Abstract
The role which Ca2+-activated K+(KCa) 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 μMS-(−)-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 KCa 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.
Footnotes
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This work was submitted by M.T.F. in partial fulfillment of the requirements for the Ph.D. degree in Pharmacology and Toxicology as part of the combined degree Medical Scientist Training Program in the College of Osteopathic Medicine at Michigan State University. A portion of these results was presented at the 2000 Annual Meeting of the Society for Neuroscience (New Orleans, LA) on November 4 to 9, 2000 and published in abstract form in Soc Neurosci Abstr26:88. Supported by National Institutes of Health Grant ES05822 (Bethesda, MD) and a Viets Fellowship from the Myasthenia Gravis Foundation to Michael T. Flink.
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DOI: 10.1124/jpet.102.046102
- Abbreviations:
- ACh
- acetylcholine
- KCa
- calcium-activated potassium
- LEMS
- Lambert-Eaton myasthenic syndrome
- MEPP
- miniature end-plate potential
- DAP
- 3,4-diaminopyridine
- EPP
- end-plate potential
- BSA
- bovine serum albumin
- Cav
- voltage-activated calcium channel
- DHP
- dihydropyridine
- Received October 28, 2002.
- Accepted January 21, 2003.
- The American Society for Pharmacology and Experimental Therapeutics
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