An Explanation of the Effects of Varying the Arterial Pressure
How are these effects of increasing and decreasing the arterial blood pressure most reasonably explained? There is abundant evidence that fatigue products accumulate in a muscle which is doing work, and also that these metabolites interfere with efficient contraction. As Ranke[3] long ago demonstrated, if a muscle, deprived of circulating blood, is fatigued to a standstill, and then the circulation is restored, the muscle again responds for a short time to stimulation, because the waste has been neutralized or swept away by the fresh blood. When the blood pressure is at its normal height for warm-blooded animals (about 120 millimeters of mercury, see [Fig. 13]), the flow appears to be adequate to wash out the depressive metabolites, at least in the single muscle used in these experiments, because a large rise of pressure produces but little change in the fatigue level. On the other hand, when the pressure is abnormally low, the flow is inadequate, and the waste products are permitted to accumulate and clog the action of the muscle. Under such circumstances a rise of pressure has a very striking beneficial effect.
It is noteworthy that the best results of adrenin on fatigued muscle reported by previous observers were obtained from studies on cold-blooded animals. In these animals the circulation is maintained normally by an arterial pressure about one-third that of warm-blooded animals. Injection of adrenin in an amount which would not shut off the blood supply would, by greatly raising the arterial pressure, markedly increase the circulation of blood in the active muscle. In short, the conditions in cold-blooded animals are quite like those in the pithed mammal with an arterial pressure of about 50 millimeters of mercury (see [Fig. 16]). Under these conditions the improved circulation causes a remarkable recovery from fatigue. That notable results of adrenin on fatigue are observed in warm-blooded animals only when they are deeply anesthetized or are deprived of the medulla was claimed by Panella.[4] He apparently believed that in normal mammalian conditions adrenin has little effect because quickly destroyed, whereas in the cold-blooded animals, and in mammals whose respiratory, circulatory, and thermogenic states are made similar to the cold-blooded by anesthesia or pithing, the contrary is true. In accordance with our observations of the effects of blood pressure on fatigued muscle, we would explain Panella’s results not as he has done but as due to two factors. First, the efficiency of the muscle, when blood pressure is low, follows the ups and downs of pressure much more directly than when the pressure is high. And second, a given dose of adrenin always raises a low blood pressure in atonic vessels. The improvement of circulation is capable of explaining, therefore, the main results obtained in cold-blooded animals and in pithed mammals.
Oliver and Schäfer reported unusually effective contractions in muscles removed from the body after adrenal extract had been injected. As shown in [Fig. 16], however, the fact that the circulation had been improved results in continued greater efficiency of the contracting muscle. Oliver and Schäfer’s observation may reasonably be accounted for on this basis.