Effects of extracellular pH on the interaction of sipatrigine and lamotrigine with high-voltage-activated (HVA) calcium channels in dissociated neurones of rat cortex

Acidic extracellular pH reduced high-voltage-activated (HVA) currents in fr eshly isolated cortical pyramidal neurones of adult rats, shifting activati on to more positive voltages (V-1/2=-18 mV at pH 7.4, -11 mV at pH 6.4). Si patrigine inhibited HVA currents, with decreasing potency at acidic pH (IC5 0 8 muM at pH 7.4, 19 muM at pH 6.4) but the degree of maximal inhibition w as >80% in all cases (pH 6.4-8.0). Sipatrigine has two basic groups (pK(A) values 4.2, 7.7) and at pH 7.4 is 68% in monovalent cationic form and 32% u ncharged. From simple binding theory, the pH dependence of sipatrigine inhi bition indicates a protonated group with pK(A) 6.6. Sipatrigine (50 muM) sh ifted the voltage dependence of channel activation at pH 7.4 (-7.6 mV shift ) but not at pH 6.4. Lamotrigine has one basic site (pK(A) 5.5) and inhibit ed 34% of the HVA current, with similar potency over the pH range 6.4-7.4 ( IC50 7.5-9 muM). These data suggest that the sipatrigine binding site on HV A calcium channels binds both cationic and neutral forms of sipatrigine, in teracts with a group with pK(A)=6.6 and with the channel activation process , and differs from that for lamotrigine. Crown Copyright (C) 2001 Published by Elsevier Science Ltd. All rights reserved.