Proton movement accompanying monocarboxylate permeation in hemoglobin-free perfused rat liver

Abstract

While considerable information has accumulated recently from studies with isolated liver mitochondria on transport systems for diand tricarboxylates (cf. reviews [ 1,2] ), comparatively little is known on the properties of uptake and release by the liver cell of the major metabolic fuels which are either uncharged (e.g. glucose) or weak monocarboxylic acids (fatty acids, lactic and pyruvic acids). The experiments to be reported here represent attempts to follow the rate and the extent of monocarboxylic acid uptake and release by the liver cell. With the help of a ‘model monocarboxylic acid’, benzoic acid (pK = 4.2), which is considered metabolically inert when compared to the compounds named above, a simple and continuous method to follow monocarboxylic acid movements into and out of the hepatocyte will be outlined. The experimental system is that of the hemoglobin-free perfused rat liver [3,4] . The underlying idea is based on the concept of Overton [5] , elaborated and applied by Conway to the yeast cell [6,7] and by Hohorst, Kreutz and Bticher to the liver cell [8] , that permeant monocarboxylates are passively transported across biological membranes in the form of the uncharged acids. Therefore, net monocarboxylate transport should be coupled to net proton transport, no matter whether the mechanism is ‘p&on symport’ or ‘hydroxyl antiport’. In other words, the compartment from which monocarboxylic acid is leaving is expected to be rendered more alkaline, and vice uersa. On this basis, extracellular pH is explored as an indicator of monocarboxylate gradient equilibration in an open (non-recirculating) system of perfusion.