Direct calorimetry exposes inadequacies of respirometry in the measurement of resting metabolic rate

Abstract

Resting metabolic rate (RMR) in animals is frequently estimated by respirometry, but the dependence of this method upon untested or untestable assumptions (such as negligible anaerobic metabolism, simultaneously measured or negligible protein oxidation, and no substrate inter‐conversion) render respirometry inappropriate or inadequate for examining novel genetically‐manipulated animal models. A Seebeck‐style direct calorimeter apparatus was constructed to allow simultaneous measurement of total heat dissipation and respirometry gasses from unrestrained mice. Double‐transgenic “sRA” mice exhibit elevated brain renin‐angiotensin system activity by expression of human renin via the synapsin promoter and human angiotensinogen via its own promoter. Respirometry underestimated the RMR in sRA mice by 16% (n=6, male, 11.0±0.6 wks, 20.97±0.68 g; 0.889±0.036 vs 1.057±0.034 kcal/100g lean/hr, P=0.008), and by 14% in control littermates and C57Bl/6J mice (n=10, male, 11.6±0.5 wks, 27.02±0.60 g; 0.712±0.025 vs 0.828±0.018 kcal/100g lean/hr, P=0.005). While respirometry faithfully detected the 26% increase in sRA RMR vs control (P<0.001 by both methods), the magnitude of RMR estimation error by respirometry was similar (~15%). Reanalysis of various other genetically‐manipulated models by direct calorimetry may therefore uncover previously unappreciated physiological regulators of RMR.