Cardiovascular responses to intravenously infused tyramine and noradrenaline were measured in five normal subjects, five insulin-dependent diabetics and five insulin-dependent diabetics with autonomic neuropathy. Tyramine infusion produced a statistically significant increase in systolic blood pressure (BP) in the autonomic neuropaths only (p less than 0.001). No change occurred in diastolic BP. Noradrenaline infusion produced a statistically significant increase in systolic BP in the normal subjects (p less than 0.01) and in the autonomic neuropaths (p less than 0.001). The increase in systolic BP in the neuropaths was significantly greater (p less than 0.001) than in normal subjects. Diastolic BP rose significantly only in the normal subjects (p less than 0.05). There was no change in heart rate in response to either agent. Thus super-sensitivity to noradrenaline occurred in patients with diabetic autonomic neuropathy indicating post-denervation hypersensitivity. Tyramine hypersensitivity also occurred indicating that denervation is not complete and suggesting dysfunction at a pre-synaptic level.
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