Unique cytochalasin B binding characteristics of the hepatic glucose carrier

Abstract

Cytochalasin B inhibits uptake of 3-O-methylglucose into isolated rat hepatocytes with a Ki = 1.9 .mu.M. The nature of this inhibition was characterized by studies of [3H]cytochalasin B binding to liver plasma membranes. Scatchard analysis of [3H]cytochalasin B binding reveals a complex curvilinear binding pattern. This pattern can be resolved into 3 components: a high-affinity (.apprx. 10-8 M) cytochalasin E sensitive site unrelated to glucose uptake, a glucose-sensitive site, and a low-affinity site. When 5 .mu.M cytochalasin E is employed to mask the high-affinity site, glucose displaces 40-60% of the remaining [3H]cytochalasin B binding. Analysis of this glucose-sensitive cytochalasin B binding according to Scatchard reveals a Kd = 1.7 .mu.M, indistinguishable from the concentration of cytochalasin B which half-maximally inhibits hepatic glucose uptake. These data identify a glucose-sensitive cytochalasin B binding site in liver plasma membranes which corresponds to the glucose carrier in the intact hepatocyte. The Ki of 1.9 .mu.M for inhibition of hepatic glucose uptake by cytochalasin B and the Kd of 1.7 .mu.M for [3H]cytochalasin B binding to liver plasma membranes are values 1 order of magnitude higher than values for the same parameters determined in all previous studies of facilitated hexose diffusion systems. The hepatic hexose carrier is therefore unique, and this uniqueness may be of regulatory significance with regard to glucose homeostasis.