Calcineurin-independent inhibition of the delayed rectifier K+ current by the immunosuppressant FK506 in rat hippocampal neurons

The immunosuppressant drug FK506 was found to be a potent neuroprotective agent in animal models of brain ischemia. However, the mechanisms underlying the action remain to be elucidated. The delayed rectifier K+ channel has been implicated in ischemic injury and neuronal death in the brain. The aim of the present study is to investigate whether the neuroprotective action of FK506 results from blocking the K+ channel. In acutely dissociated CA1 pyramidal neurons of rat hippocampus, superfusion of FK506 (0.01–100 μM) selectively inhibited the delayed rectifier K+ current (IK) with an IC50 value of 13.2 ± 4.9 μM. The inhibition of IK by FK506 (10 μM) had a rapid onset, and then gradually reached a steady-state level. The inhibition was voltage-dependent, became more potent when the currents were elicited by strong depolarization. Moreover, FK506 (10 μM) caused marked negative shifts of the steady-state activation and inactivation curves of IK, and accelerated its recovery from inactivation. Intracellular dialysis of FK506 (30 μM) was ineffective. The inhibition of IK by FK506 (10 μM) persisted under the low-Ca2+ conditions that blocked the basal activity of protein phosphatase 2B (calcineurin). Rapamycin did not antagonize FK506 but mimicked it. Cyclosporin A inhibited IK only at 30 and 100 μM. Taken together, the results suggest that FK506 exert a direct inhibition on the delayed rectifier K+ channel without involvement of calcineurin.