EARLY SIGNALS IN SERUM-INDUCED INCREASES IN OUABAIN-SENSITIVE NA-K+ PUMP ACTIVITY AND IN GLUCOSE-TRANSPORT IN RAT SKELETAL-MUSCLE ARE AMILORIDE-SENSITIVE()

The acute effects of serum on sodium-potassium (Na+-K+) pump activity and glucose uptake in cultured rat skeletal muscle were studied. Addit ion of serum to myotubes in phosphate-buffered saline caused Na+-K+ pu mp activity (as measured by changes in the ouabain-sensitive component of both membrane potential and Rb-86 uptake) to increase, with peak e ffects obtained after 30 min. The effect was blocked completely by tre atment with amiloride, but not by tetrodotoxin, which blocks voltage-d ependent Na+ channels. On transfer of myotubes to Na+-free, choline bu ffer, resting Na+-K+ pump activity decreased to about 10% of that in p hosphate-buffered saline. Addition of regular serum, but not Na+-free serum, caused Na+-K+ pump activity to increase slightly. Similar resul ts were obtained with serum on glucose uptake, the peak effect being r eached within 15 min. Stimulation of glucose uptake by serum was parti ally reduced by amiloride and was not altered by tetrodotoxin. Removal of external Na+ also eliminated serum effects on glucose uptake. The results demonstrate that there are similar signals involving Na+-H+ ex change for serum-induced increases in Na+-K+ pump activity and glucose transport. The lack of complete blockade of serum-induced elevation o f glucose transport suggests an additional, as yet undefined, intracel lular signal for stimulation of this transport system.