Inhibition of neuronal noradrenaline transport (uptake1) and desipramine binding by amiloride and ethylisopropylamiloride.

The diuretic amiloride and its N-5 substituted analogue ethylisopropylamiloride (EIPA) inhibit both the specific high affinity desipramine binding to isolated plasma membranes of PC12 rat phaeochromocytoma cells and the carrier-mediated neuronal uptake of noradrenaline into PC12 cells. The inhibition by EIPA of both desipramine binding (Ki = 5.6 mumol/l) and noradrenaline uptake (Ki = 24 mumol/l) inversely depend on the extracellular sodium concentration. The degree of inhibition increased with decreasing sodium concentration. A more detailed analysis of the mode of interaction revealed a competitive interaction between EIPA and desipramine binding but an "uncompetitive" interaction between EIPA and noradrenaline uptake. EIPA is the first inhibitor of uptake1 known so far, which reduces both Km and Vmax of neuronal noradrenaline transport. Extracellular alkalinization from pH 7.4 to 7.9 during incubation with EIPA markedly increased the effects on the kinetics of noradrenaline transport. A model has been proposed to explain the kinetic phenomena. It is based on the hypothesis that EIPA diffuses through the plasma membrane and binds to the inward facing sodium binding site of the neuronal noradrenaline carrier.