The effects of ketamine on renal sympathetic nerve activity and phrenic nerve activity in rabbits (with vagotomy) with and without afferent inputs from peripheral receptors.

One reason for the reported conflicting results of the effect of ketamine on hemodynamics and respiration may be variations in afferent inputs from peripheral receptors to the central nervous system. In order to evaluate unmasked direct effects of ketamine on sympathetic nerve and phrenic nerve outflow, totally deafferented (involving vagus, sinus nerve, aortic depressor nerve) rabbits (n = 18), rabbits with vagotomy (n = 21), and neuraxis-intact rabbits (n = 6) were used in this study. The animals were anesthetized with urethane and mechanically ventilated. Ketamine 0.5, 1, or 2 mg/kg was injected intravenously and mean arterial pressure (MAP), heart rate (HR), and integrated renal sympathetic nerve and phrenic nerve activity (IRSNA, IPNA) were recorded before, and 1, 2, 3, 5, and 10 min after injection. MAP and IRSNA were significantly decreased, even by the smallest dose of ketamine, in the totally deafferented group. IPNA was decreased by the largest dose of ketamine only in the totally deafferented group. On the other hand, spontaneous respiratory frequency was decreased in the totally deafferented and vagotomy groups, but more so in the totally deafferented group. In the neuraxis-intact group, the only significant change with the largest dose of ketamine, 2 mg/kg was a slight increase in HR. We conclude that ketamine can suppress vasomotor and respiratory centers directly, and that the suppression is counterbalanced by afferent inputs from peripheral receptors.