Identification of the single channels that underlie the N-type and L-type calcium currents in bullfrog sympathetic neurons.
Most of the whole-cell calcium current of frog sympathetic neurons is an N-type current, blocked by omega-conotoxin GVIA (omegaCGVIA). Thus, these cells should be an excellent system to study the properties of single N-type channels. However, a channel that is active near -10 mV in isotonic Ba2+, originally identified as "N-type," corresponds more closely to a omegaCGVIA-resistant component of the whole-cell current observed in 100 mM Ba2+. That conclusion would imply that the true single-channel correlate of the macroscopic N-current remains to be identified in frog sympathetic neurons. I report here recordings from cell-attached patches of a calcium channel that activates in the appropriate voltage range (>0 mV, in isotonic Ba2+) and is blocked by omegaCGVIA. This channel has a slope conductance of 20 pS (range, 17-25 pS) and a single-channel current of -1.3 pA at 0 mV. Other channels active in the same voltage range (24 pS, -1.3 pA at 0 mV) were identified as L-type channels because they exhibited long openings after repolarization in the presence of 1 microM Bay K 8644 and were resistant to omegaCGVIA. A third channel type (13-19 pS) was distinguished by current amplitude (-0.6 pA at 0 mV) and strong inactivation at -40 mV. The similarity in slope conductance among these channels demonstrates that distinguishing them requires the consideration of additional properties. The omegaCGVIA-sensitive channel can be identified as an N-type calcium channel.
Elmslie
K S
KS
Department of Physiology, Tulane University Medical School, New Orleans, Louisiana 70112, USA.
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000818:Animals; D001464:Barium / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D020746:Calcium Channels, L-Type; D002478:Cells, Cultured; D002645:Chickens; D005728:Ganglia, Sympathetic / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects; D009419:Nerve Tissue Proteins / Q000502:physiology; D009474:Neurons / Q000502:physiology*; D010455:Peptides / Q000494:pharmacology; D011892:Rana catesbeiana; D051381:Rats; D013045:Species Specificity; D020866:omega-Conotoxin GVIA
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D009419:Nerve Tissue Proteins; D010455:Peptides; D001464:Barium; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D020866:omega-Conotoxin GVIA