The depolarizing afterpotential of crab muscle fibres. A sodium-dependent process mediated by intracellular calcium.

1. A study was made of the depolarizing afterpotential (d.a.p.) which follows the initial graded electrogenesis of crab muscle fibres. 2. Increasing the strength, duration or amplitude of the stimulating current pulses enhanced both the d.a.p. and the local contractions. 3. Arsenazo III was injected intracellularly and changes in light absorbance by the dye were used to monitor the increase in free sarcoplasmic Ca concentration during excitation-contraction (E-C) coupling. The onset of the absorbance changes occurred during the depolarizing phase of the initial electrogenesis and the maximum value coincided with the peak of the d.a.p. An exponential decay of the absorbance signal occurred during the repolarizing phase of the d.a.p. 4. Ionophoretic injection of EGTA into the sarcoplasm did not affect the initial electrogenesis but did reduce changes in dye absorbance, blocked tension development and abolished the d.a.p. 5. Caffeine (0.1--0.4 mM) markedly enhanced both the d.a.p. and the local contractions, but had no effect on the initial electrogenesis. 6. Replacement of extracellular Na ions with Li, Tris or choline abolished the d.a.p. The initial electrogenesis was enhanced in the choline-containing medium, but was not affected by Li or Tris. The rate of relaxation of the local contractions and the rate of decay of the light absorbance changes were slowed in Na-free saline. 7. Tetrodotoxin (10(-5) g/ml.) had no effect on either the membrane responses or tension development. 8. For initial graded responses of comparable peak amplitude a threefold reduction of [Ca-a1o shortened the d.a.p., but had little effect onits peak amplitude. A fivefold increase in [Ca]o reduced both the amplitude and duration of the d.a.p. 9. Changes in [Mg]o had little effect on the d.a.p., but both Mn (4--10 mM) and La (0.1 mM) blocked the initial electrogenesis and the d.a.p. 10. It is concluded that distinct ionic mechanisms give rise to the initial electrogenesis and the d.a.p. While the former is due to activation of Ca conductance, the d.a.p. is a Na-dependent phenomenon that is tetrodotoxin-insensitive, and mediated by the rise of intracellular Ca concentration during E--C coupling.