Temperature Dependence of Nucleoside Membrane Transport in Rabbit Alveolar Macrophages and Polymorphonuclear Leukocytes

The effects of temperature on two membrane transport systems in alveolar macrophages and one system in polymorphonuclear leukocytes which have been well characterized at 37° were analyzed. The results show a discontinuity in the Arrhenius plots (transition temperature) for both the alveolar macrophage systems but none in the polymorphonuclear leukocytes. A detailed analysis of the specificity of nucleoside transport in alveolar macrophages reveals a marked temperature dependence which is reflected mainly in requirements at the 3' (ribose)-position. Pretreatment of the alveolar macrophages with colchicine alkaloids results in a transport system for nucleosides which shows no transition temperature and has the specificity and activation energy of the untreated system in the low temperature range. On the basis of these findings it is proposed that the nucleoside carrier exists as two or more conformers, one of which is favored by high temperatures, and the other by low temperatures or colchicine alkaloids.