Activity-stimulated glucose uptake in the rat diaphragm: the exception to the rule?

Skeletal muscle glucose uptake is regulated by insulin and contractile activity.  The diaphragm is constantly active and may rely on higher basal glucose uptake rates.  We tested the hypothesis that glucose uptake by the rat diaphragm is not regulated by contractile activity.  We used diaphragm bundles from adult Sprague-Dawley rats to measure 3H-glucose uptake under basal conditions, insulin (100 nM), or under the following activity protocols:  (1) 0.1 train/s @ 40 Hz, 500 ms train duration for 20 min (parameters to increase glucose uptake in other muscles); (2) 40 Hz for 10 min; (3) 20 Hz for 10 min; and (4) 10 Hz for 10 min.  Protocols 2-4 were done at 0.5 train/s and 500 ms train duration.  We found that insulin increased glucose uptake 4-fold over basal rates: 0.6±0.1 vs. 2.8±0.7 μmol/g/h basal and insulin respectively (p=0.01).  Activity protocol #1 did not change glucose uptake: 1.1±0.12 vs. 1.1±0.15 μmol/g/h basal and activity respectively (p=0.5).  Activity protocols #2-4 increased glucose significantly over the basal rates:  #2: 1.6±0.2 vs. 2.1±0.2; #3: 2.2±0.11 vs. 3.4±0.3; #4: 1.9±0.3 vs. 2.7±0.2 μmol/g/h basal and activity respectively, (p≤0.01 for each comparison).  In summary, glucose uptake in the rat diaphragm does not increase in response to an activity protocol used in other muscles.  Most strenuous activity protocols increase glucose uptake, but not to the same extent as insulin.  Supported by R01 EY012998 to FHA.