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Glucose tolerance appears to follow a daily rhythm in dairy cows.

I. Salfer

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06-24-2020

Abstract:

379
Glucose tolerance appears to follow a daily rhythm in dairy cows.
I. Salfer*1,2, C. Matamoros2, K. Harvatine2. 1South Dakota State University Brookings, SD, 2The Pennsylvania State University University Park, PA.

Glucose is an important metabolic fuel for dairy cows that is required for milk lactose and fat synthesis, and other essential processes. Glucose metabolism is regulated by circadian rhythms, repeating cycles of ~24 h that govern many aspects of behavior and physiology. It is well-established that blood glucose and insulin concentrations follow 24 h rhythms in dairy cows, but it is unknown if glucose tolerance and insulin sensitivity follow a 24 h rhythm. To determine if glucose tolerance and insulin sensitivity follow a 24 h rhythm in dairy cows, 12 multiparous lactating Holstein cows were enrolled in a randomized block design (2 blocks; n = 6 per block). Within each block, cows were subjected to intravenous glucose tolerance tests (IVGTT) at 0300, 0900, 1500 and 2100. During IVGTT, cows were fed 12x/d at 2 h intervals to stabilize feed intake across the day. For each IVGTT, 250 g of a 50% wt/vol D-glucose solution were infused via jugular catheter and blood was collected at −15, −5, immediately before, 0, 5, 10, 15, 20, 30, 45, 60, 90, 120, and 240 min relative to infusion. Insulin and glucose kinetics were modeled using WinSAAM ver. 3.0.8. Glucose concentration fit a 2-compartment model with a delay component for glucose recycling. Effect of time of day on baseline glucose and insulin concentrations, glucose half-life, and insulin resistance - measured by post-IVGTT increase in insulin concentration- were analyzed in SAS 9.4 using a mixed model including main effects of time of day and block, and the random effect of cow. Moreover, time course data were fit to the linear form of cosine functions using SAS 9.4 and analyzed for fit of a 24 h rhythm using a zero-amplitude test. Half-life of glucose was affected by time (P = 0.05), with half-life being 25% greater at 0300 than 0900 and 1500 (P < 0.05), and half-life being 25% greater at 2100 than 1500 (P < 0.05). Moreover, half-life of glucose fit a cosine function with a period of 24 h, indicating a daily rhythm (P = 0.005). Baseline insulin concentration differed across the day with insulin concentration being greatest at 0300, and lowest at 0900 (P = 0.0002), while baseline glucose concentration was not affected by time of day (P = 0.30). Insulin sensitivity differed across the day, being greater at 0300 and 1500 than 0900 and 2100 (P < 0.05). Results suggest that glucose tolerance follows a daily rhythm in dairy cows, with greater glucose tolerance in morning and afternoon compared with overnight.

Keywords: daily rhythm, glucose metabolism, circadian.