STRUCTURAL DISRUPTION OF THE TRANS-GOLGI NETWORK DOES NOT INTERFERE WITH THE ACUTE STIMULATION OF GLUCOSE AND AMINO-ACID-UPTAKE BY INSULIN-LIKE GROWTH-FACTOR-I IN MUSCLE-CELLS

The effects of insulin-like growth factor I(IGF-I) on glucose and amin o acid uptake were investigated in fully differentiated L6 muscle cell s, in order to determine whether the two processes are functionally re lated. Transport of both glucose and amino acid (methylaminoisobutyric acid, MeAIB) was activated rapidly in response to IGF-I. Stimulation reached a peak within 30 min and was sustained for up to 90 min. Maxim al activation of either glucose or MeAIB transport was achieved at 3 n M IGF-I; the half-maximal activation (ED(50)) of glucose transport was at 107 pM and that of MeAIB transport was at 36 pM. Stimulation of am ino acid uptake occurred in the absence or presence of glucose, sugges ting that this response is not secondary to increased glucose intake. Incubation of cells for 1 h with Brefeldin A (5 mu g/ml), which disass embles the Golgi apparatus and inhibits the secretory pathway in eukar yotic cells, had no effect on the acute IGF-I activation of glucose an d MeAIB transport. More over, Brefeldin A caused wide redistribution o f the trans-Golgi antigen TGN38, as assessed by subcellular fractionat ion, without affecting the distribution of glucose transporters. The f inding that the degree of activation, time response and sensitivity to IGF-I and Brefeldin A were similar for both glucose and MeAIB transpo rt suggests commonalities in the IGF-I mechanism of recruitment of glu cose transporters and stimulation of amino acid transport through Syst em A. An integral trans-Golgi network does not appear to be required f or the acute IGF-I stimulation of glucose or amino acid transport, eve n though stimulation of glucose transport occurs through recruitment o f glucose transporters from intracellular stores in these cells. We pr opose that the donor site of glucose transporters (and perhaps of amin o acid transporters) involved in the acute response to IGF-I lies beyo nd the trans-Golgi network, perhaps in an endosomal compartment in clo se proximity to the plasma membrane.