1. Using venous occlusion plethysmography, we have investigated the forearm blood flow response in healthy subjects to the acute plasma volume expansion caused by a rapid intravenous infusion of saline. The contribution made to this response by nitric oxide has been investigated using local intra-arterial infusions of the nitric oxide synthase inhibitor NG-monomethyl-l-arginine.

2. The infusion of 1000 ml of saline over 25 min caused plasma volume to increase by about 7%, and resulted in a rise in forearm blood flow, with no change in arterial blood pressure. The onset of the blood flow response occurred within 10 min and blood flow remained elevated above baseline 20 mm after the end of the saline infusion.

3. Local intra-arterial infusion of NG-monomethyl-l-arginine alone caused a reduction in forearm blood flow which was maximal at the end of the infusion and gradually recovered to baseline levels over 40 min.

4. When local intra-arterial infusion of NG-monomethyl-l-arginine was followed by plasma volume expansion, the calculated effect of NG-monomethyl-l-arginine was such as to abolish the vasodilator response to saline.

5. The effect of local intra-arterial infusion of NG-monomethyl-l-arginine on forearm blood flow was greater when the drug was given after volume expansion had occurred, than when it was given before the administration of saline. However, in control experiments the vasoconstrictor response to noradrenaline was also enhanced after the administration of the volume load in comparison with the response to noradrenaline given alone.

6. These results are consistent with the possibility that increased local synthesis of nitric oxide contributes to the vasodilator response to volume expansion.

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