Energy Substrate Profiles during Fasting in Horned Larks (Eremophila alpestris)

Overnight fasting in the passerine horned lark (Eremophila alpestris) is characterized by rapid changes in the concentrations of circulating energy substrates. These changes suggest an increased reliance on lipid fuels combined with sparing of carbohydrate and protein. During fasting, free fatty acid levels rose 36%, and β-hydroxybutyrate levels rose fourfold, both within 5 h. Plasma triglyceride concentrations were elevated in the fed state and only decreased after 10 h of fasting The levels of both α-amino nitrogen and glucose remained constant throughout fasting, the latter at the relatively high levels seen in many passerines (ca. 20 mM). Both liver and pectoral glycogen were significantly depleted after pvernight fasting. The quick onset of these protein sparing adjustments (as compared with larger birds) is probably a common feature in small passerines, because of their high mass-specific metabolic rates and their need to conserve muscle protein.