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The Journal of Experimental Biology 205, 1819-1830 (2002)
© 2002 The Company of Biologists Limited

A comparison of propagated action potentials from tropical and temperate squid axons: different durations and conduction velocities correlate with ionic conductance levels

Joshua J. C. Rosenthal and Francisco Bezanilla^*

Departments of Physiology and Anesthesiology, UCLA School of Medicine, Los Angeles, CA 90095, USA

^* Author for correspondence (e-mail: fbezanil{at}ucla.edu )

Accepted 5 April 2002

To determine which physiological properties contribute to temperature adaptation in the squid giant axon, action potentials were recorded from four species of squid whose habitats span a temperature range of 20°C. The environments of these species can be ranked from coldest to warmest as follows: Loligo opalescens>Loligo pealei>Loligo plei>Sepioteuthis sepioidea. Action potential conduction velocities and rise times, recorded at many temperatures, were equivalent for all Loligo species, but significantly slower in S. sepioidea. By contrast, the action potential's fall time differed among species and correlated well with the thermal environment of the species (`warmer' species had slower decay times). The biophysical underpinnings of these differences were examined in voltage-clamped axons. Surprisingly, no differences were found between the activation kinetics or voltage-dependence of Na⁺ and K⁺ currents. Conductance levels, however, did vary. Maximum Na⁺ conductance (gNa) in S. sepiodea was significantly less than in the Loligo species. K⁺ conductance (gK) was highest in L. pealei, intermediate in L. plei and smallest in S. sepiodea. The time course and magnitude of gK and gNa were measured directly during membrane action potentials. These data reveal clear species-dependent differences in the amount of gK and gNa recruited during an action potential.

Key words: squid, giant axon, Loligo pealei, Loligo opalescens, Loligo plei, Sepioteuthis sepioidea, temperature adaptation, action potential, conduction velocity, K⁺ conductance, Na⁺ conductance, K⁺ current, Na⁺ current, conduction velocity, action potential broadening

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