Effect of changing the frequency of conditioning tetanus on depressor responses evoked by stimulation of the aortic nerve.

The effect of changes in frequency of the conditioning tetanus on the magnitude of the testing depressor response was studied in rabbits anaesthetized with urethane. Conditioning and testing stimulations were applied to the same aortic nerve. The duration of the conditioning tetani was set at 3 and 60 sec and the interval between stimulations amounted to 40 and 120 sec. At the testing interval of 40 sec the increase in frequency both of short and long conditioning tetani reduces the magnitude of the testing response which attains a minimum at frequency of about 30 cycles/sec. Conditioning stimulations of higher frequency are gradually less effective and cause the testing response to increase. Similar depression is observed at the testing interval of 120 sec but only following long-lasting conditioning tetanus. Short conditioning trains at the testing interval of 120 sec facilitate the testing response. The frequency of the conditioning stimulation which produces the greatest reduction of the depressor response indicates the range of control exerted by the conditioning tetanus over the testing blood pressure effect. The size of this control is determined by the lowest level of depression and the highest value of facilitation of the testing response.