David and Goliath: A Mitochondrial Coupling Problem?

Abstract

An organism's size, known to affect biological structures and processes from cellular metabolism to population dynamics, depends upon the duration and rate of growth. However, it is still poorly understood how mitochondrial function affects the energetic basis of growth, especially in ectotherms, which represent a huge majority of animal biodiversity. Here, we present an intraspecies comparison of neighboring populations of frogs (Rana temporaria) that have large differences in body mass even at the same age. By investigating liver mitochondrial bioenergetics, we find that frogs with high growth rates and large body sizes exhibit higher ATP synthesis rates and more efficient oxidative phosphorylation compared to the smaller frogs with low growth rates. This higher energy transduction efficiency is not associated with significant increased oxidative capacity or membrane potential values, but instead may rely on a higher mitochondrial phosphorylation system activity in combination with a lower inner membrane proton leakage. Overall, the present study introduces the mitochondrial energy transduction system as an important mechanism for balancing physiological and ecological trade-offs associated with body size. Whether phenotype differences in mitochondrial function result from local ecological constraints or reflect a natural genetic variability within wild populations of common frogs remains an open question. However, our findings highlight the need for closer consideration of all aspects of mitochondrial metabolism for a better understanding of the physiological basis of the link between size, metabolism, and energy production in wild-dwelling organisms.

Oxidative phosphorylation efficiency, proton conductance and reactive oxygen species production of liver mitochondria correlates with body mass in frogs

“…The respiratory control ratios (RCRs) were significantly different between species and were significantly dependent on M b (Table 1). Importantly, mean RCR values obtained (3.2±0.1) in this study fall within the range of values previously published for ectotherm liver mitochondria respiring on succinate (1.4 to 5.8; Akhmerov, 1986;Brand et al, 1991;Hulbert et al, 2002;Savina et al, 2006;Salin et al, 2012b). Calculated fractional mitochondrial aerobic scope (fAS m ) was significantly higher in larger individuals (Table 1), with a significant dependence on M b (fAS m =2.4×M b 0.14 , where M b is in g; R 2 =0.28, P<0.001).…”

“…Such a difference in mitochondrial coupling efficiency has previously been shown to correlate with intraspecific variation in M b in natural populations of common frogs (Salin et al, 2012b). It has been suggested that a change in mitochondrial membrane proton leak is one of the underlying biochemical mechanisms involved in the variability of mitochondrial efficiency in common frogs (Salin et al, 2012b). To support this, we have found that there is a strong negative correlation between proton conductance and M b , similar to previous studies on avian and mammalian liver mitochondria (Porter and Brand, 1993;Brand et al, 2003).…”

“…This suggests that larger frogs have a lower cost of cellular functioning, as their mitochondria are set to minimize the cost of ATP production and therefore optimize energy output for cellular processes. Such a difference in mitochondrial coupling efficiency has previously been shown to correlate with intraspecific variation in M b in natural populations of common frogs (Salin et al, 2012b). It has been suggested that a change in mitochondrial membrane proton leak is one of the underlying biochemical mechanisms involved in the variability of mitochondrial efficiency in common frogs (Salin et al, 2012b).…”

“…Liver mitochondria was isolated using differential centrifugation in ice-cold isolation buffer containing 250 mmol l −1 sucrose, 1 mmol l −1 EGTA and 20 mmol l −1 Tris-HCl brought to pH 7.4 at 4°C (Salin et al, 2012b). The protein content of the mitochondrial preparation was assayed at 540 nm using the biuret method with bovine serum albumin used as a standard.…”

“…Mitochondrial ATP synthesis was determined from the glucose 6-phosphate content of the samples, which was measured enzymatically by spectrophotometry as previously described (Salin et al, 2012b). Briefly, after recording the respiration rate, four aliquots (60 µl each) of mitochondrial suspension were withdrawn from the respiratory chamber every 2 min and immediately added to 40 µl of perchloric acid solution (10% HClO 4 , 25 mmol l −1 EDTA) to stop the reaction.…”