The effects of thyroxine treatment of rats on neuronal and extraneuronal uptake and metabolism of catecholamines in the heart

The effects of thyroxine (T) treatment of rats on the neuronal and extraneuronal uptake and subsequent metabolism of catecholamines in the heart were examined, to determine whether changes in the local dissipation of catecholamines might contribute to the enhanced sympathetic cardiac responses that occur when thyroid hormone levels are elevated. T‐treated rats were injected subcutaneously with L‐thyroxine sodium 1 mg kg on days 1, 3 and 5, and controls were injected with the normal saline vehicle on the same days. The experiments on isolated, perfused hearts were carried out on day 8. The T‐treated rats had only 50% of the growth rate of the controls and their heart weights were 18% greater than the controls. The experimental data were adjusted to allow for the increase in heart weight caused by the T treatment. The initial rates of neuronal uptake of noradrenaline and of extraneuronal uptake of noradrenaline and isoprenaline in the hearts from T‐treated rats were not significantly different from those in hearts from control rats. The steady‐state rates of extraneuronal O‐methylation of isoprenaline and of extraneuronal deamination of noradrenaline in hearts from T‐treated rats were not significantly different from those in hearts from control rats. The steady‐state rate of neuronal deamination of noradrenaline was significantly lower and the accumulation of unmetabolized noradrenaline in the hearts was significantly greater in T‐treated rats than in the controls. These findings could be explained by a decrease in neuronal monoamine oxidase activity or by an increase in intraneuronal binding of noradrenaline in hearts from T‐treated rats. If the first of these two speculations should be correct, the changes in neuronal dissipation could contribute to cardiac supersensitivity to noradrenaline or adrenaline, since these amines are substrates for neuronal uptake. However, it could not explain cardiac supersensitivity to isoprenaline in T‐treated rats. The study has shown that it is unlikely that increased plasma thyroid hormone levels cause cardiac supersensitivity to catecholamines by affecting the local dissipation processes for those amines in the heart. 1986 British Pharmacological Societ