Insulin-induced cytoplasmic alkalinization and glucose transport in muscle cells

Abstract

Insulin stimulates glucose uptake into muscle within minutes, preceding stimulation of glycolysis. Signals involved in stimulation of glycolysis include cytoplasmic alkalinization and specific intracellular proteolytic products. In contrast, the signals that mediate stimulation of glucose transport remain unknown. Here we explore whether the insulin-induced cytoplasmic alkalinization is an early event that precedes activation of sugar uptake, whether such alkalinization is causally related to stimulation of sugar uptake, and whether proteolytic activity mediates stimulation of hexose transport. Cytoplasmic pH (pHi) was measured in suspended skeletal muscle cells of the L6H9 line with the intracellularly trapped fluorescent pH indicator bis(carboxyethyl)carboxy fluorescein. At 37 degrees C, insulin (1 X 10(-7) M) produced an increase in pHi of 0.11 units in 10 min. This increase became apparent 2 min after addition of the hormone, and maximal elevation of pHi was observed after 10 min, remaining elevated for up to 60 min. Removal of the hormone with anti-insulin antiserum did not reverse pHi back to the resting level. The alkalinization was prevented by amiloride, by 5-(N,N'-disubstituted)amiloride analogues, and by isosmotic replacement of Na+ with N-methylglucamine+ or choline+. This suggests that insulin activates Na+-H+ exchange. In contrast, stimulation of 2-deoxy-D-glucose transport by insulin was not affected by replacement of external Na+ or by addition of amiloride. Monensin, an exogenous Na+-H+ exchanger, did not stimulate sugar transport even though it increased pHi. Proteinase inhibitors that block hormonal stimulation of glycolysis were ineffective in preventing stimulation of 2-deoxy-D-glucose transport by insulin.(ABSTRACT TRUNCATED AT 250 WORDS)