ISOLATION OF MYOCARDIAL L-TYPE CALCIUM-CHANNEL GATING CURRENTS WITH THE SPIDER TOXIN OMEGA-AGA-IIIA

The peptide omega-agatoxin-IIIA (omega-Aga-IIIA) blocks ionic current through L-type Ca channels in guinea pig atrial cells without affectin g the associated gating currents. omega-Aga-IIIA permits the study of L-type Ca channel ionic and gating currents under nearly identical ion ic conditions. Under conditions that isolate L-type Ca channel current s, omega-Aga-IIIA blocks all ionic current during a test pulse and aft er repolarization. This block reveals intramembrane charge movements o f equal magnitude and opposite sign at the beginning of the pulse (Q(o n)) and after repolarization (Q(off)). Q(on), and Q(off) are suppresse d by 1 mu M felodipine, saturate with increasing test potential, and a re insensitive to Cd. The decay of the transient current associated wi th Q(on) is composed of fast and slow exponential components. The slow component has a time constant similar to that for activation of L-typ e Ca channel ionic current, over a broad voltage range. The current as sociated with Q(off) decays monoexponentially and more slowly than ion ic current. Similar charge movements are found in guinea pig tracheal myocytes, which lack Na channels arid T-type Ca channels. The kinetic and pharmacological properties of Q(on) and Q(off) indicate that they reflect gating currents associated with L-type Ca channels. omega-Aga- IIIA has no effect on gating currents when ionic current is eliminated by stepping to the reversal potential for Ca or by Cd block. Gating c urrents constitute a significant component of total current when physi ological concentrations of Ca are present and they obscure the activat ion and deactivation of L-type Ca channels. By using omega-Aga-IIIA, w e resolve the entire time course of L-type Ca channel ionic and gating currents. We also show that L- and T-type Ca channel ionic currents c an be accurately quantified by tail current analysis once gating curre nts are taken into account.