Glucose uptake in rat extraocular muscles: effect of insulin and contractile activity.
OBJECTIVE
Extraocular muscles show specific adaptations to fulfill the metabolic demands imposed by their constant activity. One aspect that has not been explored is the availability of substrate for energy pathways in extraocular muscles. In limb muscles, glucose enters by way of GLUT1 and GLUT4 transporters in a process regulated by insulin and contractile activity to match metabolic supply to demand. This mechanism may not apply to extraocular muscles because their constant activity may require high basal (insulin- and activity-independent) glucose uptake. The authors tested the hypothesis that glucose uptake by extraocular muscles is not regulated by insulin or contractile activity.


METHODS
Extraocular muscles from adult male Sprague-Dawley rats were incubated with 100 nM insulin or were electrically stimulated to contract (activity); glucose uptake was measured with 2-deoxy-d[1,2-(3)H]glucose. The contents of GLUT1, GLUT4, total and phosphorylated protein kinase B (Akt), phosphorylated AMP-activated protein kinase (AMPK), and glycogen synthase kinase 3 (GSK3) underwent Western blot analysis.


RESULTS
Insulin and activity increased glucose uptake over the basal rate to 108% and 78%, respectively. GLUT1 and GLUT4 were detectable in extraocular muscles. Phosphorylated AKT/total AKT increased by twofold after insulin stimulation, but there was no change with activity. AMPK phosphorylation increased 35% with activity. Phosphorylated-GSK3/total GSK3 did not change with insulin or activity.


CONCLUSIONS
Glucose uptake in extraocular muscles is regulated by insulin and contractile activity. There is evidence of differences in the insulin signaling pathway that may explain the low glycogen content in these muscles.
Garcia-Cazarin
Mary L
ML
Department of Physiology, University of Kentucky, Lexington, Kentucky 40536-0298, USA.
Fisher
Tatijana M
TM
Andrade
Francisco H
FH
D055372:AMP-Activated Protein Kinases / Q000378:metabolism; D000818:Animals; D015153:Blotting, Western; D003847:Deoxyglucose / Q000378:metabolism; D004558:Electric Stimulation; D005947:Glucose / Q000378:metabolism*; D051272:Glucose Transporter Type 1 / Q000378:metabolism; D051275:Glucose Transporter Type 4 / Q000378:metabolism; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism; D007328:Insulin / Q000494:pharmacology*; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D009801:Oculomotor Muscles / Q000187:drug effects* / Q000378:metabolism; D010766:Phosphorylation; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley
D051272:Glucose Transporter Type 1; D051275:Glucose Transporter Type 4; D007328:Insulin; C495679:Slc2a1 protein, rat; C495694:Slc2a4 protein, rat; D003847:Deoxyglucose; D051057:Proto-Oncogene Proteins c-akt; D038362:Glycogen Synthase Kinase 3; D055372:AMP-Activated Protein Kinases; D005947:Glucose