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Last Updated: 2001-08-06 15:53:38 EDT (Reuters Health)

By David Douglas

WESTPORT, CT (Reuters Health) – Steroid-naive asthmatics show an increased level of nitric oxide (NO) exhalation following inhalation of both L-arginine and D-arginine, British researchers report in the August issue of Thorax.

Drs. Jon G. Ayres and D. C. Chambers, of Birmingham Heartlands NHS Trust, note that NO is found in both normal and asthmatic airways. Administration of L-arginine, the substrate for the enzyme nitric oxide synthase, increases airway NO production in asthmatics.

However, the researchers suggest that NO production in these circumstances "may be at least partially mediated through a nonenzymatic route." If this happens, they speculate, D-arginine administration should also "lead to an increase in respiratory tract NO production."

To investigate, the team studied eight steroid-naive asthmatics, who on separate occasions were randomized to receive 2.5 g L-arginine, 2.5 g D-arginine, or 2.0% saline by ultrasonic nebulizer.

L-arginine caused a maximum drop in lung function (FEV1) of 11.9%. The corresponding figure for D-arginine was 10.0% and for saline 8.5%. Exhaled NO declined in proportion to the degree of bronchoconstriction. This resolved within 15 minutes.

However, the investigators were unable to determine whether the temporary reduction in exhaled NO was "causal or merely associated with bronchoconstriction."

Nevertheless, compared with placebo, arginine administration caused an increase in exhaled NO. After exposure to L-arginine, the mean exhaled NO concentration was 15.75, ppb; after D-arginine, 15.16 ppb; and after saline, 12.74 ppb.

Commenting on the findings, Dr. Ayres told Reuters Health that it appears that "not all exhaled NO is from activity of the enzyme NO synthase, but that some comes from chemical reactions in the fluid which lines the respiratory tract."

Arginine may react directly with airway hydrogen peroxide to form NO, the investigators conclude, and changes in airway chemistry may modulate this release "with potential pathophysiological consequences."

Thorax 2001;56:602-606.

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