Calcium-induced calcium release in crayfish skeletal muscle

1. Cut crayfish skeletal muscle fibres were mounted in a triple Vaseline-gap voltage clamp with the Ca(2+)-sensing dye Rhod-2 allowed to diffuse in via the cut ends. Ca2+ currents across the surface/T-tubule membranes (ICa) were recorded simultaneously with changes in myoplasmic Ca2+ concentration (Ca2+ transients). 2. Excitation-contraction coupling in crayfish skeletal muscle fibres is abolished when calcium in the extracellular solution is replaced by Mg2+. 3. The amplitude of the Ca2+ transients elicited by voltage clamp pulses closely followed the amplitude of the peak calcium currents recorded simultaneously across the surface/T-tubule membranes. This included decreases in both parameters as the pulse potential approached ECa (reversal potential for Ca2+), as well as secondary Ca2+ transients accompanying large tail calcium currents occurring upon repolarization from very large depolarizations. 4. A large contribution of sarcoplasmic reticulum (SR) Ca2+ release to the Ca2+ transients was revealed by a large decrease in the transient caused by the calcium-induced calcium release (CICR) blockers procaine and tetracaine. 5. Short pulses which interrupted the calcium current while SR Ca2+ release was in progress at high rates caused the Ca2+ transient to stop rising nearly immediately after the end of the pulse in most fibres. In about 15% of the fibres the Ca2+ transients continued to rise, albeit at a slower rate, for 10-20 ms after the end of the pulse, as if released Ca2+ was able to elicit some further Ca2+ release from the SR for a while. 6. Even with fibres displaying little sign of continued release after termination of short pulses under control conditions, procaine accelerated the decay of Ca2+ transients elicited by short pulses, indicating that continued release was taking place even as the transient was declining. 7. These results suggest that CICR in crayfish fibres is more closely controlled by a small entry of Ca2+ via surface/T-tubule membrane Ca2+ current than by a larger amount of Ca2+ released from the SR. The limited positive feedback of released Ca2+ on further Ca2+ release allows CICR to remain graded (according to ICa) rather than all-or-none.