Impact of ergot alkaloid and steroidal implant on whole-body protein turnover and expression of mTOR pathway proteins in muscle of cattle

Authors: FERGUSON, TAYLOR - UNIVERSITY OF KENTUCKY; LOOS, CAROLINE - UNIVERSITY OF KENTUCKY; VANZANT, ERIC - UNIVERSITY OF KENTUCKY; URSCHEL, KRISTINE - UNIVERSITY OF KENTUCKY; Klotz, James; MCLEOD, KYLE - UNIVERSITY OF KENTUCKY

Submitted to: Frontiers in Veterinary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2023
Publication Date: 4/18/2023
Citation: Ferguson, T.D., Loos, C.M., Vanzant, E.S., Urschel, K.L., Klotz, J.L., McLeod, K.M. 2023. Impact of ergot alkaloid and steroidal implant on whole-body protein turnover and expression of mTOR pathway proteins in muscle of cattle. Frontiers in Veterinary Science. 10. Article 1104361. https://doi.org/10.3389/fvets.2023.1104361.
DOI: https://doi.org/10.3389/fvets.2023.1104361

Interpretive Summary: Cattle suffering from ergot alkaloid exposure or fescue toxicosis frequently have lower rates of average daily gain (ADG). One strategy to improve the gains in stocker cattle has been to use steroidal implants to increase ADG while on toxic endophyte-infected tall fescue pastures. This study used cattle with a fixed feed intake that were or were not receiving a synthetic ergot alkaloid, bromocriptine and were or were not implanted with a growth promoting anabolic steroid to study the effects that ergot alkaloids (found in the common forage - tall fescue) on muscle protein synthesis and how this is affected by the growth promoting implants. Muscle biopsies were performed before and after a glucose tolerance test and samples were analyzed to assess the animals insulin sensitivity and regulation of proteins associated with muscle synthesis. The bromocriptine treatment successfully induced a fescue toxicosis-like syndrome and the implants improved ADG. Muscle protein synthesis was not affected by the bromocriptine treatment and steers with implants had decreased protein turnover and degradation that contributed to the improved gain. Bromocriptine did cause a disruption in glucose and insulin clearance. When feed intake is controlled, decreased performance (ADG) due to fescue toxicosis appears to be due to issues with glucose metabolism in muscle and not issues associated with nitrogen metabolism. The results of this study will be beneficial to other researchers working to understand how ergot alkaloids cause fescue toxicosis.

Technical Abstract: Holstein steers (n=32) were used to determine if fescue-derived alkaloids decrease muscle protein synthesis though inhibitory action on the mTOR pathway via a direct effect on signal proteins, and if these negative effects can be alleviated by implantation with anabolic agents. Steers were treated with intramuscular administration of bromocriptine (vehicle or 0.1 mg/kg BW) and a subdermal estradiol implant (with or without), in a 2x2 factorial design. During the 35-day experiment, intake was restricted to 1.5 times maintenance energy requirement. On days 27 through 32, steers were moved to metabolism stalls for urine collection, and whole-body protein turnover was determined using a single pulse dose of [15N] glycine into the jugular vein on day 28. On day 35, skeletal muscle samples were collected before (basal state) and 60 mins after (stimulated state) an i.v. glucose challenge (0.25 g glucose/kg). Blood samples were collected at regular intervals before and after glucose infusion for determination of circulating concentrations of glucose and insulin. Bromocriptine reduced insulin and glucose clearance following the glucose challenge, indicating decreased insulin sensitivity and possible disruption of glucose uptake and metabolism in the skeletal muscle. Conversely, analysis of whole-body protein turnover demonstrated that bromocriptine does not appear to affect protein synthesis or N retention. Western immunoblot analysis of skeletal muscle showed that it did not affect abundance of S6K1 or 4E-BP1, so does not appear to inhibit activation of the mTOR pathway or protein synthesis. Implantation improved N retention, decreased protein turnover, and had no effect on protein synthesis, suggesting that steroidal implants promote protein accretion through unchanged rates of synthesis and decreased degradation, even in the presence of bromocriptine, resulting in improved daily gains. Implanted steers likely experienced increased IGF-1 signaling, but downstream activation of mTOR, S6K and 4E-BP1, and thus increased protein synthesis did not occur as expected. Overall, this data suggests that fescue derived alkaloids do not have a negative impact on muscle protein synthetic pathways, independent of DMI.