Vol. 305, Issue 2, 646-652, May 2003

Iberiotoxin-Induced Block of Ca²⁺-Activated K⁺ Channels Induces Dihydropyridine Sensitivity of ACh Release from Mammalian Motor Nerve Terminals

Michael T. Flink and William D. Atchison

Department of Pharmacology and Toxicology, Michigan State University, East Lansing, Michigan

The role which Ca²⁺-activated K⁺ (K_Ca) channels play in regulating acetylcholine (ACh) release was examined at mouse motor nerve terminals. In particular, the ability of the antagonist iberiotoxin to recruit normally silent L-type Ca²⁺ channels to participate in nerve-evoked release was examined using conventional intracellular electrophysiological techniques. Incubation of cut hemidiaphragm preparations with 10 µM nimodipine, a dihydropyridine L-type Ca²⁺ channel antagonist, had no significant effect on quantal content of end-plate potentials. Nevertheless, 1 µM S-()-1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl]phenyl)-3-pyridine carboxylic acid methyl ester (Bay K 8644) enhanced quantal content to 134.7 ± 3.5% of control. Iberiotoxin (150 nM) increased quantal content to 177.5 ± 9.9% of control, whereas iberiotoxin plus nimodipine increased quantal content to only 145.7 ± 10.4% of control. Coapplication of 1 µM Bay K 8644 with iberiotoxin did not significantly increase quantal content further than did treatment with iberiotoxin alone. The effects of iberiotoxin and nimodipine alone or in combination on the miniature end-plate potential (MEPP) frequency following KCl-induced depolarization were examined using uncut hemidiaphragm preparations. Nimodipine alone had no effect on MEPP frequency from preparations incubated in physiological saline containing 5 to 20 mM KCl. Moreover, iberiotoxin alone or combined with nimodipine also had no effect on MEPP frequency in physiological salines containing 5 to 15 mM KCl. At 20 mM KCl, however, iberiotoxin significantly increased MEPP frequency to 125.6% of iberiotoxin-free values; combined treatment with nimodipine and iberiotoxin prevented this increase in MEPP frequency. Thus, loss of functional K_Ca channels unmasks normally silent L-type Ca²⁺ channels to participate in ACh release from motor nerve terminals, particularly under conditions of intense nerve terminal depolarization.