Potassium-induced depolarizations and generation of tension in barnacle muscle fibres: effects of external calcium, strontium and barium.

Potassium-induced contractions were studied in muscle fibres from Megabalanus psittacus (Darwin) in an attempt to establish the requirement of extracellular calcium for the development of force. A sudden elevation of external potassium induced the production of contractures and/or twitches only if calcium was present. In calcium-free solutions no contraction or action potential occurred. When external calcium was replaced by barium, potassium-induced depolarizations accompanied by several action potentials did not generate either contractures or twitches. With strontium in place of calcium, action potentials of different duration were observed. Only the long-lasting ones (a few seconds) induced twitches.