Action potential afterdepolarization mediated by a Ca2+-activated cation conductance in myenteric AH neurons.
We investigated the nature of afterdepolarizing potentials in AH neurons from the guinea-pig duodenum using whole-cell patch-clamp recordings in intact myenteric ganglia. Afterdepolarizing potentials were minimally activated following action-potential firing under normal conditions, but after application of charybdotoxin (40 nM) or tetraethyl ammonium (TEA; 10-20 mM) to the bathing solution, prominent afterdepolarizing potentials followed action potentials. The whole-cell current underlying afterdepolarizing potentials (I(ADP)) in the presence of TEA (10-20 mM) reversed at -38 mV and was not voltage-dependent. Reduction of NaCl in the bathing (Krebs) solution to 58 mM shifted the reversal potential of the I(ADP) to -58 mV, suggesting that the current underlying the afterdepolarizing potential was carried by a mixture of cations. The relative contributions of Na(+) and K(+) to this current were estimated to be about 1:5. Substitution of external Na(+) with N-methyl D-glucamine blocked the current while replacement of internal Cl(-) with gluconate did not block the I(ADP). The I(ADP) was also inhibited when CsCl-filled patch pipettes were used. The I(ADP) was blocked or substantially decreased in amplitude in the presence of N-type Ca(2+) channel antagonists, omega-conotoxin GVIA and omega-conotoxin MVIIC, respectively, and was eliminated by external Cd(2+), indicating that it was dependent on Ca(2+) entry. The I(ADP) was also inhibited by ryanodine (10-20 microM), indicating that Ca(2+)-induced Ca(2+) release was involved in its activation. Niflumic acid consistently inhibited the I(ADP) with an IC(50) of 63 microM. Using antibodies against the pore-forming subunits of L-, N- and P/Q-type voltage-gated Ca(2+) channels, we have demonstrated that myenteric AH neurons express N- and P/Q, but not L-type voltage-gated Ca(2+) channels. We conclude that the ADP in myenteric AH neurons, in the presence of an L-type Ca(2+)-channel blocker, is generated by the opening of Ca(2+)-activated non-selective cation channels following action potential-mediated Ca(2+) entry mainly through N-type Ca(2+) channels. Ca(2+) release from ryanodine-sensitive stores triggered by Ca(2+) entry contributes significantly to the activation of this current.
Vogalis
F
F
Department of Anatomy and Cell Biology, University of Melbourne, Parkville, Vic. 3010, Australia. fivos@unimelb.edu.au
Harvey
J R
JR
Lohman
R-J
RJ
Furness
J B
JB
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000032:analysis / Q000502:physiology*; D002586:Cesium / Q000494:pharmacology; D002712:Chlorides / Q000378:metabolism; D016861:Cyclooxygenase Inhibitors / Q000494:pharmacology; D004553:Electric Conductivity; D005942:Gluconates / Q000494:pharmacology; D006168:Guinea Pigs; D007150:Immunohistochemistry; D009197:Myenteric Plexus / Q000737:chemistry / Q000502:physiology*; D009544:Niflumic Acid / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism
D015220:Calcium Channels; D002712:Chlorides; D016861:Cyclooxygenase Inhibitors; D005942:Gluconates; D002586:Cesium; D009544:Niflumic Acid; C030691:gluconic acid; D011188:Potassium; D002118:Calcium