Iberiotoxin-induced block of Ca2+-activated K+ channels induces dihydropyridine sensitivity of ACh release from mammalian motor nerve terminals

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 muM nimodipine, a dihydropyridine L-type Ca2+ channel antagonist, had no significant effect on quantal content of end-plate potentials. Nevertheless, 1 muM 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 muM 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 iberio-toxin-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.