A study of the crustacean axon repetitive response. 3. A comparison of the effect of veratrine sulfate solution and potassium-rich solutions

The effect of temperature on veratridine action in squid giant axons

Resting membrane potential and intracellular sodium and potassium concentrations were determined at 5 and 21 degrees C in normal and veratridine-treated axons of the squid Doryteuthis plei. 300 microM veratridine produced an increase in the intracellular sodium concentration, which changed from 52 to 284 mM in 10 min of exposure at 21 degrees C, and from 76 to 260 mM at 5 degrees C. Under the same treatment the intracellular potassium concentration changed from 357 to 221 mM (21 degrees C) and from 334 to 194 mM (5 degrees C). All the changes could be prevented by adding 1 microM tetrodotoxin. Veratridine (30, 100 and 300 microM) increased the resting sodium permeability of the giant axon, and the effect was greater at 21 degrees C. The affinity of the membrane for veratridine increases when the nerves are cooled, the three concentrations tested produce maximum activation of the sodium channels at 5 degrees C. But only the higher two concentrations are saturating at 21 degrees C.

Fluorescence changes of rhodamine 6G associated with changes in membrane potential in synaptosomes

The intensity of rhodamine 6G fluorescence was found to be a useful scale for measuring the membrane potential in synaptosomes. The fluorescence of rhodamine 6G in synaptosomal suspensions increases with depolarization in the synaptosomes induced by the replacement of cations in the medium or by the addition of agents known to depolarize the membrane potential. Considering the character of the dye, we have derived an equation which gives the relation between the fluorescence intensity of the dye and the membrane potential. The change in membrane potential (diffusion potential) of synaptosomes was calculated using the equation. The calculated membrane potential was proportional to the logarithm of the K+ concentration above 20 mM, and the slope of membrane potential against log [K+] was about 52 mV per decade of concentration. The permeability ratio (Px/Pk; the ratio of the permeability constants of a given cation, X+, and K+) was estimated from the calculated membrane potential.

The effects of protoveratrine and germines on the release of acetylcholine from the Auerbach plexus of the guinea-pig ileum

The effect of protoveratrine A and germine-3-acetate (GMA) on the release mechanism of acetylcholine from the nerve terminals of the Auerbach plexus in the longitudinal muscle layer of the guinea-pig ileum was studied. Protoveratrine A, GMA and germine potentiated neuroeffector transmission in Auerbach's plexus in the longitudinal muscle preparation provided the neurons were stimulated at low frequencies (less than 10 Hz). Protoveratrine A and GMA enhanced the release of acetylcholine from the nerve terminals during the resting period and at low frequency of stimulation (less than 10 Hz). At continous stimulation with high frequency they were ineffective (greater than 10 Hz). When trains of 20 stimuli, with a pulse interval of 0.1 s (10 Hz) were repeatedly applied with intervals of 0.1 to 20 s between trains, the effect of GMA to increase ACh release depended on the length of the train interval; the longer the resting period between trains the higher was the output of ACh. This fact indicates that the release of ACh increased primarily during the resting periods following single stimuli or trains. The effect of GMA on ACh release proved to be highly temperature-dependent: in the presence of GMA Q10 increasing from 3.25 to 4.92. A high Ca concentration, removal of Mg or lowering of the Na concentration abolished the effect of GMA to enhance ACh release.

Increase in PNa and PK of cultured heart cells produced by veratridine

Noninnervated cultured chick embryonic heart cells are depolarized by veratridine (10(-5) 10(-6) g/ml) within a few minutes to membrane potentials of -12 +/- 2 mv. Action potentials and beating cease. Before depolarization begins, the repolarizing phase of the action potential is prolonged and leads to a long-lasting depolarizing afterpotential, probably due to a holding open of Na(+) channels. There is no direct effect on automaticity. Maximum rate of rise of the action potential decreases as a function of the depolarization. The inexcitability is transiently reversed by repolarizing current pulses and by 5 mM Ba(++) (but not Sr(++)) which increases membrane resistance (R(m)) and produces a small transient repolarization. Cocaine does not reverse the depolarization. The depolarization also occurs in Cl(-)-free Ringer and in Na(+)-free Li(+)-Ringer, but not in Na(+)-free sucrose-Ringer. In most cases, R(m), measured in the presence and absence of Cl(-), initially decreases but sometimes increases. Some of the decrease or increase in g(K) may be indirectly produced by anomalous or delayed rectification, respectively. Tetrodotoxin, although having no effect on the action potential magnitude or rate of rise, prevents the depolarizing action of veratridine but not its effect on decreasing R(m). It is concluded that veratridine depolarizes by increasing the resting Na(+) permeability (P(Na)); it also tends to increase P(K), but this action may be obscured by anomalous rectification when E(m) is allowed to change. The equilibrium potential for veratridine action is about halfway between E(Na) and E(K), similar to that of acetylcholine at the vertebrate neuromuscular junction.

Bioelectric phenomena related to protein-fixed charge in a crab nerve fiber

The bioelectrical characteristics of a crab nerve fiber subjected to solutions containing the impermeant anions ferrocyanide, glutamate, or ethanolsulfate substituted for chloride include prolonged slow depolarizations that elicit prolonged trains of impulses sulperimposed on the plateau portion. Propionate and nitrate depress repetitive firing while thiocyancite has only a slighit effect. Nerves treated with ferrocyanide lose potassium and gain sodium, which fact may account for their depolarization. On the other hand, studies with interference microscopy reveal shift by ferrocyanide in the interference fringe pattern of the sheath material; this suggests a reorientation of fixed charges in the protein layers by direct action of the anion on these layers. This may also account for the electrical manifestations observed.