Interaction between catecholamines and neuropeptides in the carotid body: evidence for dopamine modulation of neutral endopeptidase activity.

Carotid body (CB) contains multiple neurochemicals that include catecholamines (CA) and neuropeptides. They are involved in the modulation of sensory response of the carotid body. Based on observations from the central nervous system, we hypothesized that CA modulates neuropeptide metabolism in CB. To test our hypothesis, fetal calf carotid body model was used. Immunocytochemical analysis showed that fetal calf carotid body expresses both tyrosine hydroxylase, and neutral endopeptidase-like immunoreactivity. To assess the effect of CA, thin slices of fetal calf carotid body were incubated with 50-500 microM of dopamine (DA) at 37 degrees C for 1 hour. As an index of neuropeptide metabolism, the activity of neutral endopeptidase (NEP), a major degrading enzyme of neuropeptides in CB was determined in the membrane-enriched and soluble fractions of the carotid body. CBs incubated with medium lacking DA served as control. On average, NEP activities of the membrane-enriched, and soluble fractions of the untreated CB were 4.8 +/- 0.2 and 6.7 +/- 0.2 pmole per hour per mg of CB respectively. At concentrations less than 200 microM, DA enhanced NEP activity of the membrane fraction (approximately 60%) whereas inhibition of NEP was observed in the soluble fraction (approximately 62%). At concentrations > 200 microM, DA inhibited NEP activity of the two fractions. When CBs were incubated with DA in the presence of sodium dithionite, an oxygen scavenger, DA, even at higher concentrations, stimulated NEP activity of the membrane-enriched fraction. The above results demonstrate that DA modulates neuropeptide metabolism in CB via a non-receptor-mediated mechanism involving a direct interaction with NEP.