Evaluation of 5-hydroxytryptamine concentration in portal vein measured by microdialysis

Free 5-hydroxytryptamine (5-HT) concentration in the portal vein was monitored in anesthetized rats using microdialysis. The 5-hydroxytryptamine metabolite 5-hydroxyindoleacetic acid (5-HIAA) was also measured. Concentration of 5-HT in the portal vein decreased gradually after insertion of a microdialysis probe but fluctuated markedly throughout the entire perfusion, whereas 5-HIAA values did not fluctuated in the same manner. An electron microscopic study of the probe membrane after dialysis showed platelet adhesion and aggregation on part of the membrane. In contrast, 5-HT concentration measured by puncture and direct blood sampling of the portal vein did not fluctuate in the same way as that determined by microdialysis. These results suggest that measurement of free 5-HT concentration in the blood by microdialysis might be inaccurate due to contamination of 5-HT from adhesion and aggregation of platelets on the probe membrane.