The energy output of normal and anoxic cardiac muscle.

A myothermic technique has been used to investigate the energy output of rabbit papillary muscles that were either hypoxic or metabolically inhibited by iodoacetic acid and nitrogen. At room temperature, 18–22°C, hypoxia reduced resting heat production from a mean of 30.1 to 18.4 mcal/g muscle · min. Tension development fell from 3.29 to 2.48 (g) and work output under a 1.0 g load decreased from 0.069 to 0.044 g.cm. The mean isometric heat coefficient rose from 3.6 to 5.3. 30 min exposure to 0.5 mM IAA followed by 10 min of anoxia, oxygen tension < 2 mm Hg, caused the resting heat production to fall from a mean of 29.8 to 16.5 mcal/g muscle · min and developed tension to fall from 3.18 to 1.97 g. The total heat per isometric contraction fell from a mean of 2.5 to 0.83 mcal/g muscle and the activation heat decreased from 0.48 to 0.18 mcal/g muscle. The mean isometric heat coefficient rose from 3.7 to 6.7. Evidence is presented that the fast phase of heat production associated with the contractile event is unchanged by hypoxia or anoxia and this phase may therefore be taken to be the equivalent of skeletal muscle initial heat.