Plasma dopamine beta-hydroxylase (DBH) activity, noradrenaline tissue levels and blood pressure were monitored in the pithed rat following electrical stimulation of the complete sympathetic outflow of the spinal cord. Stimulation at 10 or 30 Hz evoked marked increases in mean blood pressure which averaged 121 mmHg and were abolished by phenoxybenzamine. 2. Stimulation for 1 hr at 10 or 30 Hz did not change the noradrenaline content of the spleen nor the catecholamine content of the adrenals, but the heart noradrenaline content was doubled. 3. Plasma DBH activity was increased by 27% after 1 hr stimulation at 30 Hz, but remained unchanged after stimulation at 10 Hz, or at 30 Hz in phenoxybenzamine-treated rats. 4. We conclude that the pressor responses evoked by sympathetic nerve stimulation are due to the release of noradrenaline probably from adrenergic nerve terminals supplying the entire vasculature, and that acute alterations of circulating DBH activity are not dependent on the rate of catecholamine release evoked by direct electrical stimulation of sympathetic nerves in the whole pithed rat. The rat seems not to be a good model to study circulating noradrenaline and DBH levels as an index of exocytotic noradrenaline release from adrenergic neurones, and therefore of sympathetic activity.
