Role of the different calcium sources in the excitation-contraction coupling in crab muscle fibers.

Excitation-contraction coupling in crab muscle fibers was studied in voltage-clamp conditions. Extracellular calcium is essential for the mechanical activity. Two calcium influxes induced by membrane depolarization contribute to tension development: one is the inward calcium current responsible for the phasic tension, the other is a calcium influx dependent on extracellular sodium and calcium concentrations and is responsible for the tonic tension. These calcium influxes are not sufficient to activate contractile proteins. Experiments with procaine and caffeine show that a calcium release from the sarcoplasmic reticulum is required.