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 freshly isolated cortical pyramidal neurones of adult rats, shifting activation to more positive voltages ( V 1/2=−18 mV at pH 7.4, −11 mV at pH 6.4). Sipatrigine inhibited HVA currents, with decreasing potency at acidic pH (IC 50 8 μM at pH 7.4, 19 μM at pH 6.4) but the degree of maximal inhibition was >80% in all cases (pH 6.4–8.0). Sipatrigine has two basic groups (p K A values 4.2, 7.7) and at pH 7.4 is 68% in monovalent cationic form and 32% uncharged. From simple binding theory, the pH dependence of sipatrigine inhibition indicates a protonated group with p K A 6.6. Sipatrigine (50 μM) shifted 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 (p K A 5.5) and inhibited 34% of the HVA current, with similar potency over the pH range 6.4–7.4 (IC 50 7.5–9 μM). These data suggest that the sipatrigine binding site on HVA calcium channels binds both cationic and neutral forms of sipatrigine, interacts with a group with p K A=6.6 and with the channel activation process, and differs from that for lamotrigine.