Hormonal Modulation of Amino Acid Transport in Rat Hepatoma Cells in Tissue Culture

Abstract

Abstract 1. HTC cells in suspension culture actively transport the model amino acid α-aminoisobutyric acid (AIB) by a saturable, energy-dependent process that demonstrates Michaelis-Menten kinetics. 2. Dexamethasone markedly decreases AIB transport, reducing uptake by 75% within 2 hours and by 85% after 18 to 24 hours incubation with the hormone. Dexamethasone has no effect on AIB transport for 30 min, after which initial velocity of transport declines exponentially. The effect is half-maximal in 1 hour. After 2 hours of exposure of cells to dexamethasone, Vmax for AIB transport declines to 25% of the control value, while Km is essentially unchanged. After 18 hours, Vmax declines further to 20% of the control value, and Km increases to three times the control value. Dexamethasone decreases AIB influx into HTC cells and does not increase efflux from the cells. 3. Insulin increases AIB transport only slightly in untreated cells. In contrast, in cells previously incubated with dexamethasone, insulin markedly increases AIB uptake. In cells incubated with dexamethasone for 18 hours and then with insulin for a further 2 hours, AIB transport increases 5-fold to 75% of the control value. The effect of insulin is apparent within 30 min and is maximal in 2 hours; it has declined by 4 hours. After 2 hours of exposure of cells to insulin, Vmax increases nearly 4 times to 70% of the control value, and Km decreases 2 times. 4. Both hormonal effects on AIB transport are dependent on protein synthesis, for cycloheximide added 30 min prior to either hormone completely blocks its effect. Addition of actinomycin D 30 min prior to either hormone partially blocks its effect.