Energy partitioning in lactating cows affects milk production, feed efficiency, and body reserves, with the latter having health implications for the transition into the following lactation. One molecule likely involved in the regulation of energy partitioning is serotonin. The objective of this experiment was to explore how increasing circulating serotonin, by intravenous infusion of the serotonin precursor 5-hydroxytryptophan (5-HTP), affects metabolic responses to a glucose challenge in midlactation cows as a means to manipulate energy partitioning. We intravenously infused Holstein cows with 5-HTP (1 mg/kg bodyweight dissolved in saline, n = 11) or saline alone as control (n = 9) over 1 h/day for 3 days. Cows were fasted overnight on day 2. On day 3, fasted cows were given an intravenous bolus of glucose (0.092 g/kg bodyweight). Blood samples were collected for the following 120 min for metabolic and hormonal analysis. Infusion of 5-HTP elevated circulating concentrations of serotonin and free fatty acids, reduced the concentration of insulin and amino acids, and did not affect the concentration of glucose and glucagon before the glucose challenge. Surrogate insulin sensitivity indices indicated improved insulin sensitivity in 5-HTP cows, but due to the unique metabolism of lactating ruminants, these index changes may instead reflect effects in insulin-independent glucose disposal, like milk synthesis. Challenging 5-HTP-treated cows with a glucose bolus reduced the insulin spike and blunted the decrease in free fatty acids, compared to saline cows, without changing glucose dynamics. Overall, these results suggest that serotonin stimulates insulin-independent glucose disposal, requiring less insulin to maintain normoglycemia.
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