Dissociation between the effect of nitrous oxide on spontaneous and reflexly evoked sympathetic activity in dogs

We have examined the effect of nitrous oxide on spontaneous sympathetic activity and A delta- and C-fibre mediated somatosympathetic reflexes in renal nerves, evoked by supramaximal electrical stimulation of radial nerves in anaesthetized, paralysed dogs undergoing mechanical ventilation. In six preparations, nitrous oxide was administered at end-tidal concentrations of 10%, 30%, 50% and 70%, each for 20 min. Spontaneous renal sympathetic activity increased significantly to 147.8% and 151.2% of control values with 50% and 70% nitrous oxide, respectively (P < 0.05), but there were no significant changes in A delta and C reflexes. We conclude that the large increase in spontaneous sympathetic activity was dissociated from somatosympathetic reflexes which remained unchanged at these concentrations of nitrous oxide.