A cardiac-like sodium current in motor neurons of a jellyfish.
1. Whole cell voltage-clamp recordings from isolated swimming motor neurons (SMNs) reveal a rapidly activating and inactivating sodium current. 2. Permeability ratios of PLi/PNa = 0.941 and P(guanidinium)/PNa = 0.124 were measured for the mediating channel, which was impermeable to rubidium. 3. The conductance/voltage and steady state inactivation curves are shifted in a depolarizing direction by approximately 45 mV relative to most neuronal sodium currents in higher animals. 4. Activation could be fitted with two exponents and maximal current peaked at 0.74 +/- 0.06 ms (mean +/- SD). 5. Inactivation could be fitted with fast (Tau 1 = 1.91 +/- 0.07 ms at +10 mV) and slow (Tau 2 = 11.65 +/- 0.55 ms at +10 mV) exponents. 6. Half-recovery from inactivation occurred slowly (52.6 +/- 2.9 ms). 7. A second class of identifiable neurons, "B" neurons, possesses a distinctly different population of sodium channels. they showed different inactivation kinetics and far more rapid recovery from inactivation (half-recovery < 5 ms). 8. We conclude that there was physiological diversification of sodium channels early in metazoan evolution and that there has been considerable cell-specific selection of channel properties.
Department of Biological Sciences, University of Alberta, Edmonton, Canada.
N G
NG
Grigoriev
J D
JD
Spafford
J
J
Przysiezniak
A N
AN
Spencer
D000818:Animals; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D004553:Electric Conductivity; D006321:Heart / Q000502:physiology*; D007700:Kinetics; D009046:Motor Neurons / Q000502:physiology*; D018408:Patch-Clamp Techniques; D037401:Scyphozoa; D015222:Sodium Channels / Q000502:physiology*
D015222:Sodium Channels