Maintained depressor responses evoked by continuous and intermittent stimulation of aortic nerve in the rabbit

Stability of depressor responses evoked by long-lasting continuous and intermittent stimulation of the aortic nerve was studied in rabbits anaesthetized with urethane. Continuous stimulation produces blood pressure falls whose stability at low frequencies (1-10 cycles/sec) ranges from 91 to 86%. With rise of the stimulation frequency stability is decreased : at 500 cycles/sec, it amounts to 19%. Intermittent stimulation consisting in switching excitation on and off every 10 sec increases stability of depressor responses and at 500 cycles/sec, it is significantly higher than stability of effects produced by continuous stimulation. Following transection of aortic nerves, stability is increased at all frequencies of continuous stimulation and at all but the lowest frequency of intermittent stimulation. Vagotomy performed after section of the aortic nerves does not significantly affect the changes in stability observed after severing the aortic afferents. It is suggested that at high frequencies of stimulation, stability of depressor responses is reduced by homosynaptic depression. During intermittent stimulation, its effect is counteracted by post-tetanic changes occurring at intervals when the stimulation is switched off. The increase in stability after section of aortic nerves is probably related to its effect on excitability of the vasomotor centres.