HEAT, WORK AND CHEMICAL CHANGE IN MUSCLE.

Abstract

The relation between chemical analysis and measurement of heat production as methods for investigating muscular contraction is best illustrated by a simple example. During the single twitch of a frog’s muscle about 3 mcal/g of heat is liberated. At the same time, approximately 0.3 μmole/g of phosphocreatine ( PC ) are broken down—a total of perhaps 0.03 μmole in a whole sartorius. This change, superimposed as it is on a normal PC level of about 25 μmole/g, is at the limit of what can be detected chemically, whereas the temperature rise of 3 x 10 -3 degC can be measured quite easily. Against the high sensitivity of heat measurements must be set the disadvantage that heat measurements are entirely non-specific; the heat observed may arise from one or from several chemical reactions; it may represent degradation of chemical energy or merely the heat movement that results from entropy changes. Heat measurements thus complement chemical analysis, which is generally specific but in consequence adapted to detect only those reactions whose existence is already suspected. One can only be reasonably sure that all important reactions are being detected if the sum of their heats of reaction tallies with the heat production actually observed. For this accounting, only the first law of thermodynamics is needed: measured h eat + w ork = n ( - ∆ H ) for one reaction, h + w = ∑ a k n j . ( - ∆ H j ) for several reactions. -∆ H is the heat (strictly enthalpy) of reaction in kcal/mole; n is the number of moles of each reaction. Only when we have a fairly complete balance sheet of this kind shall we be able to move on to the more interesting question of the exact way in which the muscle functions as a thermodynamic machine. This is the realm of the second law; it is necessary to know the free energies of the reactions as well as their heats. At present, such information is lacking, so that even the efficiency of conversion of chemical into mechanical energy during contraction cannot be calculated. Likewise, it is not clear at present how much of the recovery heat represents waste of energy, i.e. entropy production, and how much of it merely arises from entropy exchanges between the reactants and the environment. Such evidence as there is suggests that entropy exchange is quite important and in consequence that recovery is more efficient than it appears superficially to be.