Effects of Mg2+ on Ca2+ release from sarcoplasmic reticulum of skeletal muscle fibres from yabby (crustacean) and rat

1. The role of myoplasmic [Mg2+] on Ca2+ release from the sarcoplasmic reti culum (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 mater decapod crustacean Cherax: destructor (yabby) and in the f ast-twitch rat muscle fibres using the mechanically skinned muscle fibre pr eparation. 2. A robust Ca2+-induced Ca2+-release (CICR) mechanism was present in both long and short sarcomere fibres and 1 mM Mg2+ exerted a strong inhibitory a ction on the XR Ca2+ release in both fibre types. 3. The XR displayed different properties with respect to Ca2+ loading in th e long and the short sarcomere fibres and marked functional differences wer e identified with respect to Mg2+ inhibition between the two crustacean fib re types. Thus, in long sarcomere fibres, the submaximally loaded XR was ab le to release Ca2+ when [Mg2+] was lowered from 1 to 0.01 mw in the presenc e of 8 mM ATP(total) and in the virtual absence of Ca2+ (< 5 nM) even when the CICR was suppressed. In contrast, negligible Ca2+ was released from the submaximally loaded SR of short sarcomere yabby fibres when [Mg2+] was low ered from 1. to 0.01 mM under the same conditions as for the long sarcomere fibres. Nevertheless, the rate of XR Ca2+ release in short sarcomere fibre s increased markedly when [Mg2+] was lowered in the presence of [Ca2+] appr oaching the normal resting levels (50-100 nM). 4. Rat fibres were able to release SR Ca2+ at a faster rate than the long s arcomere yabby fibres when [Mg2+] was lowered from 1 to 0.01 mM in the virt ual absence of Ca2+ but, unlike with yabby fibres, the net rate of Ca2+ rel ease was actually increased for conditions that were considerably less favo urable to CICR. 5. In summary it is concluded that crustacean skeletal muscles have more th at one functional type of Ca2+-release channels, that these channels displa y properties that are intermediate between those of mammalian skeletal and cardiac isoforms, that the inhibition exerted by Mg2+ at rest on the crusta cean SR Ca2+-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 XR Ca2+-release channels in the rat skeletal mus cle.