The specific sodium and potassium permeability mechanisms in the nodal membrane were analysed. The current to potential relations were measured with various external concentrations of NaC1, KC1 and CaC1 2 . P Na and P K were calculated. The measurements showed that the permeability to potential curve was shifted along the potential axis and in the same direction for changes in the uni‐univalent salt concentration as well as for changes in [Ca]. The shift caused by changes in [Ca] was larger at low uni‐univalent salt concentrations and the shift caused by [K] and [Na] increased with decreased [Ca]. It was assumed that the membrane has negative charges fixed to its outside surface. The permeability shifts were approximately predicted by a charge density of‐5.5 μC μ cm ‐2 . A decrease in [Na], but not in [Ca] caused a decrease in P K . This behaviour was not predicted.
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Tom Brismar (1973) studied this question.