Effects of Mg super(2+) on Ca super(2+) release from sarcoplasmic reticulum of skeletal muscle fibres from yabby (crustacean) and rat

The role of myoplasmic [Mg super(2+)] on Ca super(2+) release from the sarcoplasmic reticulum (SR) was examined in the two major types of crustacean muscle fibres, the tonic, long sarcomere fibres and the phasic, short sarcomere fibres of the fresh water decapod crustacean Cherax destructor (yabby) and in the fast-twitch rat muscle fibres using the mechanically skinned muscle fibre preparation. A robust Ca super(2+)-induced Ca super(2+)-release (CICR) mechanism was present in both long and short sarcomere fibres and 1 mM Mg super(2+) exerted a strong inhibitory action on the SR Ca super(2+) release in both fibre types. The SR displayed different properties with respect to Ca super(2+) loading in the long and the short sarcomere fibres and marked functional differences were identified with respect to Mg super(2+) inhibition between the two crustacean fibre types. Thus, in long sarcomere fibres, the submaximally loaded SR was able to release Ca super(2+) when [Mg super(2+)] was lowered from 1 to 0 times 01 mM in the presence of 8 mM ATP sub(total) and in the virtual absence of Ca super(2+) (< 5 nM) even when the CICR was suppressed. In contrast, negligible Ca super(2+) was released from the submaximally loaded SR of short sarcomere yabby fibres when [Mg super(2+)] was lowered from 1 to 0 times 01 mM under the same conditions as for the long sarcomere fibres. Nevertheless, the rate of SR Ca super(2+) release in short sarcomere fibres increased markedly when [Mg super(2+)] was lowered in the presence of [Ca super(2+)] approaching the normal resting levels (50-100 nM). Rat fibres were able to release SR Ca super(2+) at a faster rate than the long sarcomere yabby fibres when [Mg super(2+)] was lowered from 1 to 0 times 01 mM in the virtual absence of Ca super(2+) but, unlike with yabby fibres, the net rate of Ca super(2+) release was actually increased for conditions that were considerably less favourable to CICR. In summary, it is concluded that crustacean skeletal muscles have more that one functional type of Ca super(2+)-release channels, that these channels display properties that are intermediate between those of mammalian skeletal and cardiac isoforms, that the inhibition exerted by Mg super(2+) at rest on the crustacean SR Ca super(2+)-release channels must be removed during excitation-contraction coupling and that, unlike in crustacean fibres, CICR cannot play the major role in the activation of SR Ca super(2+)-release channels in the rat skeletal muscle