CRAYFISH SKELETAL MUSCLE REQUIRES BOTH INFLUX OF EXTERNAL Ca2+ AND Ca2+ RELEASE FROM INTERNAL STORES FOR CONTRACTION

The isometric tension and membrane potential of single skeletal muscle fibres from the flexor muscle of the carpopodite in the meropodite of crayfish Procambarus clarkii (Girard) were studied to determine whether crayfish muscle contraction requires Ca2+ release from the sarcoplasmic reticulum. Contraction elicited by brief extracellular electrical stimulation was reduced by the removal of Ca2+ or by the addition of 25 micromolar nicardipine in crayfish Ringer's solution. Addition of 30 micromolar ryanodine with 1 mmol l-1 caffeine induced a transient contracture, the peak tension of which was 10–30 % of that of the high-K+ induced contracture and which declined to the pretreatment level in 20–60 min. After ryanodine-caffeine treatment, 30 mmol l-1 caffeine failed to induce contraction, suggesting that intracellular Ca2+ stores had been exhausted by the treatment. Extracellular electrical stimulation also failed to induce contraction after ryanodine-caffeine treatment, although the resting potential was not changed. These results suggest that Ca2+ release from the sarcoplasmic reticulum, together with Ca2+ influx via nicardipine-sensitive Ca2+ channels, is essential to the contraction of crayfish leg muscle fibres after a brief membrane depolarization.