The effects of temperature and ethanol on the properties of the fast excitatory axon to the crab limb bender muscle

Feeling the heat: source–sink mismatch as a mechanism underlying the failure of thermal tolerance

A mechanistic explanation for the tolerance limits of animals at high temperatures is still missing, but one potential target for thermal failure is the electrical signaling off cells and tissues. With this in mind, here I review the effects of high temperature on the electrical excitability of heart, muscle and nerves, and refine a hypothesis regarding high temperature-induced failure of electrical excitation and signal transfer [the temperature-dependent deterioration of electrical excitability (TDEE) hypothesis]. A central tenet of the hypothesis is temperature-dependent mismatch between the depolarizing ion current (i.e. source) of the signaling cell and the repolarizing ion current (i.e. sink) of the receiving cell, which prevents the generation of action potentials (APs) in the latter. A source–sink mismatch can develop in heart, muscles and nerves at high temperatures owing to opposite effects of temperature on source and sink currents. AP propagation is more likely to fail at the sites of structural discontinuities, including electrically coupled cells, synapses and branching points of nerves and muscle, which impose an increased demand of inward current. At these sites, temperature-induced source–sink mismatch can reduce AP frequency, resulting in low-pass filtering or a complete block of signal transmission. In principle, this hypothesis can explain a number of heat-induced effects, including reduced heart rate, reduced synaptic transmission between neurons and reduced impulse transfer from neurons to muscles. The hypothesis is equally valid for ectothermic and endothermic animals, and for both aquatic and terrestrial species. Importantly, the hypothesis is strictly mechanistic and lends itself to experimental falsification.

The second sodium pump: from the function to the gene

Transepithelial Na(+) transport is mediated by passive Na(+) entry across the luminal membrane and exit through the basolateral membrane by two active mechanisms: the Na(+)/K(+) pump and the second sodium pump. These processes are associated with the ouabain-sensitive Na(+)/K(+)-ATPase and the ouabain-insensitive, furosemide-inhibitable Na(+)-ATPase, respectively. Over the last 40 years, the second sodium pump has not been successfully associated with any particular membrane protein. Recently, however, purification and cloning of intestinal α-subunit of the Na(+)-ATPase from guinea pig allowed us to define it as a unique biochemical and molecular entity. The Na(+)- and Na(+)/K(+)-ATPase genes are at the same locus, atp1a1, but have independent promoters and some different exons. Herein, we spotlight the functional characteristics of the second sodium pump, and the associated Na(+)-ATPase, in the context of its role in transepithelial transport and its response to a variety of physiological and pathophysiological conditions. Identification of the Na(+)-ATPase gene (atna) allowed us, using a bioinformatics approach, to explore the tertiary structure of the protein in relation to other P-type ATPases and to predict regulatory sites in the promoter region. Potential regulatory sites linked to inflammation and cellular stress were identified in the atna gene. In addition, a human atna ortholog was recognized. Finally, experimental data obtained using spontaneously hypertensive rats suggest that the Na(+)-ATPase could play a role in the pathogenesis of essential hypertension. Thus, the participation of the second sodium pump in transepithelial Na(+) transport and cellular Na(+) homeostasis leads us to reconsider its role in health and disease.

Ethanol decreases the velocity of spike propagation along a fast motor axon

Intracellular recordings were made from the fast bender excitor motor axon in autotomized crab limbs bathed in normal saline, and in salines made with up to 240 mM of ethanol. The presence of ethanol reduced the amplitude, the rise time and the decay time of the evoked action potential, and decreased the velocity at which the spike was conducted down the axon. There was a linear relationship between each of these four parameters and the concentration of ethanol in the saline. The close relationship between spike rise time and conduction velocity suggests that ethanol slows the rate of membrane depolarization by the spike and thus decreases the velocity at which action potentials are propagated along the axon.

The effects of ethanol on intracellular potassium and the membrane potential of an identified crab motor axon

1. Observations were made on the fast bender excitor axon in autotomized crab walking limbs bathed in normal crab saline, and in salines made up with 0.2 M sucrose or 0.2 M ethanol. Microelectrode techniques were used to measure the resting membrane potential and the intracellular level of potassium. 2. Sucrose-saline had little effect on the membrane potential or the intracellular level of potassium. Ethanol-saline hyperpolarized the membrane potential by about 3 mV and increased the level of intracellular potassium. 3. The ethanol-induced changes in intracellular potassium levels and membrane potential took place with the same time course. Further, the changes in membrane potential could be accounted for by changes in the equilibrium potential for potassium, Ek as predicted by the Nernst equation.

Differential effects of alcohols on the spike threshold of an identified motor axon in a crab (Pachygrapsus crassipes)

Observations were made on the fast bender excitor (FBE) axon in autotomized crab limbs bathed in salines made up with different alcohols. It has been shown previously that the presence of ethanol at a certain level causes a single action potential to generate additional spikes in the peripheral axon branches. The present study examines the level of different alcohols required to induce peripheral spike generation. For primary alcohols, increasing the molecular weight decreased the level of alcohol required to produce peripheral spike generation. The threshold level of 2-butanol was greater than 1-butanol, but less than tertiary-butanol. These results are explained in terms of the partition coefficient, so that an alcohol with a higher partition coefficient enters the lipid bilayer more readily, thus a lower threshold level of that alcohol is required in the saline to generate additional spikes.