A Cardiac-like Sodium Current in Motor Neurons of a Jellyfish. GRIGORIEV, NIKITA G., J. DAVID SPAFFORD, JAN PRZYSIEZNIAK AND ANDREW N. SPENCER. Department of Biological Sciences, The University of Alberta, Edmonton, Alberta T6G 2E9 and Bamfield Marine Station, Bamfield, B.C., V0R 1B0 1.
APStracts 3:0071N, 1996.
SUMMARY AND CONCLUSIONS
1. Whole-cell voltage clamp recordings from isolated swimming motor neurons (SMNs) reveal a rapidly activating and inactivating sodium current. 2. Permeability ratios of P Li /P Na = 0.941 and P guanidinium /P Na = 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 about 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. 5. Inactivation could be fitted with fast ( t 1 =1.91+/-0.07 ms at + 10 mV) and slow ( t 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, possess 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.

Received 3 August 1995; accepted in final form 29 March 1996.
APS Manuscript Number J509-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 23 April 96