[Plasma tryptophan bioavailability during chronic urticaria]

Chronic idiopathic urticaria is known to have psychogenic component with a triggering or favoring effect. Different tests or evaluation scales have been unable to identify a specific psychological profile. Erythrocyte-specific membrane transport of tyrptophan (TRP), the main plasma precursor of cerebral serotonin synthesis, controls, by a erythrocyte-specific storage and release mechanism, circulating TRP homeostasis. Bioavailability of circulating TRP is a factor controlling serotonin synthesis in the brain. An evaluation of the rate of TRP transfer could be a biochemical approach to chronic urticaria more informative than psychological tests.

A kinetic study of L-TRP influx into circulating erythrocytes was conducted in 17 patients with chronic urticaria with no detectable cause and in 35 healthy controls. Blood samples were marked with 3H-TRP. Maximum L-TRP-specific influx (Vmax) was expressed in mumol/cell/min. The urticaria patients also underwent psychological testing to determine anxiety and depression scores using standardized scales (Hamilton).

Mean Vmax was not significantly difference between the two groups. Vmax values were quite similar in all the control subjects but showed wide dispersion in the urticaria group. Three subgroups were found in the urticaria patients depending on Vmax: those with Vmax equivalent in control levels (+2 SD), those with Vmax less then 2 SD (29% of the patients) and those with Vmax greater than 2 SD of control levels (23% of the patients). Thus more than 50% of the urticaria patients had perturbed erythrocyte-specific L-TRP influx. The anxiety and depression scores obtained from the psychological evaluation were not correlated with Vmax.

Erythrocyte-specific TRP membrane transport, evaluated by Vmax. Would not appear to be perturbed in chronic urticaria. Even though the urticaria patients could be divided into three groups according to their Vmax, the mean value was not significantly different from that in controls. These findings do not allow a conclusion concerning a perturbation of bioavailability of plasmatic TRP and any possible central serotoninergic dysfunction in chronic urticaria.