Relationship between mitochondria and oxygen consumption in isolated cat muscles.

1. Oxygen consumption, mitochondrial content and composition, intracellular lipid stores and fibre size were studied in isolated cat muscles: predominantly glycolytic gracilis, purely oxidative soleus and gracilis transformed into an oxidative muscle by chronic low-frequency (10 Hz) electrical stimulation. 2. Oxygen consumption in control gracilis at rest (0.303 +/- 0.050 ml O2 min-1 100 g-1; mean +/- S.E. of mean) was three to five times lower than in either stimulated gracilis (1.16 +/- 0.40) or soleus (1.57 +/- 0.56); it was about two times lower during maximal contractions in control gracilis (5.15 +/- 0.24) than in either stimulated gracilis (11.6 +/- 2.0) or soleus (9.34 +/- 0.78). 3. The volume density of mitochondria in control gracilis (2.75 +/- 0.12%) was half that of soleus (6.23 +/- 0.76) and only one-third that of stimulated gracilis (8.35 +/- 0.71). Subsarcolemmal mitochondria represented a significantly smaller fraction of the total mitochondrial volume in control gracilis than in either soleus or stimulated gracilis. 4. The surface area of inner and outer mitochondrial membranes per unit volume of mitochondria ranged from 23.4 to 26.1 and from 14.0 to 16.5 m2 cm-3, respectively. Mean values of these variables were not significantly different among experimental groups. 5. The volume density of the intracellular lipid stores in control gracilis (0.232 +/- 0.041%) was one-fourth of that in stimulated gracilis (0.860 +/- 0.12) and one-fifth of that in soleus (1.17 +/- 0.27). 6. The fibre cross-sectional area was 1670 +/- 260 micron 2 in control gracilis, 2250 +/- 280 in stimulated gracilis and 2390 +/- 110 in soleus. The difference was statistically significant only between control gracilis and soleus. 7. There was a significant correlation between the volume density of mitochondria and maximal oxygen consumption for all three muscles combined. 8. It was found that mitochondrial structure was similar in muscles with different oxidative capacities and that equal amounts of mitochondria consumed equal amounts of oxygen under limiting conditions of maximal in vivo respiration.