Effect of breathing helium oxygen on autonomic function

Following a recent suggestion that the anti arrhythmic effect of breathing helium oxygen (He O₂) is mediated by changes in sympathetic activity, the effect of He O₂ (80:20) on autonomic function was investigated. The unanesthetized, unrestrained rat with indwelling cannulae, responded to pressor doses of norepinephrine (NE), tyramine and dimethylphenylpiperazinium in a similar fashion whether in an environment of He O₂ at 31°C or nitrogen oxygen (N₂ O₂; 80:20) at 26°C ambient temperature. Again using the same rat as its own control, it was found that He O₂ did not alter the fall in arterial pressure which followed a 45° taildown tilt in animals anesthetized with pentobarbital. Likewise, He O₂ had no effect on the response of the nictitating membrane to preganglionic stimulation in the cat anesthetized with pentobarbital. Finally the effect of He O₂ was examined on the cardiac turnover of ³H L NE. Rats were injected with ³H L NE (33.3 μCi/kg; tail vein) and 20 rats were exposed to He O₂ at 31δC while another group of 20 rats was exposed to N₂ O₂ at 26°C. Rats were killed at 2, 6, 12 and 23 hr after injection and the levels of both endogenous and labelled NE determined; the absolute level of NE and the rate of decline of tritiated amine were similar in both groups. It was concluded that He O₂ does not affect autonomic nervous function in the species studied.