title: Evidence for two types of rat liver microsomes with differing permeability to glucose and other small molecules.
abstract: Radioisotope flux measurements using Millipore filtration revealed two populations of rat liver microsomes designated type A and B. Type A and B vesicle are similar in that both are essentially impermeable to sucrose yet permeable to Cl-. About 70% of the microsome (type A) are permeable to D-glucose, L-glucose, 2-deoxy-D-glucose, D-mannose, D-mannitol, uridine, glycine, L-leucine, choline+, Tris+, Rb+, K+, and Na+. Other solutes such as D-gluconate-, D-glucosamine+, N-acetyl-D-glucosamine, L-glutamate-, L-lysine+, sulfate2-, oxalate2-, and phosphate anions transverse type A vesicles with an intermediate rate. All of the above solutes except Cl- pass with a comparatively slow rate the remaining 30% type B vesicles. Both type A and B microsomes are relatively impermeable to glucose 6-phosphate and related monophosphates. Membrane potential measurements using liver microsomes and control membrane vesicles derived from rabbit skeletal muscle sarcoplasmic reticulum indicated that type A liver microsomes, despite being permeable to K+ and Na+, either lack or contain only a small number of highly conducting K+ and Na+ structures, such as the K,Na channel of sarcoplasmic reticulum. Treatment with the anion transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid lowered the permeability of type A vesicles to several uncharged and negatively charged solutes including D-glucose and gluconate-. These results suggest that a large fraction of liver microsomes is rendered permeable to various biologically relevant solutes and ions, perhaps through the presence of one or more channels with a maximal diameter of approximately 7-8 A which select(s) against solutes on the basis of their size and charge.
mesh_terms: D000818:Animals; D002712:Chlorides / Q000378:metabolism; D005947:Glucose / Q000378:metabolism*; D008564:Membrane Potentials; D008854:Microscopy, Electron; D008862:Microsomes, Liver / Q000378:metabolism* / Q000648:ultrastructure; D009997:Osmotic Pressure; D010539:Permeability; D011188:Potassium / Q000378:metabolism; D051381:Rats; D012964:Sodium / Q000378:metabolism
chemical_list: D002712:Chlorides; D012964:Sodium; D005947:Glucose; D011188:Potassium