Summary Isolated rat hepatocytes were incubated with 0.05 mol/l or 0.2 mol/l
3H-(–)-noradrenaline or 0.05 mol/l
3H-(–)-adrenaline for 15 min and the content of amines as well as the formation of metabolites was measured.The removal Of both amines from the incubation medium was quantitatively similar, and mainly due to metabolism (which represented 96% of the removal of
3H-adrenaline and 98% of the removal of
3H-noradrenaline). O-methylation predominated for
3H-adrenaline: O-methylated and deaminated metabolites (
3H-OMDA) and
3H-metanephrine (
3H-MN) were the most abundant metabolites, accounting for 63% and 34% of total metabolite formation, respectively. Deamination predominated for
3H-noradrenaline:
3H-OMDA and
3H-dihydroxymandelic acid (
3H-DOMA) were the most abundant metabolites, representing respectively 56% and 36% of total metabolite formation. The following activities of monoamine oxidase and catechol-O-methyl transferase were determined for
3H-noradrenaline: k
COMT 0.70±0.15 min
–1 and k
MAO 2.27±0.14 min
–1 In experiments with
3H-noradrenaline, inhibition of monoamine oxidase reduced the formation of
3H-OMDA and deaminated metabolites [
3H-dihydroxyphenylglycol (
3H-DOPEG) and
3H-DOMA] and increased the formation of
3H-normetanephrine (
3H-NMN). Inhibition of catechol-O-methyl transferase, On the Other hand, decreased
3H-NMN and increased
3H-DOPEG formation. When both enzymes were inhibited, the formation of all metabolites was strongly reduced but surprisingly there was no accumulation of
3H-amines in the cells, as the cell: medium ratio for
3H-noradrenaline or
3H-adrenaline was about unity. In experiments with either
3H-noradrenaline or
3H-adrenaline, specific inhibitors of either uptake, or uptake
2 produced discrete effects, slightly decreasing the formation of
3H-OMDA and
3H-NMN or
3H-MN, and having no effect on
3H-amine content of the cells. Additional experiments were carried Out with rat liver slices incubated for 15 min with
3H-noradrenaline 0.2 mol/l. The pattern of metabolism of
3H-noradrenaline (
3H-OMDA and
3H-DOMA were the most abundant metabolites) as well as the degree of metabolism of the amine removed from the incubation medium (91% of the removal) were similar to those of the isolated cells. Likewise, there was no accumulation of intact
3H-noradrenaline in the tissue. Moreover, the results obtained with enzyme inhibitors as wells as with uptake inhibitors were similar to those obtained with hepatocytes.In conclusion, isolated hepatocytes remove and metabolize catecholamines very efficiently, being one of the most active systems studied in this respect. Uptake
1 and uptake
2 are responsible for part of the removal of catecholamines by hepatocytes; the system(s) involved in the remaining removal seem(s) to be active, but possess(es) characteristics that do not allow us to characterize it (them) either as uptake
1 or uptake
2.Abbreviations COMT
catechol-O-methyl transferase
- DOMA
3,4-dihydroxymandelic acid
- DOPEG
3,4-dihydroxyphenylglycol
- HEPES
4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
- MAO
monoamine oxidase
- MN
metanephrine
- NMN
normetanephrine
- OMDA
O-methylated and deaminated metabolites (i.e., MOPEG = 4hydroxy-3-methoxyphenylglycol and VMA = 4-hydroxy-3-methoxymandelic acid)
Supported by Programa STRIDE (STRDA/P/SAU/259/92)PhD student with a grant from JNICT (Programa Ciência)
Correspondence to: F. Martel at the above address
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