A series of aldehydes of varying protein crosslinking strengths have been tested on intact and internally perfused crayfish axons. Non-crosslinking aldehydes have no effect, or cause a gradual decline in resting potential and overshoot with no widening of the spike. Strong crosslinking compounds, such as acrolein, crotonaldehyde, and glutaraldehyde, widen the action potential significantly while reducing its amplitude. Differences in the shapes of the resulting action potentials and accompanying impedance changes suggest that each crosslinking aldehyde exerts different effects on the axon. Glutaraldehyde, the strongest crosslinking agent tested, slows both rising and falling phases of the spike, and also of the impedance change, suggesting a prolongation of the transient increase in sodium conductance. The ability of protein crosslinking agents to alter excitability, and particularly to slow the various phases of the action potential, provides support for the hypothesis that a conformational change in a protein or protein-phospholipid complex is involved in excitation.
Changes in conformation of a membrane protein or protein-phospholipid complex have often been postulated as a primary step in the generation of the nerve impulse. Recent studies, which allowed time resolution within a single action potential, have provided evidence for such physico-chemical transitions. Howarth et al. ('68) measured heat production of unmyelinated nerves during the action potential and found a decrease in entropy of the membrane during depolarization. Cohen et al. ('68) detected light scattering and birefringence changes in the membrane during excitation and interpreted them in terms of possible structural transitions. Tasaki et al. ('68) found an increase in the fluorescence of a bound dye on stimulation, as well as changes in light scattering and birefringence.
If, in fact, a conformational change in a protein or protein complex does occur, then excitability might be particularly sensitive to protein crosslinking reagents. Some evidence for such effects has been reported. Takahashi et al. ('58) found that certain divalent metal ions capable of crosslinking sulfhydryl groups caused the formation of a plateau in the falling phase of the action potential of toad nerve.
Cooke et al. ('68) showed that 1,5-diilu-