Catecholamine synthesis in rabbit carotid body in vitro

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Abstract
Catecholamine synthesis in rabbit carotid body was studied in vitro using [3H]dopa and [3H]tyrosine as precursors. The effects of sympathectomy and transection of the carotid sinus nerve on [3H]dopamine ([3H]DA) and [3H]noradrenaline [norepinephrine] ([3H]NA) synthesis were investigated in chronically denervated carotid bodies. When [3H]dopa was used as precursor, the synthesis of [3H]DA was linear for > 6 h. The carotid body synthesized larger amounts of [3H]catecholamines than when [3H]tyrosine was used as precursor, but most of this excess was liberated into the incubation media. Using 10 .mu.M-[3H]dopa as precursor, the synthesis rates were 6.76 and 1.51 nmol/g per h for [3H]DA and [3H]NA, respectively; with 40 .mu.M-[3H]dopa, these values increased to 19.22 and 3.23 nmol/g per h, respectively. The relationship between [3H]dopa concentration and [3H]DA synthesis was linear throughout the range 5-40 .mu.M-[3H]dopa. Sympathectomy reduced the synthesis of [3H]NA by 90% and [3H]DA by 37% when [3H]dopa was used as precursor. When [3H]tyrosine (40 .mu.M) was used as precursor, synthesis of [3H]catecholamines was linear for at least 4 h, with rates of 12.10 and 0.85 nmol/g per h for [3H]DA and [3H]NA, respectively. [3H]DA and [3H]NA synthesis from [3H]tyrosine exhibited the characteristics of saturable processes, with Km values of 16.8 and 17.6 .mu.M, respectively. 6-Methyltetrahydropterin (6-MPH4, 100 .mu.M), a synthetic analog of the natural cofactor for tyrosine hydroxylase, increased [3H]DA and [3H]NA synthesis from [3H]tyrosine in both the carotid body and superior cervical ganglion, with the greatest effect seen in the carotid body. When [3H]tyrosine was used as precursor, sympathectomy of the carotid body reduced [3H]NA synthesis by 80%, but did not alter [3H]DA or [3H]tyrosine levels in the tissue. Transection of the carotid sinus nerve had no effect on [3H]catecholamine synthesis in the carotid body.