ALAPROCLATE EFFECTS ON VOLTAGE-DEPENDENT K- STUDIES IN CULTURED RAT HIPPOCAMPAL-NEURONS AND FIBROBLAST CELLS TRANSFORMED WITH KV1.2 K+ CHANNEL CDNA( CHANNELS AND NMDA RECEPTORS )

The effects of alaproclate on voltage-dependent K+ currents and N-meth yl-D-aspartate (NMDA) and gamma-aminobutyric acid(A) (GABA(A)) recepto r currents were investigated in cultured rat hippocampal neurons using whole-cell voltage clamp recording techniques. Alaproclate produced a concentration-dependent block of the sustained voltage-dependent K+ c urrent activated by depolarization from -60 to +40mV (IC50, 6.9 mu M). At similar concentrations alaproclate also blocked the sustained volt age-dependent K+ current in fibroblast cells transformed to stably exp ress Kv1.2 K+ channels. Analysis of tail currents and the voltage-depe ndence of the alaproclate block suggested an open-channel blocking mec hanism. Alaproclate also produced a potent block of NMDA receptor curr ents in hippocampal neurons (IC50, 1.1 mu M), but did not affect GABA( A) receptor currents (concentrations up to 100 mu M). The alaproclate block of NMDA receptors occurred predominantly by an open-channel mech anism, although the drug was also able to block closed NMDA channels a t a much slower rate. The interaction of alaproclate with NMDA recepto rs (activated by 10 mu M NMDA) appeared to be governed by a first orde r binding reaction with forward and reverse rate constants of 6.7 x 10 (3) M(-1) s(-1), and 0.025 sec(-1), respectively (at -60 mV). At depol arized potentials the alaproclate-induced block of the NMDA receptor c urrent was strongly reduced, a result opposite to that seen with the v oltage-activated K+ currents, suggesting that the K+ channel block may occur at a superficial internal site, whereas the NMDA receptor block occurs at a deep external site. (+)-Alaproclate was a more potent blo cker of K+ currents than (-)-alaproclate, whereas a reversed stereosel ectivity was observed for NMDA receptor current, supporting the view t hat alaproclate block of the two channel types occurs at structurally distinct binding sites.