enzymatic basis of high metabolic rates in calling frogs

• Online Access: http://hdl.handle.net/2047/d20000610

Oxygen consumption by male spring peepers (Hyla crucifer) increased linearly with calling rate, reaching peak values of 1.5−1.7 ml O₂/(g⋅h) at the highest calling rates. The intercept of the regression line describing the relationship between metabolism and calling rate does not differ significantly from daytime resting metabolism (0.11 ml O₂/[g⋅h]). Metabolic rate during vigorous locomotor exercise at the same temperature (19 C) was only 1.1 ml O₂/(g⋅h). We measured activities of mitochondrial enzymes in the trunk muscles (internal and external obliques) involved in sound production and in mixed hind limb muscles of male and female frogs. Male trunk muscles were very large, accounting for 15% of total body mass, whereas female trunk muscle constituted only 3% of total body mass. Citrate synthase (CS) activity in male trunk muscle, indicative of oxidative capacity, was six times the CS activity in leg muscle (86 vs. 14 μmol/[min ⋅ g fresh muscle at 20 C]) and 17 times the CS activity in female trunk muscle (5 μmol/[min ⋅ g fresh muscle]). The capacity to oxidize fat, as indicated by β-hydroxyacyl-CoA dehydrogenase activity, was five times higher in male trunk muscle than in leg muscle (30 vs. 6 μmol/[min ⋅ g fresh muscle]), suggesting that fatty acid oxidation plays an important role in the energetics of vocalization in this species. Phosphofructokinase activity, a key glycolytic marker, was not significantly different in trunk and leg muscle. The capacity of male spring peeper trunk muscles for aerobic metabolism exceeds considerably the highest values yet reported for ectothermic vertebrate muscle tissue and is comparable to highly oxidative muscles of endotherms.