Calcium and lanthanum effects at the nodal membrane.

Abstract

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 A2 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.