Differential effect of baroreceptor reflexes and clonidine on frequency components of sympathetic discharge

The effects of the baroreceptor reflexes and clonidine on the 3 and 10 Hz components of spontaneously occurring renal sympathetic nervous discharge (SND) were studied in the cat. The proportion of 3 : 10 Hz SND was assessed by selective filtering and integration of activity in the 2-4 Hz and 9-11 Hz frequency bands. Baroreceptor denervation increased activity in both frequency bands while significantly lowering the 3 : 10 Hz ratio. Baroreceptor reflex activation (i.v. phenylephrine) decreased activity in both frequency bands while significantly increasing the 3 : 10 ratio. Clonidine (4-20 mug/kg, i.v.) affected the 3 : 10 Hz ratio of SND in a manner similar to baroreceptor reflex activation. These results indicate that the 3 and 10 Hz components of SND, both of which have been previously shown to be inherent to the central vasomotor system, are differentially affected by the baroreceptor reflexes and clonidine. It is concluded that it is possible to characterize certain cardiovascular states with respect to the 3 : 10 Hz ratio of SND.