PROCEEDINGS OF THE SOCIETY FOR E X P E R I M E N T A L BIOLOGY A N D M E D I C I N E 151.
372-375 (1976)
Effects of Glucagon on Gluconeogenesis from Lactate and Propionate in the Perfused Rat Liver (39213) TIMOTHY M. CHAN A N D RICHARD A. FREEDLAND Vanderbilt University, School of Medicine, Department of Physiology, Nashville, Tennessee 3 7232, and Department of Physiological Sciences, School of Veterinary Medicine, University of California, Davis, California 9.5616
Materials and methods. Male Sprague-Daw- converge at the point of malate-oxaloaceley rats, weighing 100-160 g, were fed Pur- tate, their similar responses to glucagon ina Chow pellets, then starved 48 hr prior to does not necessarily imply that the hormone use. Procedures for in situ liver perfusion acts above the point of oxaloacetate formahave been described (7). Unless otherwise tion. With propionate as substrate, glucagon stated, livers were perfused without sub- could have stimulated steps that are not in strate for the first 37 min. Addition of quin- common with pyruvate gluconeogenesis. olinate, butyrate, glucagon, or their combi- Exton et al. (4) observed a possible stimulanations, were made 2 min prior to the addi- tion, between a-ketoglutarate and succinate tion of lactate. Glucagon solutions were pre- in the tricarboxylic acid cycle, by glucagon pared as described by Exton and Park (9). in the perfused rat liver. In some experiments, portions of the liver In addition to the pyruvate carboxylase were frozen with aluminum tongs prechilled and PEP carboxykinase reaction, transport in liquid nitrogen, 40 min after perfusion of dicarboxylic acids across the mitochonwith substrate plus other additions. Extrac- drial membrane has also been suggested as a tion of tissues and assays for glucose and possible process affected by glucagon, and other metabolites either in tissue extracts or leading to stimulation of glucose production in perfusate have been described elsewhere (ref. 4; Fig. 1). It is therefore still possible (7). Statistical significance and P values that the glucagon effects are identical with were determined by the use of the Student’s both lactate and propionate. This possibility was examined by the use of butylmalonate, t test (8). Results and discussion. The effects of glu- which has been shown to inhibit both malate cose production under various conditions transport across the mitochondrial memare shown in Table I. One potential hazard brane and gluconeogenesis from lactate in in studying glucagon stimulation of glucone- the perfused rat liver (10). Results in Table ogenesis without the use of radioactive-la- I show that butylmalonate inhibited glucose beled substrates is the well documented gly- production from lactate more drastically cogenolytic effect of glucagon. The slow rate than from propionate. The addition of gluof glucose production, in the absence of cagon did not significantly alter the rate of added substrate, suggests that under these glucose production from lactate plus butylconditions the main source of glucose was malonate, while with propionate the butylendogenous substrate(s), which was not in- malonate inhibition was almost completely fluenced by glucagon. The salient points of reversed by the hormone, indicating that potential control of gluconeogenesis from glucagon action is not likely exerted on pyruvate and propionate are shown in Fig. points common to both propionate and lac1. Figure 1 also indicates some of the poten- tate. Since the directions of flow of carbon tial control points as possibly affected by from lactate and propionate to the dicarboxglucagon, as well as the site of action of ylic acids are opposite to each other, glucamany of the inhibitors used in this study. gon could have stimulated the formation of The rate of glucose production from either a dicarboxylic acid from propionate, whose lactate or propionate was increased by the transport across the mitochondrial memaddition of glucagon to approximately the brane was not inhibited by butylmalonate. same extent. Although the pathways of glu- A stimulation of some reactions resulting in coneogenesis from lactate and propionate elevated fumarate formation from propion3 72 Copyright 0 1976 by the Society for Experimental Biology and Medicine All rights reserved.
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GLUCAGON AND GLUCONEOGENESIS
TABLE I. EFFECTOF GLUCAGON ON RATESOF GLUCONEOGENESIS? Rate (pmol/min/g liver) Substrate (mM)
+ Glucagon
- Glucagon
None Lactate (1 0) Lactate + butyrate (5) Lactate + Q.A. (4.8) Lactate + Q.A. + Butyrate Propionate (10) Lactate (10) + Butylmalonate (2) Propionate (1 0) + Butylmalonate
0.20 f. 0.08 (4) 1.05 f. 0.07 (8) 1.63 2 0.04 (4) 0.51 -+ 0.06 (4) 0.55 2 0.04 (4) 0.70 f. 0.13 (8) 0.54 -+ 0.08 (4) 0.49 2 0.02 (4)
0.23 f. 0.10(4) 1.36 2 0.12 (4) 0.52 f. 0.05 (4)
-
0.95 & 0.05 (4) 0.68 2 0.04 (4) 0.81 ? 0.09 (4)
P
>0.10 0.10 0.05