Role of chloride ions in cardiac action and pacemaker potentials

¹ Instituto de Biofisica, Universidade do Brasil, Rio de Janeiro, Brasil

Evidence is presented that chloride ions are able to carry an appreciable amount of electric charge through the membrane of atrial muscle fibers, Purkinje fibers, and fibers of the S-A nodal pacemaker. An increase in the slope of diastolic depolarization of pacemaker fibers was recorded when Cl^– was replaced by larger anions (sulfate) in the external medium. The rate of repolarization decreased when larger anions were substituted for chloride ions, and it was increased in nitrate or bromide solutions. The anionic permeability of the cell membrane of S-A node, atrial muscle, and Purkinje fibers seems to follow the series: NO₃^– > Br^– > Cl^– > CH₃COO^– > SO₄⁼ Evidence is presented that chloride ions contribute to diastolic depolarization of pacemaker fibers. The K⁺ electrode properties of resting membrane of fibers of the S-A node were investigated at low and at constant extracellular Cl^– concentration. It was found that above 16 mm K₂SO₄ there is agreement between the resting potential and E_k. Below this K⁺ concentration a deviation of the resting potential from the line for a K⁺ electrode was observed. Determinations of the Cl^– content and of volume changes support the idea that the membrane of fibers of the S-A node and atrial muscle is permeable to Cl^– and to K⁺ ions.

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