Interactions between effects of W-7, insulin, and hypoxia on glucose transport in skeletal muscle

The calmodulin antagonist N-(6-aminohexyl)-5-chloro-1- naphthalenesulfonamide (W-7) stimulates glucose transport in skeletal muscle, apparently by raising cytosolic Ca²⁺ (P. Palade, J. Biol. Chem. 262: 6142- 6148 1987; J. H. Youn, E. A. Gulve, and J. O. Holloszy. Am. J. Physiol. 260 (Cell Physiol. 29): C555-C561, 1991). This study was performed to describe the interactions between the effects of W-7 and those of hypoxia and of insulin on glucose transport. The effect on 3-O-methylglucose (3-MG) transport of 50 μM W-7 was additive to the effect of a maximal insulin stimulus (2,000 μU/ml) but not to the effect of maximal (60 min) hypoxic stimulus, suggesting that W-7 stimulates glucose transport via the same pathway as hypoxia, independent of the pathway activated by insulin. The effect of 50 μM W-7 was additive to that of a submaximal (20 min) hypoxia stimulus, indicating that W-7 does not interfere with the stimulation of glucose transport by hypoxia. In contrast, 50 μM W-7 had an inhibitory effect on stimulation of 3-MG transport by submaximally effective insulin levels, causing a fivefold increase in the concentration of insulin needed to produce a half-maximal stimulation of 3-MG transport, from ~70 to ~350 μU/ml (P < 0.05). Thus these data demonstrate that W-7 selectively inhibits insulin stimulation of glucose transport. W-7 also inhibited the stimulation of glucose transport by vanadate, an insulin mimetic agent that stimulates glucose transport at a site distal to the insulin binding step, suggesting that the site of action of W-7 is at a step beyond the insulin receptor. In conclusion, these findings provide evidence that W-7 stimulates glucose transport by the same pathway as hypoxia. In addition, this calmodulin antagonist has a specific inhibitory effect on insulin action on glucose transport without affecting the effect of hypoxia.