Die Beziehungen zwischen der Wärmebildung und den im Muskel stattfindenden chemischen Prozessen

A century of exercise physiology: key concepts on coupling respiratory oxygen flow to muscle energy demand during exercise

After a short historical account, and a discussion of Hill and Meyerhof's theory of the energetics of muscular exercise, we analyse steady-state rest and exercise as the condition wherein coupling of respiration to metabolism is most perfect. The quantitative relationships show that the homeostatic equilibrium, centred around arterial pH of 7.4 and arterial carbon dioxide partial pressure of 40 mmHg, is attained when the ratio of alveolar ventilation to carbon dioxide flow ([Formula: see text]) is - 21.6. Several combinations, exploited during exercise, of pertinent respiratory variables are compatible with this equilibrium, allowing adjustment of oxygen flow to oxygen demand without its alteration. During exercise transients, the balance is broken, but the coupling of respiration to metabolism is preserved when, as during moderate exercise, the respiratory system responds faster than the metabolic pathways. At higher exercise intensities, early blood lactate accumulation suggests that the coupling of respiration to metabolism is transiently broken, to be re-established when, at steady state, blood lactate stabilizes at higher levels than resting. In the severe exercise domain, coupling cannot be re-established, so that anaerobic lactic metabolism also contributes to sustain energy demand, lactate concentration goes up and arterial pH falls continuously. The [Formula: see text] decreases below - 21.6, because of ensuing hyperventilation, while lactate keeps being accumulated, so that exercise is rapidly interrupted. The most extreme rupture of the homeostatic equilibrium occurs during breath-holding, because oxygen flow from ambient air to mitochondria is interrupted. No coupling at all is possible between respiration and metabolism in this case.

The heat liberated by the beating heart

Since myothermic methods have been improved several attempts have been made to measure the heat liberated by the beating heart. Two observers, both experienced in the difficulties and limits of myothermic experiments, A. Y. Hill (1) and K. Bürker (2), were unable to decide how far their results showed the real change of temperature of the heart, and how far they were the effects of several sources of error. On the other hand, Herlitzka (3), Snyder (4), and Bohnenkamp (5) believed themselves entitled to draw important conclusions from their experimental results. At the suggestion, therefore, of Prof. A. V. Hill, I undertook to investigate the heat production of the heart with the various means available in his laboratory. Since, as will be shown below, my results were not in accordance with those of the last named investigators, I was forced to compare my methods with theirs. According to the most reliable of my observations I believe the warming of the heart (eel or tortoise) for a single beat to be not higher than 0·00012°. For the heart of the rabbit Herlitzka found a rise of 0·004° to 0·012°; for the terrapin’s heart Snyder found 0·00075° to 0·00255°: and for the frog’s heart Bohnenkamp found 0·001 to 0·01°. Apparatus and Methods. Galvanometer .—The galvanometer employed was designed and constructed by A. C. Downing (6). The “figure of merit” of this instrument (23,000) permitted the use of a high sensitivity, without making the deflection time when critically damped too great. With a sensitivity from 1·2 X 10 -9 to 1·3 X 10 -10 amps. per mm. the galvanometer had a complete period from 0·6 to 2·5 secs. The four coils, each of resistance 6·6 ohms, could be placed in series, or parallel, or series-parallel, so that the resistance of the galvanometer as a whole could be made to approximate to that of the thermopile, or thermocouple, used in the different experiments. The galvanometer was protected by a double shield of high permeability steel, as described by Hartree and Hill (7). Records were made photographically, as described by them, but with the difference that the distance of the drum was 1·30 metres and that the light from the lamp was interrupted each half-second by a shutter carried on an electro-magnet actuated by a half-second pendulum.

The free energy of glycogen-lactic acid breakdown in muscle

It is the purpose of the present discussion to show, upon the basis of thermodynamic data obtained within the last four of five years, that the free energy of glycogen-lactic acid breakdown in muscle is considerably greater than the heat of reaction, about one and one-half to two times. It is the intention to outline merely the orders of magnitude of the various quantities involved in the evaluation of this difference. This evaluation, as will be shown, need not depend upon a knowledge of the actual heat of reaction, which is still in dispute, varying between Meyerhof's value of —180 cal. and Slater's value of —235 cal. It will depend, rather, upon the specific heat differences, or ultimately, molecular structure differences, obtaining between glycogen and lactic acid. Stated briefly, the existence of this large negative difference, designated hereafter as (ΔF — ΔH), implies that the theoretical maximum mechanical work which a muscle can perform as a consequence of this breakdown is considerably greater than the corresponding heat of reaction. The notations of Lewis and Randall (1) will be used throughout. ΔH, the heat of reaction, and ΔF, the free energy of reaction, will be negative when heat and free energy respectively are liberated. Before presenting the thermodynamic data and calculations, it will be it historical interest to point out that in 1912 A. V. Hill (2) suggested the possibility of such a difference, when he first made the observation that during anaerobic lactic acid formation in muscle the heat evolved amounted to at least three times as much as would have been predicted if the precursor were a hexose carbohydrate. He suggested, "the breakdown from this body to lactic acid may be one of those somewhat rare but by no means unknown chemical reactions which can do more mechanical work than is equivalent to their total loss of energy; by virtue of their completeness they possess the power of absorbing heat from their surroundings to do this excess of work." Meyerhof (3, 1922) reconsidered the question, and while alive to the possibility of a considerable difference, offered an opinion, based upon the Nernst heat theorem, that probably no difference did exist. In general, however, little attention has been paid to A. V. Hill's original surmise, especially since further investigation of the other hydrolysis, neutralization, and deionization reactions occurring simultaneously with the formation of lactic acid, has shifted the attention to explaining the other more immediate problem, namely, the discrepancy between the observed chemical change and the required evolution of heat.