Effect of Endurance Conditioning on Insulin-mediated Glucose Clearance in Dogs

Introduction Physical activity has been shown to improve insulin sensitivity in subjects with insulin resistance, but the effect of athletic conditioning on subjects with normal insulin sensitivity has received less scrutiny. Because strenuous exercise can be limited by the availability of substrates, it is reasonable to hypothesize that conditioning would increase the capacity for muscle uptake of substrates like glucose, and to the extent that improvement in this process would include upregulation of the portions of the glucose uptake pathway in muscle, this increased capacity would also be reflected in insulin sensitivity. Therefore, we tested the hypothesis that conditioning for endurance exercise would result in increased insulin sensitivity using elite racing sled dogs. Methods A frequent sampled intravenous glucose tolerance test was performed on these dogs before and after a full 7 month season of conditioning in preparation for a 1600-km race. Results Compared to the results in unconditioned dogs, conditioned dogs rapidly cleared the intravenous glucose bolus through increases in both glucose mediated (7.6 ± 3.4%/min vs 3.0 ± 2.2%/min, p = 0.008) and insulin-mediated (36.3 ± 18.4x 10-4 L/min/mU vs 11.5 ± 8.0 x 10-4 L/min/mU, p = 0.007) mechanisms. The more modest increase in serum insulin following the intravenous glucose bolus in Conditioned dogs failed to suppress lipolysis and serum concentrations of non-esterified fatty acids remained constant in the conditioned dogs throughout the 4-hr test. Conclusion These results, in particular the increase in insulin-independent peripheral uptake of glucose, describe novel alterations in metabolism induced by athletic conditioning that arguably result in near-continuous provision of oxidizable substrates to peripheral muscle in support of sustained muscular work typical of these dogs. Corresponding Author: Michael S. Davis, 264 McElroy Hall, Stillwater, OK, 74078, Phone: 405-744-8172, Fax: 405-744-8263, Michael.davis@okstate.edu Studies were funded by the Army Research Office Division of Life Sciences (W911NF-09-1-0549). The authors affirm that they have no professional relationships with companies or manufacturers who will benefit from the results of the present study, and that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by ACSM. Submitted for publication April 2018. Accepted for publication July 2018. © 2018 American College of Sports Medicine

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