Heat work and phosphorylcreatine break-down in muscle.

1. A new instrument, the integrating thermopile, is described for measuring the total quantity of heat produced during muscular contraction.2. This instrument has been used to investigate the relation between change of enthalpy (- (heat produced + work produced)) and break-down of phosphorylcreatine (DeltaPC) in iodoacetate-poisoned frog sartorii at 0 degrees C. In a variety of different types of contraction-series of isometric twitches, isometric tetani, contractions with positive and with negative work-the relation between enthalpy change and DeltaPC was always the same, and corresponded to an in vivo molar enthalpy change (DeltaH) of -11.0 +/- 0.23 (S.E.; n = 52) kcal/mole.3. This value of DeltaH is used to estimate the in vivo DeltaH for ATP splitting and also the number of rephosphorylations to be expected per hexose unit oxidized by normal unpoisoned muscle.