THE SPECIFIC RÔLE OF ADRENIN IN COUNTERACTING THE EFFECTS OF FATIGUE

As a muscle approaches its fatigue level, its contractions are decreased in height. Higher contractions will again be elicited if the stimulus is increased. Although these phenomena are well known, no adequate analysis of their causes has been advanced. A number of factors are probably operative in decreasing the height of contraction: (1) The using up of available energy-producing material; (2) the accumulation of metabolites in the fatigued muscle; (3) polarization of the nerve at the point of repeated electrical stimulation; and (4) a decrease of irritability. It may be that there are interactions between these factors within the muscle, e. g., the second may cause the fourth.

Variations of the Threshold Stimulus as a Measure of Irritability

The last of the factors mentioned above—the effect of fatigue on the irritability of the nerve-muscle combination, or on the muscle alone—can be tested by determining variations in the least stimulus capable of causing the slightest contraction, the so-called “threshold stimulus.” As the irritability lessens, the threshold stimulus must necessarily be higher. The height of the threshold is therefore a measure of irritability. How does fatigue affect the irritability of nerve-muscle and muscle? How is the irritability of fatigued structures affected by rest? How is it influenced by adrenin or by adrenal secretion? Answers to these questions were sought in researches carried on by C. M. Gruber[1] in 1913.

The Method of Determining the Threshold Stimulus

The neuro-muscular arrangements used in these researches were in many respects similar to those already described in the account of experiments by Nice and myself. To avoid the influence of an anesthetic some of the animals were decerebrated under ether and then used as in the experiments in which urethane was the anesthetic. The nerve (the peroneus communis) supplying the tibialis anticus muscle was bared and severed; and near the cut end shielded platinum electrodes were applied. These electrodes were used in fatiguing the muscle. Between these electrodes and the muscle other platinum electrodes could be quickly applied to determine the threshold stimulus and the tissue resistance. These second electrodes were removed except when in use, and when replaced were set always in the same position. Care was taken, before replacing them, to wipe off moisture on the nerve or on the platinum points.

For determining the threshold stimulus of the muscle the skin and other overlying tissues were cut away from the tibialis anticus in two places about 5 centimeters apart. Through these openings platinum needle electrodes could be thrust into the muscle whenever readings were to be taken. Local polarization was avoided by reinserting the needles into fresh points on the exposed areas whenever new readings were to be taken.

The tendon of the tibialis anticus was attached, as in the previous experiments, by a strong thread passing about pulleys to a lever which when lifted stretched a spring. During the determination of the threshold the spring was detached from the lever, so that only the pull of the lever itself (about 15 grams) was exerted on the muscle.

The method of measuring the stimulating value of the electric current which was used in testing the threshold was that devised by E. G. Martin[*] of the Harvard Laboratory—a method by which the strength of an induced electric shock is calculable in definite units. If the tissue resistance enters into the calculation these are called β units. When the threshold of the nerve-muscle was taken, the apparatus for the determination was connected with the nerve through the electrodes nearer the muscle. They were separated from the fatiguing electrodes by more than 3 centimeters, and arranged so that the kathode was next the muscle. When the threshold of the muscle was taken directly the apparatus was connected with the muscle through platinum needle electrodes thrust into it. The position of the secondary coil of the inductorium, in every case, was read by moving it away from the primary coil until the very smallest possible contraction of the muscle was obtained. Four of these readings were made, one with tissue resistance, the others with 10,000, 20,000, and 30,000 ohms additional resistance in the secondary circuit. Only break shocks were employed—the make shocks were short-circuited. Immediately after the determination of the position of the secondary coil, and before the electrodes were removed or disconnected, three readings of the tissue resistance were made. From these data four values for β were calculated.

The strength of the primary current for determining the threshold of the nerve-muscle was usually .01 ampere, but in a few cases .05 ampere was used. For normal muscle it was .05 ampere and for denervated muscle 1.0 ampere. The inductorium, which was used throughout, had a secondary resistance of 1400 ohms. This was added to the average tissue resistance in making corrections—corrections were made also for core magnetization.

The Lessening of Neuro-muscular Irritability by Fatigue

The threshold for the peroneus communis nerve in decerebrate animals varied from 0.319 to 2.96 units, with an average in sixteen experiments of 1.179.[*] This average is the same as that found by E. L. Porter[2] for the radial nerve in the spinal cat. For animals under urethane anesthesia a higher average was obtained. In these it varied from .644 to 7.05, or an average in ten experiments of 3.081.

The threshold for the tibialis anticus muscle varied in the decerebrate animals from 6.75 units to 33.07, or an average in fifteen experiments of 18.8. Ten experiments were performed under urethane anesthesia and the threshold varied from 12.53 to 54.9, with an average of 29.84 β units. From these results it is evident that anesthesia notably affects the threshold.

E. L. Porter proved, by experiments carried on in the Harvard Physiological Laboratory, that the threshold of an undisturbed nerve-muscle remains constant for hours, and his observation was confirmed by Gruber (see Fig. 19). If, therefore, after fatigue, a change exists in the threshold, this change is necessarily the result of alterations set up by the fatigue process in the nerve-muscle or muscle.

After fatigue the threshold of the nerve-muscle, in sixteen decerebrate animals, increased from an average of 1.179 to 3.34—an increase of 183 per cent. In ten animals under urethane anesthesia the threshold after fatigue increased from a normal average of 3.08 to 9.408—an increase of 208 per cent.

An equal increase in the threshold stimulus was obtained from the normal muscle directly. In decerebrate animals the normal threshold of 18.8 units was increased by fatigue to 69.54, or an increase of 274 per cent. With urethane anesthesia the threshold increased from 29.849 to 66.238, or an increase of 122 per cent.

Fig. 18, plotted from the data of one of the many experiments, shows the relative heights of the threshold before and after fatigue. The correspondence of the two readings of the threshold, one from the nerve supplying the muscle and the other from the muscle directly, served as a check on the electrodes. The broken line in the figure represents the threshold (in units) of the nerve-muscle, and the continuous line that of the muscle. The threshold values of the nerve-muscle have been magnified ten times in order to bring the two records close together. In this experiment the threshold of the muscle after fatigue (i. e., at 2) is 167 per cent higher than the normal threshold (at 1), while that of the nerve-muscle after fatigue is 30.5 per cent higher than its normal.

Evidently a direct relation exists between the duration of work and the increase of threshold. For instance, the threshold is higher after a muscle is fatigued for two hours than it is at the end of the first hour. The relation between the work done and the threshold is not so clear. In some animals the thresholds were higher after 120 grams had been lifted 120 times a minute for 30 minutes than they were in others in which 200 grams had been lifted 240 times a minute for the same period. The muscle in the latter instances did almost four times as much work, yet the threshold was lower. The difference may be due to the general condition of the animal.

A few experiments were performed on animals in which the nerve supplying the muscle was cut seven to fourteen days previous to the experiment. The muscle, therefore, had within it no living nerve fibres. The average normal threshold for the denervated muscle in 6 animals was 61.28 units. As in the normal muscle, the percentage increase due to fatigue was large.

The Slow Restoration of Fatigued Muscle to Normal Irritability by Rest

That rest decreases the fatigue threshold of both nerve-muscle and muscle can be seen in Fig. 18. The time taken for total recovery, however, is dependent upon the amount of work done, but this change, like that of fatigue, varies widely with different individuals. In some animals the threshold returned to normal in 15 minutes; in others, in which the same amount of work was done, it was still above normal even after 2 hours of rest. This may be due to the condition of the animals—in some the metabolites are probably eliminated more rapidly than in others. There were also variations in the rate of restoration of the normal threshold when tested on the nerve and when tested on the muscle in the same animal. In Fig. 18 (at 3) the nerve-muscle returned to normal in 30 minutes, whereas the muscle (at 4) after an hour’s rest had not returned to normal by a few β units. This, however, is not typical of all nerve-muscles and muscles. The opposite condition—that in which the muscle returned to normal before the nerve-muscle—occurred in as many cases as did the condition just cited. The failure of the two tissues to alter uniformly in the same direction may be explained as due to variations in the location of the electrodes when thrust into the muscle at different times (e. g., whether near nerve filaments or not). The results from observations made on the nerve are more likely to be uniform and reliable than are those from the muscle.

The time required for the restoration of the threshold from fatigue to normal, in denervated muscles, is approximately the same as that for the normal muscle.

The Quick Restoration of Fatigued Muscle to Normal Irritability by Adrenin

The foregoing observations showed that fatigue raises the normal threshold of a muscle, on the average, between 100 and 200 per cent (it may be increased more than 600 per cent); that this increase is dependent on the time the muscle works, but also varies with the animal; that rest, 15 minutes to 2 hours, restores the normal irritability; and that this recovery of the threshold depends upon the time given to rest, the duration of the work, and also upon the condition of the animal. The problem which was next attacked by Gruber was that of learning whether the higher contractions of fatigued muscle after splanchnic stimulation could be attributed to any influence which adrenal secretion might have in restoring the normal irritability. To gain insight into the probabilities he tried first the effects of injecting slowly into the jugular vein physiological amounts of adrenin.[*]

The normal threshold of the peroneus communis nerve varied in the animals used in this series of observations from 0.35 to 5.45 units, with an average in nine experiments of 1.3, a figure close to the 1.179 found in the earlier series on the effect of fatigue. For the tibialis anticus muscle, in which the nerve-endings were intact, the threshold varied from 6.75 to 49.3 units, with an average in the nine experiments of 22.2. This is slightly higher than that cited for this same muscle in the earlier series. By fatigue the threshold of the nerve-muscle was increased from an average of 1.3 to an average of 3.3 units, an increase of 154 per cent. The muscle increased from an average of 22.2 to an average of 59.6, an increase of 169 per cent. After an injection of 0.1 to 0.5 cubic centimeters of adrenin (1:100,000) the fatigue threshold was decreased within five minutes in the nerve-muscle from an average of 3.3 to 1.8, a recovery of 75 per cent, and in the muscle from an average of 59.6 to 42.4, a recovery of 46 per cent. To prove that this effect of adrenin is a counteraction of the effects of fatigue, Gruber determined the threshold for muscle and nerve-muscle in non-fatigued animals before and after adrenin injection. He found that in these cases no lowering of threshold occurred, a result in marked contrast with the pronounced and prompt lowering induced by this agent in muscles when fatigued.

Figs. 19 and 20, plotted from the data of two of the experiments, show the relative heights of the threshold before and after an injection of adrenin. The close correspondence of the two readings of the threshold, one from the nerve supplying the muscle, the other from the muscle directly, served to show that there was no fault in the electrodes. The continuous line in the Figures represents the threshold (in units) of the muscle, the broken line that of the nerve-muscle. The threshold of the nerve-muscle is magnified 100 times in Fig. 19 and 10 times in Fig. 20. In Fig. 19 (at 2 and 4) the threshold was taken after an intravenous injection of 0.1 and 0.2 cubic centimeter of adrenin respectively.

These examples show that adrenin does not affect the threshold of the normal non-fatigued muscle when tested either on the muscle directly or on the nerve-muscle. In Fig. 19 (at 3) the observation taken after two hours of rest illustrates the constancy of the threshold under these circumstances.

In Fig. 19 the normal threshold was increased by fatigue (at 5)—the muscle had been pulling 120 times a minute for one hour on a spring having an initial tension of 120 grams—from 30.0 to 51.6 units, an increase of 72 per cent; and in the nerve-muscle from 0.62 to 0.89 units, an increase of 46 per cent. The threshold (at 6) was taken five minutes after injecting 0.1 cubic centimeter of adrenin (1:100,000). The threshold of the muscle was lowered from 51.6 to 38.0 units, a recovery of 62 per cent; that of the nerve-muscle from 0.89 to 0.79 units, a recovery of 37 per cent. After another injection of 0.5 cubic centimeter of adrenin the thresholds (at 7) were taken; that of the nerve-muscle dropped to normal—0.59 units—a recovery of 100 per cent, and that of the muscle remained unaltered—26 per cent above its normal threshold.

In Fig. 20 the threshold (at 5) was taken five minutes after an injection of 0.1 cubic centimeter of adrenin. The drop here was as large as that shown in Fig. 19. The threshold taken from the muscle directly was lowered from 30.6 to 18 units, a recovery of 61 per cent; the nerve-muscle from 1.08 to 0.87 units, a recovery of 51 per cent. That this sudden decrease cannot be due to rest is shown in the same Figure (at 3 and 4). These readings were made after 60 and 90 minutes’ rest respectively. The sharp decline in the record (at 5) indicates distinctly the remarkable restorative influence of adrenin in promptly lowering the high fatigue threshold of neuro-muscular irritability.

The Evidence that the Restorative Action of Adrenin is Specific

As stated in describing the effects of arterial blood pressure, an increase of pressure is capable of causing a decided lowering of the neuro-muscular threshold after fatigue. Is it not possible that adrenin produces its beneficial effects by bettering the circulation?

Nice and I had argued that the higher contractions of fatigued muscle, that follow stimulation or injection of adrenin, could not be wholly due to improved blood flow through the muscle, for when by traction on the aorta or compression of the thorax arterial pressure in the hind legs was prevented from rising, splanchnic stimulation still caused a distinct improvement, the initial appearance of which coincided with the point in the blood-pressure curve at which evidence of adrenal secretion appeared. And, furthermore, the improvement was seen also when adrenin was given intravenously in such weak solution (1:100,000) as to produce a fall instead of a rise of arterial pressure. Lyman and I had shown that this fall of pressure was due to a dilator effect of adrenin. Since the blood vessels of the fatigued muscle were dilated by severance of their nerves when the nerve trunk was cut, and, besides, as previously stated (see p. 86), were being stimulated through their nerves at a rate favorable to relaxation, it seemed hardly probable that adrenin could produce its beneficial effect by further dilation of the vessels and by consequent flushing of the muscle with an extra supply of blood.[3] The lowering of blood pressure had been proved to have no other effect than to impair the action of the muscle (see p. 103). Although the chances were thus against an interpretation of the beneficial influence of adrenin through action on the circulation, it was thought desirable to test the possibility by comparing its effect with that of another vasodilator—amyl nitrite.

Figs. 21 and 22 are curves obtained from the left tibialis anticus muscle. The rate of stimulation was 240 times a minute.

The muscle in Fig. 21 contracted against a spring having an initial tension of 120 grams, and that in Fig. 22 against an initial tension of 100 grams. In Fig. 21, at the point indicated on the base line, 0.4 cubic centimeter of adrenin (1:100,000) was injected into the left external jugular vein. There resulted a fall of 25 millimeters of mercury in the arterial pressure and a concurrent betterment of 15 per cent in the height of contraction, requiring two minutes and fifteen seconds of fatigue (about 540 contractions) before it returned to the former level. In Fig. 22, at the point indicated by the arrow, a solution of amyl nitrite was injected into the right external jugular vein. There resulted a fall of 70 millimeters of mercury in arterial pressure and a betterment of 4.1 per cent in the height of muscular contraction, requiring fifteen seconds of fatigue (about 60 contractions) to decrease the height of contraction to its former level. In neither case did the blood pressure fall below the critical region (see p. 104).[*]

Although the fall in arterial pressure caused by dilation of the vessels due to amyl nitrite was almost three times as great as that produced by the adrenin, yet the resultant betterment was only about one-fourth the percentage height and lasted but one-ninth the time. In all cases in which these solutions caused an equal fall in arterial pressure, adrenin caused higher contractions, whereas amyl nitrite caused no appreciable change.

The Point of Action of Adrenin in Muscle

From the evidence presented in the foregoing pages it is clear that adrenin somehow is able to bring about a rapid recovery of normal irritability of muscle after the irritability has been much lessened by fatigue, and that the higher contractions of a fatigued muscle after an injection of adrenin are due, certainly in part, to some specific action of this substance and not wholly to its influence on the circulation. Some of the earlier investigators of adrenal function, notably Albanese,[4] and also Abelous and Langlois,[5] inferred from experiments on the removal of the glands that the rôle they played in the bodily economy was that of neutralizing, destroying or transforming toxic substances produced in the organism as a result of muscular or nervous work. It seemed possible that the metabolites might have a checking or blocking influence at the junction of the nerve fibres with the muscle fibres, and might thus, like curare, lessen the efficiency of the nerve impulses. Radwánska’s observation[6] that the beneficial action of adrenin is far greater when the muscle is stimulated through its nerve than when stimulated directly, and Panella’s discovery[7] that adrenin antagonizes the effect of curare, were favorable to the view that adrenin improves the contraction of fatigued muscle by lessening or removing a block established by accumulated metabolites.

The high threshold of fatigued denervated muscle, however, Gruber found was quite as promptly lowered by adrenin as was that of normal muscles stimulated through their nerves. Fig. 23 shows that the height of contraction, also, of the fatigued muscle is increased when adrenin is administered. In this experiment the left tibialis anticus muscle was stimulated directly by thrusting platinum needle electrodes into it. The peroneus communis nerve supplying the muscle had been cut and two centimeters of it removed nine days previous to the experiment. The rate of stimulation was 120 times per minute and the initial tension of the spring about 120 grams. At the point indicated by the arrow an injection of 0.1 cubic centimeter of adrenin (1:100,000) was made into a jugular vein. A fall in arterial pressure from 110 to 86 millimeters of mercury and a simultaneous betterment of 20 per cent in the height of contraction were obtained. It required four minutes of fatigue (about 480 contractions) to restore the muscle curve to its former level. Results similar to this were obtained from animals in which the nerve had been cut 7, 9, 12, 14, and 21 days. In all instances the nerve was inexcitable to strong faradic stimulation.

In Radwánska’s experiments, mentioned above, the muscle was stimulated directly when the nerve endings were intact. It seems reasonable to suppose, therefore, that in all cases he was stimulating nerve tissue. Since a muscle is more irritable when stimulated through its nerve than when stimulated directly (nerve and muscle), a slight change in the irritability of the muscle by adrenin would naturally result in a greater contraction when the nerve was stimulated. Panella’s results also are not inconsistent with the interpretation that the effect of adrenin is on the muscle substance rather than on the nerve endings. A method which has long been used to separate muscle from nerve is that of blocking the nervous impulses by the drug curare. Gruber found that when curare is injected the threshold of the normal muscle is increased as was to be expected from the removal of the highly efficient nervous stimulations. And also, as was to be expected on that basis, curare did not increase the threshold in a muscle in which the nerve endings had degenerated. Adrenin antagonizes curare with great promptness, decreasing the heightened threshold of a curarized muscle, in five minutes or less, in some cases to normal. From this observation it might be supposed that curare and fatigue had the same effect, and that adrenin had the single action of opposing that effect. But fatigue raises the threshold of a curarized muscle, and adrenin then antagonizes this fatigue. Langley[8] has argued that curare acts upon a hypothetical “receptive substance” in muscle. If so, probably curare acts upon a substance, or at a point, different from that upon which fatigue acts; for, as the foregoing evidence shows, fatigue increases the threshold of a muscle whether deprived of its nerve supply by nerve section and degeneration or by curare, whereas curare affects only the threshold of a muscle in which the nerve endings are normal.[9] And since adrenin can oppose the effects of both curare and fatigue, it may be said to have two actions, or to act on two different substances or at two different points in the muscle.

The evidence adduced in the last chapter indicated that the greater “head” of arterial pressure produced by the more rapid heart beat and the stronger contraction of many arterioles in times of great excitement would be highly serviceable to the organism in any extensive muscular activity which the excitement might involve. By assuring an abundant flow of blood through the enlarged vessels of the working muscle, the waste products resulting from the wear and tear in contraction would be promptly swept away and thus would be prevented from impairing the muscular efficiency. The adrenin discharge at such times would, as was pointed out, probably reënforce the effects of sympathetic impulses. The evidence presented in this chapter shows that adrenin has also another action, a very remarkable action, that of restoring to a muscle its original ability to respond to stimulation, after that has been largely lost by continued activity through a long period. What rest will do only after an hour or more, adrenin will do in five minutes or less. The bearing of this striking phenomenon on the functions of the organism in times of great need for muscular activity will be considered in a later discussion.

REFERENCES