Heart mitochondrial properties and aerobic capacity are similarly related in a mammal and a reptile

The heart mitochondrial properties and the aerobiccapacity (V˙O∑max) of the rat (Sprague-Dawley breed) andthe Cuban iguana (Cyclura nubila) were used to evaluatethe relationship between the oxidative capacity of the heartand the maximum oxygen delivery rate. Both species areactive at body temperatures of 37–39 ˚C, have similar heartmitochondrial volumes [Vmt; 0.43±0.02 ml (S.E.M.) in the ratand 0.48±0.02 ml in the iguana] and differ less than twofoldin V˙O∑max (29.2±1.6 and 16.9±0.6 ml min21, respectively).We found that Vmt was closely correlated with V˙O∑max inthe rat (r2=0.77, P<0.005) and the iguana (r2=0.82;P<0.001). Furthermore, the inner mitochondrialmembrane (cristae) area (Sim) per unit V˙O∑max did not differbetween the rat and the iguana (0.60±0.02 and 0.71±0.02 m2min ml21O2, respectively). This correspondence ofSim/V˙O∑max indicates that the rat and the iguana have thesame cardiac oxidative capacity at the maximum oxygendelivery rate. These results suggest that, despite thedifferences between the cardiovascular systems of thesespecies, the cardiac cost of delivering oxygen at the aerobiccapacity is similar in this mammal and this reptile.