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Novel Pharmacological Sensitivity of the Presynaptic Calcium Channels Controlling Acetylcholine Release in Skate Electric Organ
Author(s) -
Richardson C. Mark,
Dowdall Michael J.,
Green A. Chris,
Bowman David
Publication year - 1995
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1046/j.1471-4159.1995.64020944.x
Subject(s) - electric organ , acetylcholine , skate , calcium , sensitivity (control systems) , neuroscience , chemistry , biology , pharmacology , acetylcholine receptor , fishery , biochemistry , engineering , torpedo , receptor , organic chemistry , electronic engineering
The presynaptic terminals of skate ( Raja montagui ) electric organ were tested for their sensitivity to calcium channel antagonists. Acetylcholine (ACh) release and the elevation of intraterminal Ca 2+ concentrations triggered by K + depolarisation were studied. ACh release was measured as 3 H efflux from slices of organ prelabelled with [ 3 H]choline. Depolarisation caused a marked, Ca 2+ ‐dependent increase in 3 H efflux that was completely blocked by 100 µ M Cd 2+ and by 300 n M ω‐conotoxin‐MVIIC (MVIIC). Inhibition by MVIIC was concentration dependent (IC 50 of ∼20 n M ) and reversible. No inhibition was seen with nifedipine (5 µ M ) or the two other peptide antagonists studied: ω‐conotoxin‐GVIA (GVIA) at 5 µ M and ω‐agatoxin‐IVA (Aga‐IVA) at 1 µ M . In a “nerve plate” preparation (a presynaptic plexus of nerve fibres, Schwann cells, and nerve terminals) changes in intraterminal Ca 2+ concentrations were measured by microfluorimetry using fluo‐3. An increase in fluorescence, indicating a rise in the free [Ca 2+ ], rapidly followed K + depolarisation, and this change was restricted to the nerve terminals. This response was insensitive to nifedipine (5 µ M ), GVIA (5 µ M ), and Aga‐IVA (300 n M ) but almost completely abolished by MVIIC (1 µ M ). MVIIC inhibition was concentration dependent and partially reversible. These results show that the nerve terminals in skate electric organ have calcium channels with a pharmacological sensitivity that is markedly different from the established L, N, and P types in other systems but shares some, but not all, of the features of the recently described Q type.