DESENSITIZATION BY NEUROPEPTIDE-Y OF EFFECTS OF SYMPATHETIC-STIMULATION ON CARDIAC VAGAL ACTION IN ANESTHETIZED DOGS

The effects of a long-lasting intravenous infusion of neuropeptide Y ( NPY, 180 +/- 8 min, 53 +/- 4 mug/kg/h) on the prolonged inhibition of cardiac vagal action evoked by cardiac sympathetic nerve stimulation a nd bolus intravenous injections of NPY were investigated in anaestheti sed dogs. Sympathetic stimulation and NPY injection were performed on four separate occasions: once in control conditions, then once early a nd again late in the period of NPY infusion, and then on a final occas ion 60-90 min after the cessation of NPY infusion. The maximum inhibit ion of cardiac vagal action evoked by an injection of NPY was signific antly less late in the NPY infusion when compared with the other three injection groups (ANOVA, P < 0.001). Also the time to half-recovery o f this response was significantly less than that seen in the other thr ee injection groups (ANOVA, P < 0.001). The maximum inhibition of card iac vagal action evoked by sympathetic stimulation was significantly r educed late in the NPY infusion when compared with the other three sti mulation groups (ANOVA, P < 0.0001). The time for half-recovery of thi s response was also less than that of the other three stimulation grou ps (ANOVA, P < 0.001). The results indicate that desensitisation of th e vagal attenuation to both exogenous NPY and sympathetic stimulation occurred during a long-lasting period of NPY infusion. This is consist ent with the proposal that NPY is a mediator of this sympathetic-evoke d vagal attenuation.