Sensitivities of rat primary sensory afferent nerves to magnesium: implications for differential nerve blocks

Contrasting findings have been published regarding the role of magnesium sulphate used as an additive to local anaesthetics in peripheral nerve blocks.

To clarify the effect of magnesium sulphate on nerve excitability.

C and Aβ compound action potentials were recorded extracellularly in vitro in saphenous nerves from adult rats.

Saphenous nerves (n = 30) from male Wistar rats (n = 19), 12 to 16 weeks old.

Primary sensory afferents were tested with a computerised threshold tracking program (QTRAC) with a supramaximal 1 ms current pulse either alone or after 300 ms of conditioning polarising ramp currents in the presence and absence of 10 mmol l magnesium sulphate, 80 μmol l lidocaine and a combination of both.

Changes in current thresholds to elicit compound action potential amplitudes of 40% of the maximal response.

Magnesium sulphate increased excitability thresholds to a greater extent in Aβ fibres than in C fibres. It enhanced the effects of lidocaine in both Aβ fibres [mixture 0.470 mA (SD 0.105) versus lidocaine 0.358 mA (SD 0.080), P < 0.001] and C fibres [mixture 2.531 mA (SD 0.752) versus lidocaine 2.385 mA (SD 0.656), P = 0.008]. Preconditioning experiments also showed that magnesium sulphate had an enhancing effect with lidocaine in Aβ fibres [mixture 0.620 mA (SD 0.281) versus lidocaine 0.543 mA (SD 0.315), P = 0.005], but not in C fibres [mixture 2.412 mA (SD 0.641), lidocaine 2.461 mA (SD 0.693), P = 0.17].

These results suggest that the binding of magnesium ions depends on both the type and conformational state of voltage-gated sodium channels. They also may help to explain the conflicting reports regarding the clinical effects of magnesium sulphate as an additive to lidocaine in peripheral nerve blocks.