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Volume 17, Number 8, Issue of April 15, 1997 pp. 2658-2668
Copyright ©1997 Society for Neuroscience

Identification of the Single Channels that Underlie the N-Type and L-Type Calcium Currents in Bullfrog Sympathetic Neurons

Received Nov. 14, 1996; revised Jan. 29, 1997; accepted Jan 30, 1997.

Keith S. Elmslie

Department of Physiology, Tulane University Medical School, New Orleans, Louisiana 70112

Most of the whole-cell calcium current of frog sympathetic neurons is an N-type current, blocked by -conotoxin GVIA (CGVIA). 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 CGVIA-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 CGVIA. 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 µM Bay K 8644 and were resistant to CGVIA. 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 CGVIA-sensitive channel can be identified as an N-type calcium channel.

Key words: N-type calcium channels; L-type calcium channels; -conotoxin GVIA; BayK 8644; sympathetic neurons; cell-attached patch clamp

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