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...

In skeletal muscle, contractile force can be transmitted laterally between the z-disks and M-lines of neighboring myofibrils, across the sarcolemma, and through the extracellular matrix to the tendon. Here we examine the data that support this model and the sarcolemmal properties and structures, termed "costameres," that are consistent with it.

To determine if chronic heart failure (CHF) leads to functional or structural alterations of skeletal muscle, we compared intracellular Ca2+ signaling, contractility, and the rate of fatigue development, together with electron microscopy (EM), in skeletal muscle preparations from rats with myocardial infarction-induced CHF versus sham-operated control rats. Bundles of 100 to 200 cells were dissected from the extensor digitorum longus (EDL) muscle of control (n = 13) and CHF (n = 19) rats and were either loaded with aequorin or fixed for EM. Muscles from CHF rats exhibited depressed tension development compared with control muscles during twitches (1.4 +/- 0.2 versus 2.8 +/- 0.7 g/mm2, P < .05) and maximal tetani (5.3 +/- 1.4 versus 10.7 +/- 2.4 g/mm2, P < .05). Depressed tension in CHF was accompanied by reduced quantitative [Ca2+]i release during twitches (0.7 +/- 0.1 versus 0.4 +/- 0.1 microM, P < .05) and during maximal tetani (1.8 +/- 0.3 versus 0.9 +/- 0.2 microM, P < .05). Skeletal muscle from CHF rats also demonstrated prolonged intracellular Ca2+ transients during twitches and tetani and accelerated fatigue development. EM revealed a lack of cellular atrophy in the CHF rats. In conclusion, EDL skeletal muscle from rats with CHF had intrinsic abnormalities in excitation-contraction coupling unrelated to cellular atrophy. These findings indicate that CHF is a condition accompanied by EDL skeletal muscle dysfunction.

SUMMARY1. The excitatory process travelling along the T-system may be either electrotonic or regenerative. If Na+ dependent action potential is present in the tubular membranes, high frequency of stimulation might cause a Na+ depletion in the tubules sufficient to abolish this process.2. We tested this hypothesis by recording tension in isolated muscle fibres stimulated tetanically (up to 60 shocks/sec). In low [Na+] solutions, output tension was initially similar to that in normal Ringer, but then fell smoothly to a substantially lower value. 3. The activity of individual myofibrils was recorded directly with cinemicrographs during isotonic contractions while the fibres were stimulated at high frequencies. In low [Na+]o wavy myofibrils appeared in the centre of the fibre and spread towards the periphery, indicating failure of activation. Wavy myofibrils never appeared in normal Ringer.4. Intracellular action potentials recorded during the tetanic stimulation indicated that the inactivated myofibrils present in low [Na+] solutions cannot be explained by the changes in size and duration of the action potential. 5. Our results strongly suggest the existence of a regenerative Na+ conductance in the tubular membrane during the inward spread of an excitatory process.

We used four antibodies to regions of obscurin isoforms A and B, encoded by the obscurin gene, to investigate the location of these proteins in skeletal myofibers at resting and stretched lengths. Obscurin A (800 kDa) which was recognized by antibodies generated to the N-terminal, Rho-GEF, and the non-modular C-terminal domain that lacks the kinase-like domains, localizes at the level of the M-band. Obscurin B (900 kDa) which has the N-terminal, Rho-GEF, and the C-terminal kinase-like domains, localizes at the level of the A/I junction. Additional isoforms, which lack one or more of these epitopes, are present at the Z-disk and Z/I junction.