EFFECT OF TEMPERATURE AND IONOPHORES ON THE PERMEABILITY OF SYNAPTOSOMES

Synaptosomes swell rapidly in isosmotic solutions of glycerol or urea, but the swelling in solutions of larger non‐electrolytes, such as erythritol, glucose or sucrose is slower. The permeability of synaptosomes to non‐electrolytes is temperature dependent, and the low activation energies for the permeation of urea (13 kcal/mol) and erythritol (9.5 kcal/mol) indicate that the penetration of non‐electrolytes into the synaptosomes does not imply complete dehydration of the molecules. The relative permeability of synaptosomes to cations, as measured by the rate of swelling in isosmotic solutions of acetate salts is in the order: NH+4 > Na+ > Li+ > K+ > Ca2+. The ionophores, X‐537A and nigericin, or valinomycin + FCCP, which promote exchange of cations for H+, cause swelling of synaptosomes in solutions of potassium salts of acetate or propionate, but not in KCI, whereas H+ release is higher in KCI medium. This suggests that the organic unions cross the membrane after combining with H+ to form the respective weak acids. The relative permeability to anions is in the order: acetate ≃ propionate > Cl− > SO2‐4≃ maleate ≃ succinate. The energies of activation for the permeability of synaptosomes to potassium acetate in the presence of X‐537A or gramicidin D are 13 kcal/mol and 7.5 kcal/mol, respectively, which reflects different mechanisms of action for the two ionophores in the membranes. Copyright © 1979, Wiley Blackwell. All rights reserved