Norepinephrine transporter regulation by reserpine and potassium chloride stimulation: Role of intracellular signaling cascades
The norepinephrine transporter (NET) is present on the plasma membrane of noradrenergic neurons and is responsible for norepinephrine (NE) reuptake. The NET is under intrinsic regulation by cellular processes that affect concentrations of intracellular NE, by receptors, and by kinase-linked pathways. Clarification of these pathways may implicate candidate proteins for pharmacological targeting that could ultimately lead to the development of novel therapeutic agents. The overall goal of this research was to elucidate the potential role of intracellular NE stores in regulating NET function and the second messenger systems that are recruited in this process. Three different procedures for reducing intracellular NE were studied, NE synthesis inhibition, vesicular monoamine transporter (VMAT) inhibition, and repeated KC1 stimulation. NET function was assessed in two cell lines, rat PC12 and human SK-N-SH cells. Treatment of cells with a tyrosine hydroxylase inhibitor to inhibit NE synthesis did not affect NET function. In contrast, treatment with a VMAT inhibitor (reserpine) decreased NE uptake in both cell lines. KCl stimulation increased NE uptake in PC12 cells, but had no effect on SK-N-SH cells. Experiments demonstrated that reserpine did not alter the plasma membrane surface expression of the NET, but failed to identify an intracellular mediator of the effect of reserpine on NET function, leaving the possibility that reserpine directly binds the NET at a novel and as yet unknown site on the protein. The KCl stimulated increase in NE uptake was dependent on Ca2+ and was blocked by inhibitors of Ca 2+-calmodulin dependent kinase and myosin light chain kinase. The increase in NE uptake following KCl stimulation was not accompanied by a change in the surface expression of the transporter. Given that the three NE depleting procedures had different effects on NET function, the findings do not support the hypothesis that NET function is regulated to support intracellular stores of NE. Different degrees of NE depletion produced by each method and/or direct binding to the NET and interference with its function could have contributed to the different effects on NET function.