The oxygen consumption of the frog's heart: I

Cardiac efficiency and Starling's Law of the Heart

The formulation by Starling of The Law of the Heart states that 'the [mechanical] energy of contraction, however measured, is a function of the length of the muscle fibre'. Starling later also stated that 'the oxygen consumption of the isolated heart … is determined by its diastolic volume, and therefore by the initial length of its muscular fibres'. This phrasing has motivated us to extend Starling's Law of the Heart to include consideration of the efficiency of contraction. In this study, we assessed both mechanical efficiency and crossbridge efficiency by studying the heat output of isolated rat ventricular trabeculae performing force-length work-loops over ranges of preload and afterload. The combination of preload and afterload allowed us, using our modelling frameworks for the end-systolic zone and the heat-force zone, to simulate cases by recreating physiologically feasible loading conditions. We found that across all cases examined, both work output and change of enthalpy increased with initial muscle length; hence it can only be that the former increases more than the latter to yield increased mechanical efficiency. In contrast, crossbridge efficiency increased with initial muscle length in cases where the extent of muscle shortening varied greatly with preload. We conclude that the efficiency of cardiac contraction increases with increasing initial muscle length and preload. An implication of our conclusion is that the length-dependent activation mechanism underlying the cellular basis of Starling's Law of the Heart is an energetically favourable process that increases the efficiency of cardiac contraction. KEY POINTS: Ernest Starling in 1914 formulated the Law of the Heart to describe the mechanical property of cardiac muscle whereby force of contraction increases with muscle length. He subsequently, in 1927, showed that the oxygen consumption of the heart is also a function of the length of the muscle fibre, but left the field unclear as to whether cardiac efficiency follows the same dependence. A century later, the field has gained an improved understanding of the factors, including the distinct effects of preload and afterload, that affect cardiac efficiency. This understanding presents an opportunity for us to investigate the elusive length-dependence of cardiac efficiency. We found that, by simulating physiologically feasible loading conditions using a mechano-energetics framework, cardiac efficiency increased with initial muscle length. A broader physiological importance of our findings is that the underlying cellular basis of Starling's Law of the Heart is an energetically favourable process that yields increased efficiency.

XXII.—The Metabolism of the Frog's Isolated Heart

Theories regarding the chemistry of the contraction process in striped muscle have been revolutionised during the last year. The Hill-Meyerhof theory was based on the assumption that the contraction process was due to the liberation of lactic acid by the break-down of glycogen, a non-oxidative reaction, and that the recovery process consisted in the oxidation of part of the lactic acid and the resynthesis of glycogen from the remainder. Meyerhof now believes (1, p. 305) that contraction is produced by the break-down of creatin-phosphoric acid (phosphagen), and that the energy produced by carbohydrate break-down is utilised to resynthesise phosphagen.

Contraction and action potentials of frog heart muscles soaked in sucrose solution

Isolated auricles or ventricles from the frog continue to contract, either spontaneously or when stimulated, for from 2 to 4 hours after they are placed in isotonic sucrose solution. After the muscles stop contracting in sucrose solution, contractility is partially restored when the muscles are placed in chloride Ringer's. However, contractility is usually not restored if the muscles are placed in sulfate Ringer's. Ventricles soaked in sucrose solution at 4–7°C continue to contract for 12 to 24 hours and during the first few hours in sucrose solution the contractions often are enhanced. Several types of experiment indicate that the sucrose solution does replace the Ringer's in the extracellular space. Auricles and ventricles also continue to conduct action potentials, with an overshoot, for from 30 to 360 minutes after being placed in sucrose solution. Muscles soaked in sucrose until they are inexcitable rapidly recover in chloride Ringer's but often fail to recover in sulfate Ringer's. The results are discussed in relation to theories about the generation of the action potential in cardiac muscle, and the role of the extracellular fluid in contraction.

The energy liberated by an isolated muscle during the performance of work

The investigation of Fenn (1) (2) established the fundamental fact that the total energy liberated by a stimulated muscle depends upon the amount of work which it does, and also upon the manner, and the phase of contraction, in which that work is performed. The extreme importance of Fenn's results, in any theory of muscular contraction, has long made a further study of the subject desirable. This, and the development of various improvements in technique, particularly of the ergometer of Levin and Wyman (3) which we used in the experiments described in a recent paper (9), have caused us during the last 18 months make the experiments described below; these have led us to a new standpoint in relation to the thermodynamics of muscle. We had no difficulty in confirming Fenn's most important conclusion, that the performance of work by a muscle causes a considerable increase in the total energy liberated, but with the provision that the contraction is of a tetanic nature . Most unexpectedly, however, we found that in a single twitch the total energy is the same whether work be done or not . This point was clearly of great theoretical significance. One statement of Fenn's, moreover, we were unable to verify, viz., that work done during the phase of relaxation causes a decrease in the total energy set free. Finding it impossible to explain this disagreement, and wishing to confirm our observation that the energy liberated in a twitch is unaffected by work done, we made a further series of 34 complete experiments, of which 23 employed the thermopile wound on a silver-frame described recently (11). By using this thermopile we hoped to eliminate any possibility of temperature differences along the length of the shortening muscle. The second series of experiments confirmed the first.

The individual effects of CO2, bicarbonate and pH on the electrical and mechanical activity of isolated rabbit auricles

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