Changes in Sympathetic Nerve Terminals in the Heart of Cold‐Exposed Rats

Changes in sympathetic nerve terminals of the heart after varying periods of exposure of rats to 4 degrees C were investigated. Two indices were used for changes in the number of noradrenaline storage vesicles, i.e., vesicular dopamine beta-hydroxylase (DBH) activity and noradrenaline storage capacity. The latter was obtained after uptake of [3H]noradrenaline; endogenous content, uptake of exogenous noradrenaline, and degree of saturation of the vesicles were calculated using the specific activity of the [3H]noradrenaline. As a measure of tyrosine hydroxylase activity, whole ventricular noradrenaline, dopamine, and dihydroxyphenylacetic acid content were used. After 4 h of cold exposure there was an increase in vesicular endogenous noradrenaline content, uptake, storage capacity, and DBH activity as well as a large increase in whole ventricular dopamine. After 6 h in the cold, vesicular endogenous noradrenaline content, storage capacity, and DBH activity were decreased. The results suggest that during cold exposure there is an initial increase followed by a decrease in the number of functional vesicles in the nerve terminal, which could explain the fluctuations in the rate of noradrenaline release.