The in vivo dopamine-β-hydroxylase (DBH) activity in peripheral tissues of the whole animal and the in vitro DBH activity in serum, adrenal glands, peripheral sympathetically innervated tissue, and brain were examined in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) at 12 weeks of age before and after daily immobilization stress. In vitro DBH activity did not differ significantly between SHR and WKY before stress, but in vivo DBH activity in SHR was significantly lower than that in WKY. After repeated immobilization stress, in vivo DBH activity was significantly elevated only in SHR but not in WKY. In vitro DBH activity in serum, adrenal gland, vas deferens, and brain was also significantly increased in SHR, whereas that in WKY was increased only in adrenal glands. These results suggest that SHR at 12 weeks of age, when hypertension is established, may have even lower sympathetic nerve activity than WKY, and that after repeated stress the response of the sympatho-adrenal system is higher in SHR than in WKY.
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