Post-tetanic changes of depressor responses evoked by stimulation of the aortic nerve.

The effect of changes in duration of the conditioning tetanus on the size of the testing depressor response was studied in rabbits anaesthetized with urethane. Depressor responses were evoked by stimulation of the aortic nerve. The interval between the conditioning and testing stimulation was fixed at 40 and 120 sec. Two frequencies of conditioning tetanus were employed. Brief conditioning tetani facilitate the testing response. With lengthening of the conditioning stimulation the size of the testing response is decreased and when duration of the conditioning amounts to 20-60 sec depression of the testing fall of blood pressure reaches a steady level. Further increase in duration of the conditioning tetanus to 180 sec does not affect the plateau of depression of the testing response. The longest duration of conditioning affecting the size of the testing response is considered to determine the range of control of the testing response exerted by preceding conditioning tetanus. Since the levels of plateau of depression are different for two used frequencies of conditioning, it is suggested that this factor may control the size of the testing response beyond the range of control executed by the duration of the conditioning tetanus.