Table I.

Quantitation of the Effects of Insulin and Muscle Contractions on GLUT4 Labeling in the Surface Membrane

		Basal		Insulin		Contractions		Insulin + contractions
Plasma membrane*		1.7 ± 0.3		12.6 ± 0.7		14.7 ± 6.0		23.1 ± 3.0
		(5/1409 μm)		(4/929 μm)		(4/1473 μm)		(5/1634 μm)
Increase over basal		× 1		× 7		× 9		× 14
T tubules (longitudinally sectioned)‡		0.6 ± 0.7		8.7 ± 0.6		13.5 ± 5.1		18.2 ± 3.8
		(3/82/34 μm)		(3/119/60 μm)		(3/161/85 μm)		(3/198/74 μm)
Increase over basal		× 1		× 15		× 23		× 30
T tubules (cross-sectioned plus		0.8 ± 0.7		19.4 ± 1.2		23.3 ± 6.4		33.4 ± 2.6
longitudinally sectioned)§		(5/1316)		(4/1010)		(4/1616)		(5/1276)
Increase over basal		× 1		× 24		× 29		× 42

Fibers from each of the four experimental groups were labeled with anti-GLUT4 and processed for immunogold electron microscopy. Silver-enhanced gold grains were counted from longitudinal sections. All results are given as means ± SEM. Numbers in parentheses represent the number of independent experiments counted, followed for rows 1 and 2 by the total length of membrane counted (in μm), and for rows 2 and 3 by the number of triads counted.  

^* Number of grains/10 μm of plasma membrane. The background labeling in the absence of primary antibody was 2.8 ± 0.2 grains/10 μm and has been subtracted from the presented data.  

^‡ Number of grains/10 μm of longitudinally sectioned junctional T tubule membrane. The background labeling in the absence of primary antibody was 1.2 ± 0.7 grains/10 μm and has been subtracted from the presented data.  

^§ Percent of junctional T tubules labeled, including both cross- and longitudinally sectioned T tubules. In the absence of primary antibody 1.4 ± 0.4% of all triads were labeled. This has been subtracted from the presented data. Cross-sectioned T tubules represent ∼85% of the total number of encountered T tubules.