On the role of extracellular calcium in triggering contraction in muscle fibres from barnacle under membrane potential control.

1. Single giant barnacle muscle fibres from Megabalanus psittacus (Darwin) were used to establish the possible role of extracellular Ca2+ in triggering the contraction. 2. Peak tension p0 during twitches elicited by depolarizing voltage clamp pulses decreases after the removal of Ca2+ from the external saline. p0 becomes too small to measure after a few minutes, when the model‐estimated level of total intracellular Ca has been reduced to 99.5% of its original value and the extracellular Ca in the clefts and/or tubules to about 10%. 3. In fibres where the outward currents were blocked by internal perfusion with Cs+, the curves of the integral of the inward current versus membrane potential and of peak tension versus membrane potential follow a similar pattern. 4. When Ca2+ in the external saline was replaced by Ba2+ or Sr2+, no twitches were generated although depolarizing voltage clamp pulses induced large inward currents comparable to those measured when Ca2+ was present. 5. It is concluded that an increase in Ca2+ in the fibre during the Ca2+ inward current is a necessary condition for the triggering of the development of tension. The data on Ba2+ inhibition is consistent with the hypothesis that there are two different intracellular sites of action for Ca2+.