Capillary and mitochondrial unit in muscles of a large lizard

We asked whether capillaries and mitochondria form a structural and functional unit in the musculature of the Cuban iguana (Cyclura nubila) similar to that found in mammals. We found a significant correlation between capillary length density [J(V)(c,f)] and mitochondrial volume density [V(V)(mt,f)] of the musculature with a slope that revealed that on average 3.5 km of capillaries were associated with each milliliter of mitochondria (vs. ~ 11 km/ml in mammals). These capillaries had a diameter of 9 μm (vs. 4.5 μm in mammals), and the mitochondria had a surface density of the inner membranes of 25 m2/ml (vs. 30-45 m2/ml in mammals). These dimensions resulted in ratios of capillary to mitochondrial volume (0.22 ml/ml) and capillary wall to mitochondrial membrane surface area (39 cm2/m2) that were similar in Cyclura to those found in mammals (~ 0.18 ml/ml and 35-52 cm2/m2, respectively). Also in agreement with mammalian values were the average oxidative capacity of the mitochondria derived from maximum rate of O2 consumption (V̇O2(max)) during exercise at 37°C and the inner mitochondrial membrane surface area [S(im)] of the musculature [V̇O2(max)/S(im) = 0.04 vs. 0.06-0.15 ml O2 · m-2 · min-1 in mammals]. These common structural and functional relationships support the notion that capillaries and mitochondria represent a similar fundamental unit in muscles of both Cyclura and mammals.