Catecholamine synthesis in rabbit carotid body in vitro.

1. Catecholamine synthesis in rabbit carotid body was studied in vitro using [(3)H]DOPA and [(3)H]tyrosine as precursors. The effects of sympathectomy and transection of the carotid sinus nerve on [(3)H]dopamine ([(3)H]DA) and [(3)H]noradrenaline ([(3)H]NA) synthesis were investigated in chronically denervated carotid bodies.2. When [(3)H]DOPA was used as precursor, the synthesis of [(3)H]DA was linear for more than 6 hr. The carotid body synthesized larger amounts of [(3)H]catecholamines than when [(3)H]tyrosine was used as precursor, but most of this excess was liberated into the incubation media. Using 10 muM-[(3)H]DOPA as precursor, the synthesis rates were 6.76 and 1.51 n-mole/g per hr for [(3)H]DA and [(3)H]NA, respectively; with 40 muM-[(3)H]DOPA, these values increased to 19.22 and 3.23 n-mole/g per hr, respectively.3. The relationship between [(3)H]DOPA concentration and [(3)H]DA synthesis was linear throughout the range 5-40 muM-[(3)H]DOPA.4. Sympathectomy reduced the synthesis of [(3)H]NA by 90% and [(3)H]DA by 37% when [(3)H]DOPA was used as precursor.5. When [(3)H]tyrosine (40 muM) was used as precursor, synthesis of [(3)H]catecholamines was linear for at least 4 hr, with rates of 12.10 and 0.85 n-mole/g per hr for [(3)H]DA and [(3)H]NA, respectively.6. [(3)H]DA and [(3)H]NA synthesis from [(3)H]tyrosine exhibited the characteristics of saturable processes, with K(m) values of 16.8 and 17.6 muM, respectively.7. 6-methyltetrahydropterine (6-MPH(4), 100 muM), a synthetic analogue of the natural co-factor for tyrosine hydroxylase, increased [(3)H]DA and [(3)H]NA synthesis from [(3) H]tyrosine in both the carotid body and superior cervical ganglion, with the greatest effect seen in the carotid body.8. When [(3)H]tyrosine was used as precursor, sympathectomy of the carotid body reduced [(3)H]NA synthesis by 80%, but did not alter [(3)H]DA or [(3)H]tyrosine levels in the tissue. Transection of the carotid sinus nerve had no effect on [(3)H]catecholamine synthesis in the carotid body.