Daily rhythms of serum lipids in dogs: influences of lighting and fasting cycles

Abstract

Circadian clocks organize a wide array of metabolic functions in a coherent daily schedule and ensure synchrony of this schedule with environmental rhythms. Daily rhythmicity of lipid metabolism occurs in rodents and ruminants. We examined daily level variations of serum lipids (nonesterified fatty acids [NEFA], triglycerides, phospholipids, total cholesterol and total lipids) in healthy dogs, particularly focusing on their temporal relationship to lighting and fasting cycles. Whereas serum NEFA levels did not change across the day, levels of total lipids, total cholesterol, phospholipids, and triglycerides occurred in dogs maintained under 12:12-h light:dark cycles and fed a single meal daily. Only the rhythmic pattern of triglycerides responded to a 6 h delay in light onset, suggesting a cardinal role of a light-entrained circadian oscillator in its generation. To investigate whether temporal variations in serum lipids depend to physiological postprandial changes, we measured lipid levels in fasted dogs. Rhythms of total lipids, total cholesterol, phospholipids, and triglycerides vanished when dogs were food-deprived, indicating that these rhythms are driven by the digestive process. Levels of serum NEFA patterns were significantly higher during fasting than after food intake. The increase of NEFA concentrations during fasting may reflect the mobilization of adipose tissue NEFA mediated by the decrease in insulin with its lypolitic effects. Elucidating the daily rhythmicity of lipid levels is fundamental to understanding the metabolism of the dog, an animal model frequently used for research in metabolic pathophysiology.

Figure 1.

Serum (A) total lipids, (B) total cholesterol, (C) phospholipids, (D) triglycerides, and (E) NEFA in dogs maintained under 12:12-h light:dark cycles (LD1, lights on at 0900; LD2, lights on at 1500) and fed a single meal in the day (LD1, filled circles; LD2, filled triangles) or fasted (empty squares). Serum samples (n = 5 for each time point) were collected every 4 h for 24 h. Mean ± SEM at each time point is shown. White and black bars indicate the durations of light and dark phases, respectively. Arrowheads indicate the time of food administration (1000) in both LD cycles.