Effect on the Guinea-pig Taenia Coli of the Substitution of Strontium or Barium Ions for Calcium Ions

Abstract

THE spontaneous discharge of the taenia coli of the guinea-pig is not inhibited by even a high concentration of tetrodotoxin (5 × 10⁻⁶ g/ml.), but it is blocked by a low concentration of ionic manganese (0.5 mM)¹, which is known to be a specific inhibitor of the calcium spikes in crustacean muscle² and also in barnacle muscle fibres³. These facts suggest that in the smooth muscle of taenia the membrane current during the action potential is carried principally by calcium ions. This would explain earlier findings that spike generation continues in solutions which are deficient in or free from sodium^4–6 and that spike height is almost independent of the external concentration of sodium ions⁶. Both the spontaneous activity and the potassium contracture are depressed by several transition metal ions such as manganese, nickel, cobalt or iron. The contraction induced by acetylcholine in normal Locke solution or in potassium-rich medium is also blocked by these divalent ions. The block is competitively antagonized by raising the external concentration of calcium, which supports the view that the entry of calcium ions across the membrane acts as a trigger for contraction⁷.