Augmentation of the effects of insulin and insulin-like growth factors I and II on glucose uptake in cultured rat skeletal muscle cells by sulfonylureas

Abstract

Summary. The effect of sulfonylureas on long-term regulation of glucose uptake by insulin and insulin-like growth factors has been studied in the L6 line of cultured skeletal muscle cells. These cells have previously been shown to possess many characteristics of differentiated skeletal muscle and to bind and respond to physiological concentrations of insulin and insulin-like growth factors I and II. Tolazamide (halfmaximal at 0.2 mg/ml) augments the effects of insulin, insulin-like growth factor I, and insulin-like growth factor II on glucose uptake, increasing both sensitivity and maximal efficacy of the hormones. In the absence of added hormone, tolazamide has no effect on glucose uptake. A similar increase in insulin-stimulated glucose uptake with unaltered basal uptake occurs with glybufide (half-maximal at 0.5 ~g/ml). The action of tolazamide requires long-term exposure to the sulfonylurea (22 h) and is inhibited by cycloheximide, suggesting a process that involves new protein synthesis. In contrast to glucose uptake, amino acid uptake in L6 cells is increased by tolazamide in the absence of hormones. Insulin and the insulin-like growth factors also stimulate amino acid uptake, but this effect is not further augmented by tolazamide. Thus, sulfonylureas appear to directly modulate amino acid uptake, but to indirectly augment glucose uptake through an effect on insulin and insulin-like growth factor stimulated pathways. Neither insulin binding nor insulin degradation is altered by tolazamide, indicating a post-binding mechanism of action. The L6 cultured skeletal muscle cell line should be useful in future studies on the mechanism of the extrapancreatic actions of sulfonylureas.

Coordinate regulation of glucose transporter function, number, and gene expression by insulin and sulfonylureas in L6 rat skeletal muscle cells.

“…The previously published observation that tolazamide increases the maximal efficacy of insulin rather than increasing insulin sensitivity also is consistent with a postbinding mechanism of action (14).…”

“…Actions of sulfonylureas on glucose utilization by peripheral tissues have been demonstrated by in vivo clamp studies in type II diabetes (12) and are further evidenced by the decreased insulin requirements of type I diabetics treated with both sulfonylureas and insulin (13). We have recently observed that sulfonylureas augment insulin-stimulated glucose uptake in cultured L6 skeletal muscle cells (14). Consistent with in vivo findings (15), sulfonylureas had no effect on the rate ofglucose transport in L6 cells in the absence ofinsulin.…”

“…Glucose transport was studied in differentiated L6 myotubes by measuring the uptake of 2-deoxyglucose. As previously described, under these experimental conditions insulin stimulates glucose uptake in L6 cells maximally after 8-10 h of incubation (14). Insulin alone (l1-0 M) stimulated glucose uptake by 30-40% after 8 h. When cells were incubated with tolazamide for 14 h and then with insulin together with tolazamide for 8 h, the effect of insulin was increased approximately twofold ( In the absence of insulin, tolazamide alone had no effect on glucose uptake.…”

“…Insulin alone (l1-0 M) stimulated glucose uptake by 30-40% after 8 h. When cells were incubated with tolazamide for 14 h and then with insulin together with tolazamide for 8 h, the effect of insulin was increased approximately twofold ( In the absence of insulin, tolazamide alone had no effect on glucose uptake. Since insulin binding has been shown to be unchanged under these conditions (14), it is apparent that the augmentation of insulin-stimulated glucose uptake by tolazamide occurs at a step subsequent to the binding of insulin.…”

“…The uptake of ['4C]2-deoxyglucose was then determined during a 10-min incubation at room temperature as previously described (14). 2-Deoxyglucose uptake was linear with time for at least 15 min in both control and treated cells.…”