1. Twelve male diabetic patients inhaled nebulized histamine in doubling concentrations from 0.03 mg/ml to 32 mg/ml until they reached the maximum concentration or until their forced expiratory volume in 1 s (FEV1.0) fell by at least 20%. Six had evidence of severe autonomic neuropathy (DAN), while the other six did not.

2. More of the DAN group decreased their FEV1.0 on histamine inhalation by at least 20% (P < 0.02) and more of them decreased their maximal flow at 50% vital capacity by at least 20% than those in the group without DAN (P < 0.05).

3. The fall in FEV1.0 and in maximal flow at 50% of vital capacity was greater (P < 0.05) in the DAN group than in the group without DAN.

4. This suggests that diabetic patients with autonomic neuropathy have increased bronchial reactivity to histamine. This could either be due to differential damage of the respiratory autonomic nerves or, alternatively, to denervation hypersensitivity.

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