EFFECTS OF PATTERNS OF SYMPATHETIC-NERVE STIMULATION ON VASOCONSTRICTING RESPONSES IN THE HINDQUARTER OF RABBITS

It is now well known that sympathetic nerve discharges (SND) of animals as well as humans oscillate at low frequencies. To determine effects of the oscillation or burst on vasoconstriction, we applied two different kinds of electrical stimulation of the lumbar sympathetic nerve, and examined the magnitude and rate of vasoconstriction in the autoperfused hindquarter of alpha-chloralose anesthetized rabbits (n = 6). In the first protocol, we obtained power spectra of lumbar SND of rabbits with sinoaortic denervation and vagotomy. The power resided over the frequency range of 0.5-5 Hz with a broad peak at 1 Hz. In the second protocol, we modulated the basal stimulus trains 5 Hz on an average with slower rhythms of 0.5, 1.0 and 2.0 Hz. (frequency modulation). This experiment revealed that, compared with the results with constant stimulation, the frequency modulation of stimulation did not affect the magnitude of the maximal vasoconstriction but augmented the rate of vasoconstriction at 0.5 and 1.0 Hz (P < 0.01). In the third protocol, we examined effects of stimulation on vasoconstriction while changing the intra-burst frequency at a fixed inter-burst interval. Since the power spectra of lumbar SND showed a peak at 1 Hz in the first protocol, we fixed the inter-burst interval at 1 Hz and varied the intra-burst frequency at 10, 20, and 40 Hz while the total number of stimuli were kept constant at 5 impulses per second. This experiment showed that the 10 Hz burst was most effective in augmenting the rate of vasoconstriction, though the magnitude of maximal vasoconstriction was not affected by any of them. These results may suggest that oscillation and burst of SND may work as an accelerator of vasoconstriction in resistance vessels.