Effects of N-ethylmaleimide on 5-hydroxytryptamine transport and sodium content in rabbit platelets.

1. The present study analysed the mechanism underlying the inhibitory action of N-ethylmaleimide (NEM) on the 5-hydroxytryptamine (5-HT) uptake by blood platelets. 2. Rabbit platelets suspended in protein-free buffer were first preincubated for 45 min in the absence and presence of NEM (20 to 160 microM) or ouabain (0.5-2.0 microM) and then either analysed for their Na+ and K+ content or incubated (15s) with various concentrations of [3H]-5-HT (0.13-4.03 microM) to determine Km and Vmax for 5-HT uptake. 3. Both NEM and ouabain produced concentration-dependent decreases in Vmax with IC50 values of 52 and 0.58 microM, respectively. Neither drug changed Km significantly. 4. Both NEM and ouabain increased the Na+ and decreased the K+ content of platelets in a concentration-dependent manner. 5. There was a linear correlation between Vmax (expressed in % of control) and the reciprocal cellular Na+ content, with the results for both drugs falling onto one and the same regression line (r = 0.992; n = 8). This regression showed that an increase in Na+ content by 69% sufficed to reduce Vmax by 50%. 6. At concentrations that reduced 5-HT uptake by about 60%, neither NEM nor ouabain altered the potency of imipramine for inhibition of 5-HT uptake. 7. Hence, NEM inhibits 5-HT transport by inhibiting the Na+/K+-ATPase and not by a direct interaction with the 5-HT carrier. The consequential increase in the intracellular Na+ concentration reduces the transmembrane Na+ gradient and, therefore, hinders 5-HT inward transport. This action of the drug does not affect the ability of the carrier to bind 5-HT or imipramine.