Modulation of voltage-gated potassium channels in human T lymphocytes by extracellular glutamate

Glutamate is present in the plasma under tightly regulated concentrations. However, under conditions of immune deficiency, such as AIDS and malignancy, its plasma levels are highly elevated. In vitro, glutamate interacts with T lymphocytes, affecting mitogen-induced calcium responses, whereas at high doses, it impairs T lymphocyte proliferation, a process strongly dependent on the activity of voltage-gated potassium channels. In this study, we demonstrate novel dose-related effects of the endogenous ligand glutamate and its metabotropic and non-N-methyl-D-aspartic acid receptor agonists on the electrophysiological properties of native Kv1.3 channels of human T lymphocytes. Glutamate, at concentrations within normal plasma levels, positively modulates Kv1.3 channel gating, causing currents to activate faster and at significantly more hyperpolarized potentials, hence rendering the T lymphocyte readily responsive to immune stimuli. This effect is maximal at 1 microM Glu and is fully mimicked by a 100 microM concentration of the metabotropic receptor agonist trans-(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid. Most importantly, Glu, at concentrations > or =100 microM, which in vitro produce suppression of mitogen-induced proliferation, significantly decreases whole-cell potassium currents by increasing current and steady-state inactivation. This effect saturates at 1000 microM and seems to result from the subsequent activation of low-affinity metabotropic Glu receptors, as suggested by specific agonist data. Therefore, the antiproliferative effects of high glutamate may, at least in part, result from its inhibitory effect on the potassium current, suggesting an in vivo immunosuppressive role of elevated plasma glutamate.