Effect of hypertonicity on hexose transporter regulation in chicken embryo fibroblasts

Abstract

The regulation of hexose transporters of cultured fibroblasts was investigated by exposing chicken embryo fibroblasts (CEF) to hypertonic culture medium, a condition known to enhance hexose transport activity. The effects of hypertonicity and the role of protein synthesis were examined with CEF in the basal (glucose fed) and transport enhanced (glucose starved) states. Glucose-fed CEF exposed to hypertonic conditions developed four-fold enhancement of hexose transport activity within 4 hrs; this declined in the following 20 hrs to a level slightly higher than the fed control. Protein synthesis was required in part for this effect, since the presence of cycloheximide during hypertonic exposure of fed CEF blocked the increase in of transport by almost 50%. Although the increased transport produced by glucose starvation was not further enhanced by hypertonicity, hypertonic treatment of starved CEF during glucose refeeding largely prevented the loss of transport activity to the basal, fed state. The hypertonic effects were concentration dependent (240mOsm optimal) and could be elicited with NaCl, KCl, or sucrose. Hypertonic treatment typically led to a greater than 50% decline in the incorporation of [3H]leucine into acid-insoluble fractions. The changes in transport were evident at the plasma membrane level, and studies of membrane vesicles prepared from hypertonically treated fed CEF showed a doubling of both [3H]cytochalasin B binding and the Vmax of D-glucose transport. These findings indicate that exposure of CEF to hypertonic conditions has some effects similar to those produced by glucose starvation and suggest that protein synthesis is to some extent involved in the regulation of hexose transporters in CEF.

Regulation of cell volume via microvillar ion channels

“…Further compelling arguments in favor of the proposed mechanism of hypo-and hypertonic volume regulation come from studies of glucose transport stimulation under hyperosmotic conditions (Clausen et al, 1975;Sorensen et al, 1980;Bigornia et al, 1986;Toyoda et al, 1986;Tillotson and Isselsbacher, 1987;Chen et al, 1997;Sakaue et al, 1997). These early findings show that hyperosmotic stimulation of ionic fluxes is always accompanied by an activation of glucose transport.…”