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.