title: On the role of extracellular calcium in triggering contraction in muscle fibres from barnacle under membrane potential control.
abstract: 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+.
mesh_terms: D000818:Animals; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D005110:Extracellular Space / Q000502:physiology; D066298:In Vitro Techniques; D008564:Membrane Potentials; D009119:Muscle Contraction / Q000187:drug effects; D009132:Muscles / Q000187:drug effects / Q000502:physiology; D013324:Strontium / Q000494:pharmacology; D001468:Thoracica / Q000502:physiology*
chemical_list: D001464:Barium; D002118:Calcium; D013324:Strontium