Enhancement of hexose entry into chick fibroblasts by starvation: differential effect on galactose and glucose.

Glucose entry, as measured by 5-min uptake into the acid-soluble fraction, is enhanced 15-30 times by long-term (12-24 hr) hexose starvation of chick fibroblasts. The rate of galactose accumulation in the cells increases only 5 times under the same conditions of starvation. Several carbon and energy sources that were tested for their effect on this "derepression" can be classified as: (i) those resembling glucose in blocking the "stimulation," (ii) those permitting full "derepression"; and (iii) those partially preventing the enhanced entry. Inhibitors of protein synthesis block enhancement under conditions otherwise conducive to it. We conclude that the glucose and galactose carrier systems are not identical, based largely on the asymmetric "repression" observed when glucose and galactose are compared as "repressors."