Effect of drugs on the synthesis of noradrenaline in guinea‐pig vas deferens

Abstract

1 . Reserpine in vitro (10⁻⁵m) caused a profound inhibition (>85%) of the formation of both ¹⁴C-catecholamine (¹⁴C-CA) and ¹⁴C-dihydroxyphenylalanine (¹⁴C-DOPA) (in the presence of the amino acid decarboxylase inhibitor brocresine) from ¹⁴C-tyrosine in guinea-pig vas deferens. The magnitude of the inhibition was similar for both ¹⁴C-CA and ¹⁴C-DOPA suggesting that the inhibition occurred primarily at the tyrosine hydroxylase step. 2 . One hour after in vivo treatment with reserpine (1 mg/kg) when tissue stores of noradrenaline (NA) were depleted by 50%, there was a significant inhibition of the formation of ¹⁴C-DOPA. Twenty-four hours after such treatment, when endogenous NA could no longer be detected, synthesis of ¹⁴C-DOPA was indistinguishable from untreated controls. However a 45% inhibition of ¹⁴C-DOPA synthesis from ¹⁴C-tyrosine could be produced in tissues which had been depleted of NA for 24 h or 48 h by the addition of reserpine, 10⁻⁵m, to the incubation medium. 3 . Addition of pteridine cofactor, 2-amino-6,7,-dimethyl-4-hydroxy-5,6,7,8-tetrahydropteridine, to the incubation medium in a concentration of 5 × 10⁻³m enhanced the formation of both ¹⁴C-CA and ¹⁴C-DOPA from ¹⁴C-tyrosine in guinea-pig vas deferens. In 52 mm KCl Krebs-Henseleit medium ¹⁴C-CA formation increased from 2·58 ± 0·20 (nmol/g)/h to 6·35 ± 0·47 (nmol/g)/h whilst ¹⁴C-DOPA formation increased from 5·04 ± 0·88 (nmol/g)/h to 11·29 ± 0·59 (nmol/g)/h. 4 . Pteridine cofactor (5 × 10⁻³m) did not reverse the inhibition of ¹⁴C-DOPA formation seen with reserpine (10⁻⁵m) in previously untreated tissues or in vasa deferentia from animals pretreated with reserpine 1 mg/kg for 24 hours. However, the inhibition did disappear in the presence of pteridine cofactor when treatment with reserpine was prolonged to 48 h and included two doses of reserpine of 2 mg/kg. 5 . Tyramine (5·8 × 10⁻⁵m) and bretylium (10⁻⁵m) in vitro inhibited the formation of ¹⁴C-CA and ¹⁴C-DOPA from ¹⁴C-tyrosine to the same extent in guinea-pig vas deferens again indicating that their major site of action is on tyrosine hydroxylase. The inhibitory effects were reversed by pteridine cofactor. 6 . Synthesis of ¹⁴C-NA from ¹⁴C-tyrosine in calf splenic nerve was not increased by incubating the tissue in 52 mm KCl-Krebs-Henseleit solution.