Effects of laser-induced hyperthermia treatment on ionic permeability of myelinated nerve

The effect of laser-induced hyperthermia on the ionic permeability of nerve membranes was studied using the nodes of Ranvier in amphibian myelinated nerve as a model. To effect a photothermal modification of nerve membrane functions, controlled laser irradiation consisting of a 5-sec thermal pulse was applied to the nodal membrane, increasing the temperature to a maximum of 48-58 degrees C at the node. Major electrophysiological changes observed in the nodal membrane following laser-induced hyperthermia were a differential reduction of the sodium and potassium permeability, an increase in the leakage current, and a negative shift on the potential axis of the steady-state Na inactivation. There was no significant change in the kinetics of ion channel activation and inactivation for treatments below 56 degrees C. The results suggest that a primary photothermal damage mechanism at temperatures below 56 degrees C could be a reduction in the number of active Na channels in the node, rather than a change in individual channel kinetics, or in the properties of the lipid bilayer of intervening nerve membrane. A differential heat sensitivity between the noninactivated and the inactivated Na channels is also suggested. For the treatments of 56 degrees C and above, a significant increase of membrane leakage current suggests an irreversible thermal damage to the lipid bilayer.