Inhibition by local anesthetics of Ca2+ channels in rat anterior pituitary cells

The characteristics of local anesthetic inhibition of voltage-dependent Ca2+ channels in a rat pituitary clonal cell line were investigated by whole-cell voltage clamp and compared with inhibition by the dihydropyridine Ca2+ channel antagonist, nicardipine. With extracellular Ba2+ (10 mM) as the current carrier, depolarization above -40 mV evoked a slowly inactivating I-Ba. Extracellularly applied Lidocaine inhibited I-Ba without changing the activation threshold, the voltage of peak current, or the reversal potential. Inhibition was greater at a holding potential of -60 mV (IC50 = 1.2 mM) than at -80 mV (IC50 = 2.6 mM). This depolarization-induced potentiation in I-Ba inhibition developed over 0.1-10 a after membrane depolarization began. Nicardipine also dose-dependently inhibited I-Ba with an IC50 = 90 nM (at a holding potential = - 80 mV). Both lidocaine and nicardipine shifted the I-Ba steady-state inactivation (availability) curves to the left. Double-pulse protocols revealed that lidocaine (1 mM) accelerated the depolarization-induced inhibition (inactivation) of I-Ba over the rate in drug-free solutions, but had no effect on the hyperpolarization-induced removal of channel inactivation. Nicardipine also accelerated the depolarization-induced inactivation of I-Ba but, in addition, it slowed the hyperpolarization-induced inactivation removal. The relative inhibitory action of lidocaine in suppressing I-Ba was unchanged in the presence of nicardipine. These results suggest that lidocaine has a direct action on membrane Ca2+ channels, similar to the voltage-dependent action of dihydropyridine, but acting at a separate and independent site. (C) 1998 Elsevier Science B.V. All rights reserved.