[Pressor effect of substance P applied to the pressor area of ventral surface of medulla oblongata and the underlying mechanism in rabbits].

Experiments were performed on urethane anesthetized and artificially ventilated rabbits. The results were as follows. Application of substance P (SP) to the pressor area of ventral surface of medulla oblongata (VSMp) elicited a dose-dependent increase in blood pressure (BP), but no significant change in heart rate. The pressor response of SP was blocked by pretreatment with SP receptor antagonist (DSP) in VSMp. Application of DSP alone to VSMp elicited a significant decrease in BP. The pressor response of SP was attenuated when VSMp was pretreated with phentolamine or prazosine, whereas pretreatment with yohimbin or propranolol was without effect. Application of SP to VSMp elicited an increase in renal sympathetic nerve discharge (RSND) in association with pressor response; both these responses were blocked by pretreatment with DSP in VSMp. Application of DSP alone to VSMp elicited a significant decrease in RSND and BP. The above results indicate that SP has a pressor effect in VSMp due to activation of SP receptor mediated by alpha 1-receptor. The pressor effect of SP may be mainly due to an increase of peripheral vascular resistance induced by the augmented sympathetic nerve activity. In summary, it appears that endogenous SP in VSMp plays an important role in maintaining sympathetic tone and BP level.