Influence of sympathetic discharge pattern on norepinephrine and neuropeptide Y release

The effects of sympathetic nerve stimulation on vasoconstrictor responses and overflow of norepinephrine (NE) and neuropeptide Y-like immunoreactivity (NPY-LI) were studied in the dog gracilis muscle and pig spleen in vivo. A continuous regular impulse activity was compared with irregular human sympathetic and regular bursting patterns. During control conditions, stimulation with the irregular activity induced larger peak vasoconstriction than the regular activity at 0.59 Hz, but not at higher frequencies in the muscle, at 0.59 and 2.0 Hz in the spleen. The nerve stimulation-evoked overflow of NE and NPY-LI from the muscle were not influenced by the pattern of stimulation. The overflow of NPY-LI, but not that of NE, from the spleen was enhanced by the irregular activity at 0.59 and 2.0 Hz, and both NPY-LI and NE overflows were enhanced by regular burst activity at 2.0 Hz. After blockade of alpha- and beta-adrenoceptors by phenoxybenzamine and propranolol, respectively, which enhanced nerve stimulation-evoked overflow of both NE and NPY-LI, the NE overflow from the muscle evoked by the irregular activity was slightly larger at 0.59 Hz but smaller at higher frequencies compared with that evoked by regular activity, whereas the detectable overflow of NPY-LI was not largely influenced by the stimulation pattern. In conclusion, both the vasoconstrictor response and the overflow of NPY-LI and NE seem to be influenced by the pattern and frequency of sympathetic nerve stimulation.