Calcium movements in single crustacean muscle fibres

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Abstract
1. Internal microinjection of the Ca‐sensitive photoprotein aequorin or the isotope 45Ca have been used to assess Ca movements in single muscle fibres from the barnacle Balanus nubilus and the crab Maia squinado.2. Progressive isosomotic replacement of external Na by Li, choline, sucrose or Tris was associated with a rapid increase in the level of light emission from internally injected aequorin. This response was dependent upon the presence of external Ca. The light output was maximal for Na concentrations < 50 m M in the replaced salines, while Na concentrations > 350 m M produced no apparent increase in the resting light emission.3. If the Ca concentration in the external saline was altered, no effect was observed in Na replaced salines when the Ca concentration was < 1 m M, but maximal effects were observed at concentrations of Ca of ca. 100 m M.4. The increased light emission from aequorin in Na‐replaced salines was substantially inhibited by 1 m M‐La3+ applied externally, but not by N‐ethyl maleimide (NEM), propanolol or D‐600.5. Following microinjection of 45Ca and 2 hr equilibration, the residual efflux of Ca was shown to be sensitive to the removal of external Ca and Na, the extent of each component being variable.6. The Na‐sensitive Ca efflux was partially inhibited by external La3+ (1 m M), but not by high concentrations of Mg2+, Ca2+, Mn2+ or Co2+. It was stimulated by NEM and ethacrynic acid, which was considered to be due to an indirect effect upon the sarcoplasmic reticulum.7. It is concluded that Ca movements in these single crustacean muscle fibres involve Na gradients and a Ca: Ca exchange and from the relative insensitivity of Ca movements to various inhibitors, in contrast to the Mg system, it seems that there may be two independent modes for divalent cation transport in this preparation.