With 90 mM Ba2+, the main Ca2+ current in frog sympathetic neurons pea
ks near +30 mV and is blocked by omega-conotoxin GVIA (omega-CgTx). It
is modulated by norepinephrine (NE) in a voltage-dependent manner via
a membrane-delimited mechanism, Surprisingly, a different current dom
inates at more negative voltages (-30 to +10 mV). That novel current i
s not sensitive to selective blockers of L- or N-type channels (respec
tively, dihydropyridines or omega-CgTx) and is inhibited weakly if at
all by NE. It is selectively inactivated at -40 mV and is selectively
blocked by Ni2+, whereas Cd2+ is slightly more potent against the main
current. The novel current is associated with a 19 pS channel (0.6 pA
at 0 mV). This channel may have been misidentified as the single-chan
nel correlate of the whole-cell N-type Ca2+ current in some previous s
tudies.