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 microM at pH 7.4, 19 microM 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 (pK(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 pK(A) 6.6. Sipatrigine (50 microM) 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 (pK(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 microM). 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 pK(A)=6.6 and with the channel activation process, and differs from that for lamotrigine.
Hainsworth
A H
AH
Section of Pharmacology, School of Pharmacy, De Montfort University, Leicester LE1 9BH, UK.
Spadoni
F
F
Lavaroni
F
F
Bernardi
G
G
Stefani
A
A
D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects* / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D000077213:Lamotrigine; D008297:Male; D019579:Neocortex / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000187:drug effects* / Q000502:physiology; D014179:Neurotransmitter Uptake Inhibitors / Q000494:pharmacology*; D010879:Piperazines / Q000494:pharmacology*; D011743:Pyrimidines / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D014227:Triazines / Q000494:pharmacology*
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D014179:Neurotransmitter Uptake Inhibitors; D010879:Piperazines; D011743:Pyrimidines; D014227:Triazines; C081980:sipatrigine; D000077213:Lamotrigine