Involvement of substance P in neutral endopeptidase modulation of carotid body sensory responses to hypoxia

Previously we showed that carotid bodies express neutral endopeptidase (NEP )-like enzyme activity and that phosphoramidon, a potent inhibitor of NEP, potentiates the chemosensory response of the carotid body to hypo;da in viv o. NEP has been shown to hydrolyze methionine enkephalin (Met-Enk) and subs tance P (SP) in neuronal tissues. The purpose of the present study is to de termine whether NEP hydrolyzes Met-Enk and SP in the carotid body and if so whether these peptides contribute to phosphoramidon-induced potentiation o f the sensory response to hypoxia. Experiments were performed on carotid bo dies excised from anesthetized adult cats (n = 72 carotid bodies). The hydr olysis of Met-Enk and SP was analyzed by HPLC. The results showed that both SP and Met-Enk were hydrolyzed by the carotid body, but the rate of Met-En k hydrolysis was approximately fourfold higher than that of SP. Phosphorami don (400 mu M) markedly inhibited SP hydrolysis (similar to 90%) but had on ly a marginal effect on Met-Enk hydrolysis (similar to 15% inhibition). Hyp oxia (PO2, 68 +/- 6 Torr) as well as exogenous administration of SP (10 and 20 nmol) increased the sensory discharge of the carotid body in vitro. Sen sory responses to hypoxia and SP (10 nmol) were potentiated by similar to 8 0 and similar to 275%, respectively (P < 0.01), in the presence of phosphor amidon. SP-receptor antagonists Spantide (peptidyl) and CP-96345 (nonpeptid yl) either abolished or markedly attenuated the phosphoramidon-induced pote ntiation of the sensory response of the carotid body to hypoxia as well as to SP. These results demonstrate that SP is a preferred substrate for NEP i n the carotid body and that SP is involved in the potentiation of the hypox ic response of the carotid body by phosphoramidon.