1. ¹Department of Anaesthesiology and Intensive Care Medicine, Justus-Liebig-University, 35392 Giessen, Germany
2. ²Department of Physiology, Justus-Liebig-University, 35392 Giessen, Germany
3. ³Instituto de Biologia Molecular e Celular (IBMC), 4150-180 Porto, Portugal

Correspondence: Boris V Safronov, Instituto de Biologia Molecular e Celular (IBMC), Rua do Campo Alegre 823, 4150-180 Porto, Portugal. E-mail: safronov@ibmc.up.pt

Received 28 December 2001; Revised 1 March 2002; Accepted 9 April 2002.

Abstract

1. Combining the patch-clamp recordings in slice preparation with the 'entire soma isolation' method we studied action of several local anaesthetics on delayed-rectifier K⁺ currents in spinal dorsal horn neurones.
2. Bupivacaine, lidocaine and mepivacaine at low concentrations (1–100 M) enhanced delayed-rectifier K⁺ current in intact neurones within the spinal cord slice, while exhibiting a partial blocking effect at higher concentrations (>100 M). In isolated somata 0.1–10 M bupivacaine enhanced delayed-rectifier K⁺ current by shifting its steady-state activation characteristic and the voltage-dependence of the activation time constant to more negative potentials by 10–20 mV.
3. Detailed analysis has revealed that bupivacaine also increased the maximum delayed-rectifier K⁺ conductance by changing the open probability, rather than the unitary conductance, of the channel.
4. It is concluded that local anaesthetics show a dual effect on delayed-rectifier K⁺ currents by potentiating them at low concentrations and partially suppressing at high concentrations. The phenomenon observed demonstrated the complex action of local anaesthetics during spinal and epidural anaesthesia, which is not restricted to a suppression of Na⁺ conductance only.