The membrane pacemaker hypothesis: novel tests during the ontogeny of endothermy

Abstract

The 'membrane pacemaker' hypothesis proposes a biochemical explanation for among-species variation in resting metabolism, based on the positive correlation between membrane docosahexaenoic acid (DHA) and metabolic rate. We tested this hypothesis using a novel model, altricial red-winged blackbird nestlings, predicting that the proportion of DHA in muscle and liver membranes should increase with the increasing metabolic rate of the nestling as it develops endothermy. We also used a dietary manipulation, supplementing the natural diet with fish oil (high DHA) or sunflower oil (high linoleic acid) to alter membrane composition and then assessed metabolic rate. In support of the membrane pacemaker hypothesis, DHA proportions increased in membranes from pectoralis muscle, muscle mitochondria and liver during post-hatch development. By contrast, elevated dietary DHA had no effect on resting metabolic rate, despite causing significant changes to membrane lipid composition. During cold challenges, higher metabolic rates were achieved by birds that had lower DHA and higher linoleic acid in membrane phospholipids. Given the mixed support for this hypothesis, we conclude that correlations between membrane DHA and metabolic rate are likely spurious, and should be attributed to a still-unidentified confounding variable.

Rapid embryonic accretion of docosahexaenoic acid (DHA) in the brain of an altricial bird with an aquatic-based maternal diet

“…In clutches of four eggs, we removed one egg or neonate hatchling to standardize parental feeding effort to three offspring. To test whether diet affects brain fatty acid composition, we randomly divided hatchlings into three groups: control birds were weighed daily but were not dosed, the fish oil group received a single daily dose of fish oil (high DHA and other ω3 fatty acids; Refined Menhaden Oil, MP Biomedicals, Santa Ana, CA, USA), and the seed oil group received a single daily dose of sunflower oil (low DHA, high 18:2ω6; A&M Gourmet Foods, Toronto, ON, Canada) (oil fatty acid compositions reported in Price et al, 2018). All nestmates received the same treatment but were collected at different ages.…”

“…Lipids were extracted and separated into total lipids, a neutral lipid fraction (mostly triglycerides) and a polar lipid fraction (mostly phospholipids), as previously described (Price et al, 2018). For yolk samples, we measured the fatty acid composition of total lipids, neutral lipids and polar lipids; for brain samples, we discarded the total and neutral lipid fractions so as to focus on membrane lipids, where DHA is proposed to be functionally important.…”

“…We added dichloromethane as a running solvent. Fatty acid separation, identification and quantification were performed using gas chromatography and mass spectrometry using calibration curves derived from fatty acid methyl ester standards as previously described (Price et al, 2018).…”

The effects of dietary linoleic acid and hydrophilic antioxidants on basal, peak, and sustained metabolism in flight‐trained European starlings

“…Results such as these suggest that (a) the fatty acids that are important micronutrients for many wild vertebrates are primarily long‐chain polyunsaturated, likely due to their unique chemical properties and limited synthesis by vertebrates (Klasing, ; Twining, Lawrence, Winkler, Flecker, & Brenna, ), (b) dietary fatty acids seem to primarily influence the regulation and scope of energy metabolism (Pierce & McWilliams, ; Price, ), and (c) the effects of dietary fatty acids have particular relevance for birds during migration, possibly due to the high metabolic demands of flight (Guglielmo, ; Martinez del Rio & McWilliams, ). However, the influence of dietary fatty acids on the performance of migratory birds remains decidedly equivocal, with inconsistent results across different measures of performance (Pierce et al, ), across different ages (Amitai, Bauchinger, McCue, & Pinshow, ; Price et al, ), and across species (Dick & Guglielmo, ; McWilliams & Pierce, ; Price & Guglielmo, ), including several studies that failed to find any effects. The inconsistency of past results suggests a need for more refined tests of the influence of individual fatty acids on multiple measures of performance.…”

“…We experimentally tested how dietary linoleic acid (18:2n6), a leading candidate to mediate the effects of dietary fatty acid composition on metabolism in songbirds (Pierce & McWilliams, ; Pierce et al, ; Price & Guglielmo, ; Price et al, ), and the dietary anthocyanins, hydrophilic antioxidants common in the diets of wild songbirds (Bolser et al, ; Cooper‐Mullin & McWilliams, ), interact to affect exercise performance of European starlings ( Sturnus vulgaris ). We measured performance in terms of metabolic rates and tissue catabolism during sustained flight in a wind tunnel to allow us to assess the effect of diet on basal, peak, and sustained metabolism as well as the relationship between these measures.…”