ISSN:
1432-1912
Keywords:
Enzyme Induction
;
Cold-Stress
;
Tyrosine Hydroxylase
;
Dopamine β-Hydroxylase
;
Dopa Decarboxylase
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Summary Cold-exposure of rats for 1–4 days leads to a gradual increase in tyrosine hydroxylase and dopamine β-hydroxylase activity in superior cervical ganglia and adrenals. This increase is neurally mediated, since it can be abolished by decentralization of the ganglia and transsection of the splanchnic fibres supplying the adrenal medulla. Enzyme kinetic data suggest that the increased enzyme activity results from an increased synthesis of enzyme protein. The activity of dopa decarboxylase, the third enzyme involved in the synthesis of norepinephrine, remains unchanged both in ganglia and in adrenals. In the adrenals, the gradual rise in tyrosine hydroxylase and dopamine β-hydroxylase activity is virtually parallel and reaches more than twice the control values. In the superior cervical ganglion, however, there is a marked difference between the rise in activity of the two enzymes. The increase in tyrosine hydroxylase activity amounts to 217% after 4 days of cold-exposure, that of dopamine β-hydroxylase to 131% of controls (100%) only. The determination of enzyme turnovers by measuring the decay in activity after inhibition of protein synthesis by cycloheximide was attempted but the turnover of tyrosine hydroxylase both in adrenals and superior cervical ganglia was too slow to be determined by this method. The same was true for dopamine β-hydroxylase in the adrenal medulla. However, in the superior cervical ganglion the turnover of the latter enzyme could be determined; the half-life amounts to 13 h. The fact that the turnover of dopamine β-hydroxylase in the superior cervical ganglion is more rapid than that of tyrosine hydroxylase could explain the difference in the increase in enzyme activity in spite of a similar rise in the rate of synthesis. It is concluded that the enzymes involved in the synthesis of norepinephrine are not regulated as an operational unit, but that the results are compatible with the assumption that the synthesis of the two enzymes, tyrosine hydroxylase and dopamine β-hydroxylase, which are specifically located in adrenergic neurons, may be regulated by a common mechanism.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00997085
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