Being able to regulate your body temperature is a luxury that most mammals take for granted, but for poikilotherms, it's a different kettle of fish!They've got to keep going, whatever their environment's temperature, so some poikilotherms thrive at near freezing temperatures, while others are perfectly happy at temperatures that would make the rest of us steam. Each of these creature's life support systems are perfectly adapted to their chosen environment: even complex neurological systems. Back in 1949, Hodgkin and Katz had noticed that tropical frog neurons failed at temperatures where temperate squid axons performed perfectly. Francisco Bezanilla and Joshua Rosenthal were intrigued to see whether the action potentials of axons varied with the temperature the organism is adapted to. The team returned to the neurobiologist's axon of choice, the squid giant axon, and found that squid from the tropics have reduced the potassium component of the nerve's conduction system to keep the potential propagating at temperatures where others would have failed (p. 1819).
Every time a nerve fires, the voltage across the cell rises at first, as sodium ions flood into the cell through sodium channels. As the potential approaches the peak, other channels open, and potassium flows out of the cell as it repolarises. Plenty of studies had found that nerve synapse function depended on temperature, but no one really knew whether the action potential was affected too.
Rosenthal and Benzanilla decided to work with the squid, a species that has colonised the globe's oceans to get a truly comparative angle on the action potential. Having chosen four specimens that lived at four different temperatures over a 20°C range, Rosenthal began working with the squid neurons to see how they'd adapted to different thermal niches.
First he recorded each axon's action potential shape over a range of temperatures. The difference was obvious when he compared the four neurons at a single temperature; the action potential had a longer duration for warmer species, and the shape of the nerve's discharge suggested that there was something different about the potassium conductance.
Rosenthal and Bezanilla were curious how the hot and cold nerve's differed to produce these different potential shapes. Rosenthal measured the nerves'abilities to conduct sodium and potassium, and found that three of the four squid nerves conducted sodium ions in the same way, but the warmer species'nerves conducted less potassium than the cold species. Either the potassium channels were modified to function in warmer waters, or there were fewer channels to carry the ions out of the nerve axon. A closer look at the speed of the channel's responses showed that they hadn't changed, so the squid probably hadn't modified the ion channels, they must simply have less than the colder species.
Rosenthal and Bezanilla did one more set of experiments to prove how the counter-flowing ion currents changed during an action potential. They sampled action potentials at microsecond intervals to directly measure both the sodium and potassium conductances during an action potential. The results proved that warm adapted squid had less potassium conductance during an action potential,by reducing the number of potassium channels. So squid have optimised their wiring to suit the latitude they live at.