Calcium and lanthanum effects at the nodal membrane

Summary

• 1. Voltage clamp experiments on single myelinated nerve fibres ofXenopus were done in Ringer solutions, in which the normal Ca concentration (2 mM) was substituted by 10 mM Ca²⁺, 0.1 mM La³⁺ or 0.5 mM La³⁺.

• 2. The activation and the inactivation curves of the Na permeability were shifted by 11 and 8 mV in positive direction on the potential axis by an e-fold increase in concentration of either Ca²⁺ or La³⁺.

• 3. These shifts were plotted versus the logarithm of the Ca or La concentration and could be fitted by straight lines under the assumption that 1 mM La³⁺ is equivalent to 55 mM Ca²⁺.

• 4. The activation curve of the K permeability was shifted 6 or 14 mV by an e-fold change in Ca or La concentration.

• 5. In high Ca and in the La solutions the maximum Na and K permeability and the leakage conductance were reduced.

• 6. Ringer solution with 0.1 mM Tb³⁺ exerted an even stronger effect than 0.1 mM La³⁺.

• 7. It is assumed that the observed shifts reflect a change in membrane surface potential due to electrostatic screening by the cations of the external solution. On this basis a negative fixed charge density of approximately 1/70 Ų is calculated for the vicinity of the Na channel; a lower density appears to apply near the K channel.

• 8. As some of the La effects cannot be interpreted in terms of screening or binding and are different from the Ca effects, La seems to be a potent Ca substituent with some specific effects in addition.