Adaptation, constraint, and the function of the gluconeogenic pathway

Abstract

and the function of the gluconeogenic pathway. Can. J . Zool. 66: 1059 -1068.Gluconeogenesis is responsible for the de novo synthesis of glucose (and glycogen) from precursors including lactate, amino acids, glycerol, and fructose. This metabolic sequence is highly constrained by design features including enzyme composition and tissue localization, but demonstrates a variety of adaptive patterns which are critical to the maintenance of blood glucose levels optimal for animal function. This review identifies the adaptive responses of gluconeogenesis when glucose levels are challenged by changes in diet (both quality and quantity) and in activity level, and by environmental disturbances. Five adaptive patterns are identified: (i) quantitative changes in gluconeogenic enzyme activities and their subcellular distribution; (ii) alterations in tissue demand for glucose; (iii) existence of in situ skeletal muscle lactate cycling; (iv) quantity and type of gluconeogenic precursors; and (v) regulation of gluconeogenesis. The validity of the omnivorous mammalian model in our understanding of gluconeogenesis is discussed. MOON, T. W. 1988. Adaptation, constraint, and the function of the gluconeogenic pathway. Can. J. Zool. 66 : 1059 -1068.La glyconCogCnkse est responsable de la synthkse de novo du glucose (et du glycogkne) a partir de precurseurs, notamment le lactate, les acides aminCs, le glycCrol et le fructose. Cette sequence mCtabolique est soumise aux contraintes imposees par des caractCristiques structurales, telles la nature des enzymes et leur repartition dans les tissus, mais fait preuve de propriCtCs d'adaptation determinantes pour que soient maintenues dans le sang des concentrations glycCmiques essentielles au fonctionnement optimal de I'animal. Cette revision a pour but d'identifier les reactions d'adaptation de la glyconCogCnkse lorsque les concentrations glycCmiques sont menacees par des changements (qualitatifs et quantitatifs) dans le rkgime alimentaire et dans l'intensitk de I'activitC, et aussi par des perturbations de l'environnement. Cinq propriCtCs d'adaptation ont pu etre reconnues : (i) des changements quantitatifs dans I'activitC des enzymes de la glyconCogCnkse et dans leur repartition au niveau infracellulaire, (ii) des modifications de la demande tissulaire en glucose, (iii) I'existence d'un cycle du lactate in situ dans les muscles squelettiques, (iv) la quantitC et le type particuliers des precurseurs glyconCogCniques et (v) la regulation de la glyconeogenkse. La validit6 du modkle du mammifkre omnivore pour faciliter la comprehension de la glyconCogCnkse fait I'objet d'une discussion.[Traduit par la revue]

Seasonal metabolic depression, substrate utilisation and changes in scaling patterns during the first year cycle of tegu lizards (Tupinambis merianae)

“…The substantial reduction in tail muscle total protein in arousing tegus (50%) is consistent with the general idea that amino acids from protein breakdown play an important role as precursors for glucose synthesis at the end of a prolonged fasting period, in addition to glycerol from fatty acid oxidation (Moon, 1988). Protein is also an important source of energy for both large and small hibernating mammals (Cherel et al, 1995;Tinker et al, 1998), and an adequate balance of fat and protein use is apparently part of a common repertory in several spontaneous fasters, the control of which is poorly known (Robin et al, 1998;Mellish and Iverson, 2001).…”

Changes in plasma glucagon and insulin associated with fasting in sea bass (Dicentrarchus labrax)

“…The supply of gluconeogenic substrates from the muscle to the liver could be related to an enhancement of gluconeogenesis. An increase in gluconeogenic enzymes has been previously found in fasted sea bass (Zammit and Newsholme 1979), plaice (Moon and Johnston 1979), and salmonids (Moon 1988) and glucagon and glucagon-like pep|ides have an important role in the activation of the gluconeogenic pathway (Moon 1988; Moon el al. 1989).…”

Effects of the parasite Probopyrus ringueleti (Isopoda) on glucose, glycogen and lipid concentration in starved Palaemonetes argentinus (Decapoda)

“…Most species reduce their metabolic rate and deplete protein, carbohydrate, glycogen and/or lipid reserves during nutritional stress (Hazlett et al 1975, Regnault 1981. The relative importance of these energetic reserves and the order of utilization vary with species, recent feeding history and length of starvation (Schirf et al 1987, Moon 1988.…”