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First published online May 24, 2004
Journal of Experimental Biology 207, 2305-2311 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01017

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Respiration rate of hepatocytes varies with body mass in birds

Paul L. Else^1,*, M. D. Brand³, N. Turner¹ and A. J. Hulbert²

¹ Department of Biomedical Science, Metabolic Research Centre, University of Wollongong, Wollongong, NSW 2522, Australia
² Department of Biological Science, Metabolic Research Centre, University of Wollongong, Wollongong, NSW 2522, Australia
³ MRC-Dunn Human Nutrition Unit, Hills Road, Cambridge CB2 2XY, UK

^* Author for correspondence (e-mail: pelse{at}uow.edu.au)

Accepted 7 April 2004

Hepatocytes were isolated from eight species of birds ranging from 13 g zebra finches to 35 kg emus. This represents a 2800-fold range of body mass (M_b). Liver mass (g) was allometrically related to species body mass by the equation: liver mass=19.6xM_b^0.91. There was a significant allometric decline in hepatocyte respiration rate (HRR; nmol O₂ mg^–1 dry mass min^–1) with species body mass (kg) described by the relationship: HRR=5.27xM_b^–0.10. The proportions of hepatocyte oxygen consumption devoted to (i) mitochondrial ATP production, (ii) mitochondrial proton leak and (iii) non-mitochondrial processes were estimated by using excess amounts of appropriate inhibitors. It was found that although hepatocyte respiration rate varied with body mass in birds, these processes constitute a relatively constant proportion of hepatocyte metabolic rate irrespective of the size of the bird species. The respective percentages were 54%, 21% and 25%. The portion of hepatocyte respiration devoted to ATP production for use by the sodium pump was estimated and found to be a relatively constant 24% of hepatocyte respiration and 45% of mitochondrial ATP production in different-sized bird species. These results are discussed in the context of competing theories to explain the metabolism–body size allometry, and are found to support the `allometric cascade' model.

Key words: allometry, body mass, metabolism, proton leak, sodium pumping, bird, hepatocyte

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