Temperature-dependent changes in activation energies of the transport systems for nucleosides, choline and deoxyglucose of cultured Novikoff rat hepatoma cells and effects of cytochalasin B and lipid solvents

Abstract

The initial rates of transport of uridine, thymidine, purines, choline and 2-deoxy-d-glucose by cultured Novikoff rat hepatoma cells were determined as a function of temperature between 5 and 41°C. Arrhenius plots of all transport systems exhibited sharp breaks in slope; between 17 and 23° for uridine, thymidine and hypoxanthine-guanine transport and between 29 and 32° for choline and 2-deoxy-d-glucose transport. The activation energies for the transport systems changed from 15–26 kcal/mole below the transition temperatures to 4–9 kcal/mole above the transition temperatures. Propagation of the cells in the presence ofcis-6-octadecenoic acid which results in marked changes in the lipid composition of cell membrane, had little effect on the temperature characteristics of the various transport systems. Similarly, propagation of the cells for 24 hr in media containing Tween 40 or nystatin had no effect on the capacity of the cells to transport the various substrates or on the temperature dependence of the transport systems. The presence of ethanol, phenethyl alcohol or Persantin at concentrations that inhibited thymidine and 2-deoxy-d-glucose transport between 40 and 70% also did not alter the transition temperatures or activation energies for the transport of these substrates. Cytochalasin B, on the other hand, shifted the transition temperature for 2-deoxy-d-glucose transport to higher temperatures in a concentration-dependent manner, whereas it had no effect on the temperature dependence of thymidine transport.