Distribution of sodium, magnesium, chlorine, calcium, potassium, phosphorus and sulphur in Z-, I- and A-bands in mammalian striated muscle.

The regularity of the striation of skeletal muscle offers a unique possibility to analyze different bands of the sarcomere in longitudinally cut semithin cryosections. The aim of the present study was to investigate the elemental content of the Z-, I- and A-bands within the sarcomere which may be related to the affinity of an element to different contractile elements and the water content in different bands. The highest potassium levels were found in the Z-band (802 mmol/kg dry weight (d.w.)) as compared to the I-band (697 mmol/kg d.w.) and the A-band (731 mmol/kg d.w.). The difference between A-band and Z-band, but not between I-band and A-band, was significant. The highest phosphorus values were detected in the Z- and I- bands and the lowest within the A-bands (632, 615 and 540 mmol/kg d.w. respectively). No significant differences were found in the concentrations of Na, S, or Cl. Ca was significantly lower in the I-band as compared to A- and Z-band. The Mg concentration in the I- and A-band was significantly higher than in the Z-band. By means of computerized densitometry, relative densities (proportional to the dry mass content) of the Z-, I- and A-band were calculated (23.9, 11.6, and 19.2, respectively). The mean value of dry mass over several sarcomeres varied between 19.5-22.5 which corresponds well with dry weight mass concentrations obtained by traditional methods. The values for dry mass content allowed recalculation of the elemental concentrations as mmol/kg wet weight.