassium,
- probe
- rana
- research
- reticulum,
- sarcoplasmic
- sodium,
- support,
- u.s.
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