AJP - Regulatory, Integrative and Comparative Physiology, Vol 248, Issue 2 133-R141, Copyright © 1985 by American Physiological Society
Glucose transport by lobster hepatopancreatic brush-border membrane vesicles
G. A. Ahearn, M. L. Grover and R. E. Dunn
Epithelial brush-border membrane vesicles (BBMV) were made from lobster
hepatopancreas by using Mg2+ precipitation. Alkaline phosphatase,
Na+-K+-ATPase, and cytochrome c oxidase activities in these vesicles were
enriched 15.0-, 1.0-, and 0.19-fold, respectively, compared with activities
of a washed original homogenate pellet, indicating a relatively pure apical
membrane preparation reduced in basolateral or organelle contamination.
Complete vesicular closure was confirmed with electron microscopy and
equilibrium [3H]D-glucose uptake experiments using various transmembrane
osmotic gradients. Glucose uptake was stimulated by a transmembrane Na+
gradient but not by an identical K+ gradient or by a Na+ gradient in the
presence of phloridzin. Electrogenicity of Na+-dependent glucose transport
was confirmed in two ways. First, an anion permeability sequence indicated
glucose uptake was stimulated in the following order: SCN- greater than Cl-
greater than gluconate- greater than SO4(2-). Second, an outwardly directed
valinomycin-induced K+ diffusion potential, rendering the vesicle interior
electrically negative, enhanced glucose uptake compared with K+-loaded
vesicles lacking the ionophore. Glucose influx occurred by a combination of
carrier-mediated transfer, illustrating Michaelis-Menten kinetics, and
nonsaturable "apparent diffusion." pH (same on both sides) strongly
influenced Na+-dependent glucose uptake according to the sequence: pH 6.0
greater than pH 7.4 greater than pH 8.0. Increased proton concentration
lowered the Michaelis constant for glucose transport and increased the
apparent diffusional permeability of the membrane to the sugar. Maximal
carrier-mediated glucose transport rate was largely unaffected by
pH.(ABSTRACT TRUNCATED AT 250 WORDS)
