Of ATP

The chemical reactions occurring in muscle generate heat that is vital for maintaining body temperature. Inversely, measuring heat in various phases of muscle contraction indicate the existence of exothermic chemical reactions. A.V. Hill has initiated sensitive and fast heat measurements at the beginning of the 20th century. The book of Carlson and Wilkie (1974) describes in detail the heat studies on muscle.

Fig. EN6 illustrates the two phases of heat production during isometric muscle contraction. The activation heat appears in a burst soon after activation of muscle and declines before the full force is reached. It parallels changes in Ca2+ concentration in the sarcoplasm. The maintenance heat parallels the force produced. This corresponds to the steady state of ATP hydrolysis catalyzed by actomyosin.

Fig. EN6. Heat production during a single isometric twitch of frog sartorius at 0o C. (From Needham, 1971). Full line, heat analysis; broken line, force curve. Not shown on the Figure is the recovery heat that occurs after contraction. This represents the recovery of energy sources.

During isotonic contraction more heat is liberated than during isometric contraction. Fenn called this extra heat the shortening heat. Fig. EN7 shows that the shortening heat is proportional to the shortening of the muscle; the larger the shortened distance, the more extra heat is produced (this is called the Fenn effect).

Fig. EN7. The Fenn effect. (From Carlson and Wilkie, 1974). Curve A, only isometric contraction.

Curves B-D, the muscle was contracting isometrically until it was released and allowed to shorten various distances.

Fig. EN7. The Fenn effect. (From Carlson and Wilkie, 1974). Curve A, only isometric contraction.

Curves B-D, the muscle was contracting isometrically until it was released and allowed to shorten various distances.

The relationship between energy output and chemical breakdown: If a muscle is completely deprived of oxygen by keeping it in pure nitrogen, and if the formation of lactic acid is prevented by applying iodoacetate, the inhibitor of glycolysis, then it is thought that only one net reaction occurs, the hydrolysis of PCr. Experimentally, the output of heat + work is directly proportional to the breakdown of PCr under many different conditions of contraction (Fig. eN8). The energy equivalent of PCr breakdown in the intact muscle was found to be 11 kcal/mol (the slope of the curve); this is in good agreement with the heat value of PCr in the test tube. Accordingly, muscle fulfills the laws of thermodynamics: it transforms mechanical energy to heat energy (first law) and it does not convert heat into work at constant temperature (second law).

Fig. EN8. Energy output and chemical breakdown. (From Carlson and Wilkie, 1974).

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