Since the early 19th century, scientists have puzzled over the factors that regulate metabolic rate. At first people suspected that an animal's metabolic rate was directly proportional to its surface area, but it became clear more recently that a creature's metabolic rate was proportional to a value close to its mass raised to the power 0.75. Although the factors that govern this convenient relationship still aren't known, Paul Else and Tony Hulbert in Wollongong, Australia, suggest that an animal's lipid membrane composition might act as a metabolic pacemaker to regulate their metabolic rate. Having previously found a strong relationship between the degree of membrane polyunsaturation and the metabolic rate of mammals, Hulbert was curious to find whether birds' metabolic rates also correlate with their lipid constituents. After all, both class of vertebrate evolved endothermy independently, so if both vertebrate's membrane compositions were highly correlated with their metabolic rates, maybe membrane composition would be the essential regulator that sets all endotherm's metabolic rates. Knowing that larger birds' lipid membranes tended to be composed of monounsaturated fatty acids, while smaller birds with high metabolic rates produce membranes with high levels of polyunsaturated fatty acids, Else and Hulbert have investigated the metabolic rate of hepatocytes from birds ranging in size from 13 g up to 35 kg, and found that small birds have higher oxygen consumption rates than large birds (p. 2305).
Fortunately, collecting birds that covered the size range the team needed wasn't too difficult; Australia boasts some of the largest birds on the planet. Else, Hulbert and Martin Brand bought some species from local suppliers and trapped others from the wild. But the emus were much less cooperative. Hulbert had to convince the huge birds to sit down by leaping on their backs before they could be safely secured in a wool sac and driven to Wollongong. Only then were the team were ready to start measuring tissue metabolic rates.
Extracting isolated hepatocytes from liver samples, they found that the larger birds' mass-specific metabolic rates were lower than the smaller species. The team also measured the relative sizes of the birds' livers and found that livers from the smaller species were proportionately larger than those from the goose and emu. Not only was their metabolic rate higher, but the tissue comprised a larger fraction of their body mass, contributing to the tiny birds' high metabolic rate.
Next the team wondered which cellular metabolic processes were increased to raise the smaller birds' metabolic rates. By systematically inhibiting mitochondrial ATP synthesis and the mitochondria's electron transport chain,they measured the hepatocytes' oxygen consumption rate and found that the cells always used the same proportion of energy for ATP production, proton leak, and other cellular processes, regardless of the animal's size. Like mammals, small birds increase all of their metabolic processes to raise their metabolic rate.
Although this work goes some way to support Else and Hulbert's metabolic pacemaker theory, they emphasise that this is one aspect of a much larger investigation into the metabolic effects of membrane composition, which may one day explain one of the most fundamental relationships in physiology.