BLOCK OF CURRENT THROUGH T-TYPE CALCIUM CHANNELS BY TRIVALENT METAL-CATIONS AND NICKEL IN NEURAL RAT AND HUMAN-CELLS

1. The effects of the trivalent cations yttrium (Y3+), lanthanum (La3+), cerium (Ce3+), neodymium (Nd3+), gadolinium (Gd3+), holmium (Ho3+), erbium (Er3+), ytterbium (Yb3+) and the divalent cation nickel (Ni2+) on the T-type voltage gated calcium channel (VGCC) were characterized by the whole-cell patch clamp technique using rat and human thyroid C cell lines. 2. All the metal cations (M3+) studied, blocked current through T-type VGCC (I(T)) in a concentration-dependent manner. Smaller trivalents were the best T-channel antagonists and potency varied inversely with ionic radii for the larger M3+ ions. Estimation of half-maximal blocking concentrations (IC50s) for I(T) carried by 10 mM Ca2+ resulted in the following potency sequence: Ho3+ (IC50 = 0.107 muM) almost-equal-to Y3+ (0.117) almost-equal-to Yb3+ (0.124) greater-than-or-equal-to Er3+ (0.153) > Gd3+ (0.267) > Nd3+ (0.429) > Ce3+ (0.728) > La3+ (1.015) much greater than Ni2+ (5.65). 3. Tail current measurements and conditioning protocols were used to study the influence of membrane voltage on the potency of these antagonists. Block of I(T) by Ni2+, Y3+, La3+ and the lanthanides was voltage independent in the range from -200 to +80 mV. In addition, the antagonists did not affect macroscopic inactivation and deactivation of T-type VGCC. 4. Increasing the extracellular Ca2+ concentration reduced the potency of I(T) block by Ho3+, indicative of competitive antagonism between this blocker and the permeant ion for a binding site. 5. The results suggest that the mechanism of metal cation block of T-type VGCC is occlusion of the channel pore by the antagonist binding to a Ca2+/M3+ binding site, located out of the membrane electric field. 6. Block of T-type VGCC by Y3+, lanthanides and La3+ differ from the inhibition of high voltage-activated VGCC block in several respects: smaller cations are more potent I(T) antagonists; block is voltage independent and the antagonists do not permeate T-type channels. These differences suggest corresponding structural dissimilarities in the permeation pathways of low and high voltage-activated Ca2+ channels.