THE EFFECTS OF CURRENT FLOW ON BIOELECTRIC POTENTIAL

The effect of direct current, of controlled direction and density, across the protoplasm of impaled cells of Halicystis, is described. Inward currents slightly increase the already positive P.D. (70 to 80 mv.) in a regular polarization curve, which depolarizes equally smoothly when the current is stopped. Outward currents of low density produce similar curves in the opposite direction, decreasing the positive P.D. by some 10 or 20 mv. with recovery on cessation of flow. Above a critical density of outward current, however, a new effect becomes superimposed; an abrupt reversal of the P.D. which now becomes 30 to 60 mv. negative. The reversal curve has a characteristic shape: the original polarization passes into a sigmoid reversal curve, with an abrupt cusp usually following reversal, and an irregular negative value remaining as long as the current flows. Further increases of outward current each produce a small initial cusp, but do not greatly increase the negative P.D. If the current is decreased, there occurs a threshold current density at which the positive P.D. is again recovered, although the outward current continues to flow. This current density (giving positivity) is characteristically less than that required to produce reversal originally, giving the process a hysteretic character. The recovery is more rapid the smaller the current, and takes only a few seconds in the absence of current flow, its course being in a smooth curve, usually without an inflection, thus differing from the S-shaped reversal curve. The reversal produced by outward current flow is compared with that produced by treatment with ammonia. Many formal resemblances suggest that the same mechanism may be involved. Current flow was therefore studied in conjunction with ammonia treatment. Ammonia concentrations below the threshold for reversal were found to lower the threshold for outward currents. Subthreshold ammonia concentrations, just too low to produce reversal alone, produced permanent reversal when assisted by a short flow of very small outward currents, the P.D. remaining reversed when the current was stopped. Further increases of outward current, when the P.D. had been already reversed by ammonia, produced only small further increases of negativity. This shows that the two treatments are of equivalent effect, and mutually assist in producing a given effect, but are not additive in the sense of being superimposable to produce a greater effect than either could produce by itself. Since ammonia increases the alkalinity of the sap, and presumably of the protoplasm, when it penetrates, it is possible that the reversal of P.D. by current flow is also due to change of pH. The evidence for increased alkalinity or acidity due to current flow across phase boundaries or membranes is discussed. While an attractive hypothesis, it meets difficulties in H. ovalis where such pH changes are both theoretically questionable and practically ineffective in reversing the P.D. It seems best at the present time to assign the reversal of P.D. to the alteration or destruction of one surface layer of the protoplasm, with reduction or loss of its potential, leaving that at the other surface still intact and manifesting its oppositely directed potential more or less completely. The location of these surfaces is only conjectural, but some evidence indicates that it is the outer surface which is so altered, and reconstructed on recovery of positive P.D. This agrees with the essentially all-or-none character of the reversal. The various treatments which cause reversal may act in quite different ways upon the surface.

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THE EFFECTS OF CURRENT FLOW ON BIOELECTRIC POTENTIAL

The Journal of General Physiology ◽

10.1085/jgp.20.2.229 ◽

1936 ◽

Vol 20 (2) ◽

pp. 229-265 ◽

Cited By ~ 23

Author(s):

L. R. Blinks

Keyword(s):

Current Density ◽

Current Flow ◽

Surface Film ◽

Outward Current ◽

Effective Resistance ◽

Bioelectric Potential ◽

Action Current ◽

Outward Currents ◽

Inward Currents ◽

Sigmoid Curve

String galvanometer records show the effect of current flow upon the bioelectric potential of Nitella cells. Three classes of effects are distinguished. 1. Counter E.M.F'S, due either to static or polarization capacity, probably the latter. These account for the high effective resistance of the cells. They record as symmetrical charge and discharge curves, which are similar for currents passing inward or outward across the protoplasm, and increase in magnitude with increasing current density. The normal positive bioelectric potential may be increased by inward currents some 100 or 200 mv., or to a total of 300 to 400 mv. The regular decrease with outward current flow is much less (40 to 50 mv.) since larger outward currents produce the next characteristic effect. 2. Stimulation. This occurs with outward currents of a density which varies somewhat from cell to cell, but is often between 1 and 2 µa/cm.2 of cell surface. At this threshold a regular counter E.M.F. starts to develop but passes over with an inflection into a rapid decrease or even disappearance of positive P.D., in a sigmoid curve with a cusp near its apex. If the current is stopped early in the curve regular depolarization occurs, but if continued a little longer beyond the first inflection, stimulation goes on to completion even though the current is then stopped. This is the "action current" or negative variation which is self propagated down the cell. During the most profound depression of P.D. in stimulation, current flow produces little or no counter E.M.F., the resistance of the cell being purely ohmic and very low. Then as the P.D. begins to recover, after a second or two, counter E.M.F. also reappears, both becoming nearly normal in 10 or 15 seconds. The threshold for further stimulation remains enhanced for some time, successively larger current densities being needed to stimulate after each action current. The recovery process is also powerful enough to occur even though the original stimulating outward current continues to flow during the entire negative variation; recovery is slightly slower in this case however. Stimulation may be produced at the break of large inward currents, doubtless by discharge of the enhanced positive P.D. (polarization). 3. Restorative Effects.—The flow of inward current during a negative variation somewhat speeds up recovery. This effect is still more strikingly shown in cells exposed to KCl solutions, which may be regarded as causing "permanent stimulation" by inhibiting recovery from a negative variation. Small currents in either direction now produce no counter E.M.F., so that the effective resistance of the cells is very low. With inward currents at a threshold density of some 10 to 20 µa/cm.2, however, there is a counter E.M.F. produced, which builds up in a sigmoid curve to some 100 to 200 mv. positive P.D. This usually shows a marked cusp and then fluctuates irregularly during current flow, falling off abruptly when the current is stopped. Further increases of current density produce this P.D. more rapidly, while decreased densities again cease to be effective below a certain threshold. The effects in Nitella are compared with those in Valonia and Halicystis, which display many of the same phenomena under proper conditions. It is suggested that the regular counter E.M.F.'S (polarizations) are due to the presence of an intact surface film or other structure offering differential hindrance to ionic passage. Small currents do not affect this structure, but it is possibly altered or destroyed by large outward currents, restored by large inward currents. Mechanisms which might accomplish the destruction and restoration are discussed. These include changes of acidity by differential migration of H ion (membrane "electrolysis"); movement of inorganic ions such as potassium; movement of organic ions, (such as Osterhout's substance R), or the radicals (such as fatty acid) of the surface film itself. Although no decision can be yet made between these, much evidence indicates that inward currents increase acidity in some critical part of the protoplasm, while outward ones decrease acidity.

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THE EFFECTS OF CURRENT FLOW ON BIOELECTRIC POTENTIAL

The Journal of General Physiology ◽

10.1085/jgp.19.4.633 ◽

1936 ◽

Vol 19 (4) ◽

pp. 633-672 ◽

Cited By ~ 20

Author(s):

L. R. Blinks

Keyword(s):

Current Density ◽

Current Flow ◽

The Other ◽

Bioelectric Potential ◽

Threshold Density ◽

Other Hand ◽

Outward Currents ◽

Inward Currents ◽

Sigmoid Curve ◽

Current Flows

The effect of direct current flow upon the potential difference across the protoplasm of impaled Valonia cells was studied. Current density and direction were controlled in a bridge which balanced the ohmic resistances, leaving the changes (increase, decrease, or reversal) of the small, normally negative, bioelectric potential to be recorded continuously, before, during, and after current flow, with a string galvanometer connected into a vacuum tube detector circuit. Two chief states of response were distinguished: State A.—Regular polarization, which begins to build up the instant current starts to flow, the counter E.M.F. increasing most rapidly at that moment, then more and more slowly, and finally reaching a constant value within 1 second or less. The magnitude of counter E.M.F. is proportional to the current density with small currents flowing in either direction across the protoplasm, but falls off at higher density, giving a cusp with recession to lower values; this recession occurs with slightly lower currents outward than inward. Otherwise the curves are much the same for inward and outward currents, for different densities, for charge and discharge, and for successive current flows. There is a slight tendency for the bioelectric potential to become temporarily positive following these current flows. Records in the regular state (State A) show very little effect of increased series resistance on the time constant of counter E.M.F. This seems to indicate that a polarization rather than a static capacity is involved. State B.—Delayed and non-proportional polarization, in which there is no counter E.M.F. developed with small currents in either direction across the protoplasm, nor with very large outward currents. But with inward currents a threshold density is reached at which a counter E.M.F. rather suddenly develops, with a sigmoid curve rising to high positive values (200 mv. or more). There is sometimes a cusp, after which the P.D. remains strongly positive as long as the current flows. It falls off again to negative values on cessation of current flow, more rapidly after short flows, more slowly after longer ones. The curves of charge are usually quite different in shape from those of discharge. Successive current flows of threshold density in rapid succession produce quicker and quicker polarizations, the inflection of the curve often becoming smoothed away. After long interruptions, however, the sigmoid curve reappears. Larger inward currents produce relatively little additional positive P.D.; smaller ones on the other hand, if following soon after, have a greatly increased effectiveness, the threshold for polarization falling considerably. The effect dies away, however, with very small inward currents, even as they continue to flow. Over a medium range of densities, small increments or decrements of continuing inward current produce almost as regular polarizations as in State A. Temporary polarization occurs with outward currents following soon after the threshold inward currents, but the very flow of outward current tends to destroy this, and to decondition the protoplasm, again raising the threshold, for succeeding inward flows. State A is characteristic of a few freshly gathered cells and of most of those which have recovered from injuries of collecting, cleaning, and separating. It persists a short time after such cells are impaled, but usually changes over to State B for a considerable period thereafter. Eventually there is a reappearance of regular polarization; in the transition there is a marked tendency for positive P.D. to be produced after current flow, and during this the polarizations to outward currents may become much larger than those to inward currents. In this it resembles the effects of acidified sea water, and of certain phenolic compounds, e.g. p-cresol, which produce State A in cells previously in State B. Ammonia on the other hand counteracts these effects, producing delayed polarization to an exaggerated extent. Large polarizations persist when the cells are exposed to potassium-rich solutions, showing it is not the motion of potassium ions (e.g. from the sap) which accounts for the loss or restoration of polarization. It is suggested that inward currents restore a protoplasmic surface responsible for polarization by increasing acidity, while outward currents alter it by increasing alkalinity. Possibly this is by esterification or saponification respectively of a fatty film. For comparison, records of delayed polarization in silver-silver chloride electrodes are included.

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