Effect of diffusion distance on measurement of rat skeletal muscle glucose transport in vitro

The relationships between muscle size, diffusion distance, and glucose uptake were studied using the Type IIb epitrochlearis (13 +/- 1 mg intact), Type I soleus (25 +/- 1 mg), and mixed Type IIa/IIb extensor digitorum longus (25 +/- 1 mg) from 60-70 g rats. Using intact muscles, the relative rates of 3-O-methyl-glucose uptake in response to 2 mUml-1 insulin were soleus = epitrochlearis greater than extensor digitorum longus, a finding inconsistent with the fibre-type compositions and the relative GLUT-4 protein levels (soleus greater than extensor digitorum longus greater than epitrochlearis). To test whether these results were influenced by substrate diffusion limitations in the tubular muscles, soleus and extensor digitorum longus were split longitudinally from tendon to tendon into strips of comparable size (13 +/- 1 mg) to the epitrochlearis. Insulin-stimulated rates of 3-O-methyl-glucose uptake were significantly enhanced in the split soleus (+120%) and split extensor digitorum longus (+200%), but not in the epitrochlearis, with the relative rates being soleus greater than extensor digitorum longus greater than epitrochlearis. Diffusion distances of the split soleus and extensor digitorum longus, as reflected by 14Cmannitol space equilibration time, were markedly enhanced (by at least 50%) relative to the intact muscles, and were comparable to that of the epitrochlearis. These results indicate that when muscles of different size and/or shape are used for in vitro measurement of glucose transport, the muscle preparations used must have similar diffusion distances for physiologically meaningful comparisons to be made.