Interactions of thiopentone, light mineral oil, and pressure at frog sciatic nerve

Combinations of thiopentone, light mineral oil, and pressure to as much as 160 ATA were applied to the isolated, thermostated (18 degrees C) frog sciatic nerve. Amplitude and rise rate of the compound action potential were examined and compared to the response in Ringer's solution. Immersion in mineral oil caused no change in the response. Pressure, whether applied by helium or after immersion in mineral oil, caused mainly reversible slowing of the response, as did thiopentone by itself, up to at least 5.1 mM. Thiopentone (3.2mM) and helium pressure up to 114 ATA were reversibly synergistic, in that the response was markedly slowed and reduced. Nerves that had been immersed in 3.2 mM thiopentone were placed in mineral oil; this procedure caused a reduction in the size of the response. Application of pressure then eliminated the response, and decompression did not lead to its recovery until the nerve was repeatedly rinsed in Ringer's solution. The findings question the inertness of mineral oil to excitable membranes and indicate that caution should be exercised in its use as a compression medium.