Abstract
BackgroundIn domestic animals many biochemical and physiological processes exhibit daily rhythmicity. The aim of the present study was to investigate the rhythmic pattern of salivary and serum urea concentrations in sheep.MethodsSix 3-year-old female sheep kept in the same environmental conditions were used. Sheep were sampled at 4 hour intervals for 48 consecutive hours starting at 08:00 of the first day and finishing at 04:00 of the second day. Blood samples were collected via intravenous cannulae inserted into the jugular vein; saliva samples were collected through a specific tube, the "Salivette". Salivary and serum urea concentrations were assayed by means of UV spectrophotometer. ANOVA was used to determine significant differences. The single Cosinor procedure was applied to the results showing significant differences over time.ResultsANOVA showed a significant effect of time on salivary and serum urea concentrations. Serum and salivary urea peaked during the light phase. In the dark phase serum and salivary urea concentrations decreased, and the diurnal trough occurred at midnight. Cosinor analysis showed diurnal acrophases for salivary and serum urea concentrations. Daily mean levels were significantly higher in the serum than in the saliva.ConclusionIn sheep both salivary and serum urea concentrations showed daily fluctuations. Urea is synthesized in the liver and its production is strongly influenced by food intake. Future investigation should clarify whether daily urea rhythms in sheep are endogenous or are simply the result of the temporal administration of food.
Highlights
In domestic animals many biochemical and physiological processes exhibit daily rhythmicity
Urea is synthesized in the liver and its production is strongly influenced by food intake
Future investigation should clarify whether daily urea rhythms in sheep are endogenous or are the result of the temporal administration of food
Summary
In domestic animals many biochemical and physiological processes exhibit daily rhythmicity. Serum concentration of urea was evaluated in cows during different feeding schedules [6] and in goats maintained under various schedules of lighting and feeding [7] in order to understand the mechanisms of entrainment of liver function Ruminants, such as sheep and cattle, secrete a large amount of saliva from the salivary glands into the rumen (≥100 litre/day in cattle and ≥10 litre/day in sheep). The amount of urea recycled to the rumen, both in saliva and across the rumen wall, is directly related to the amount of urea synthesized which, in turn, is related to nitrogen intake and the degradability of dietary nitrogen This explains how serum urea concentration is strictly related to feeding: when a nitrogen-deficient ration is ingested, urea does not pass into the urine but is converted into microbial protein in the digestive tract, to be re-utilized [10]. The defining of the liver as a site of a putative food-entrainable oscillator and the existence of a daily rhythm of serum urea concentration influenced by feeding in ruminants led us to investigate the rhythmic pattern of both salivary and serum urea concentrations in sheep
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