REPEATED RENAL AND SPLENIC SYMPATHETIC-NERVE STIMULATION IN ANESTHETIZED PIGS - MAINTAINED OVERFLOW OF NEUROPEPTIDE-Y IN CONTROLS BUT NOT AFTER RESERPINE

The overflow and the arterial vascular effects of neuropeptide Y (NPY) in response to repeated sympathetic nerve stimulation of kidney and s pleen were investigated in anaesthetized pigs. The responses under con trol conditions were compared to those evoked in pigs with tissue stor es of noradrenaline (NA) selectively depleted by reserpine pretreatmen t combined with sympathetic nerve transection. The renal and splenic s ympathetic nerves were repeatedly stimulated at 1 h intervals with one 5 Hz stimulation for 48 s and transmitter overflow determined. Betwee n these stimulations, 5 min stimulations with bursts of 20 Hz (for 1 s every 10 s) were given in order to induce a depletion of nerve transm itter. In the control group, overflow of NPY and NA and vasoconstricto r responses were almost identical for the 5 consecutive stimulations i n the kidney, whereas in the spleen the parameters showed a slight ten dency to be reduced. In the reserpine-treated group, the initial evoke d overflow of NPY was increased 8-fold and 3-fold in the kidney and sp leen, respectively, compared to the control group. Upon each subsequen t stimulation the overflow decreased gradually, in parallel with the e voked vasoconstrictor response. After a 2 h recovery period no change in evoked overflow of NPY compared to the amount released by the previ ous stimulation was observed. The present study illustrates, the high capacity of maintenance of not only NA but also NPY overflow and vascu lar responses in control conditions, whereas the enhanced release of N PY in the absence of NA cannot be maintained. It is therefore possible that the NA-mediated prejunctional feedback mechanism is important fo r the maintenance of a constant NPY release in situations of high symp athetic activation.