PFK Activity Research Articles

Changes in carbohydrate metabolism in domesticated rainbow trout ( Oncorhynchus mykiss) related to spermatogenesis

1. 1. A study of several carbohydrate metabolism pathways was carried out in three different tissues—liver, testes and kidney—from domesticated rainbow trout males ( Oncorhynchus mykiss) during spermatogenesis. 2. 2. A fall in liver glycogen levels occurred during spermatogenesis and the glucose obtained by glycogen rupture may be mainly used in liver glycolysis. 3. 3. An increase of glucose levels was observed in testes during spermatogenesis and may be mainly due to an increased gluconeogenesis. The higher glucose levels may be used not only in glycogen synthesis but also in generating reducing power. 4. 4. In kidney, only PFK activity was changed (decreased) during spermatogenesis. 5. 5. Spermatogenesis in rainbow trout is mainly associated with a higher use of glucose in the gonad and increased metabolic activity in the liver.

Molecular genetics of phosphofructokinase in the yeast Kluyveromyces lactis

We have undertaken a study of phosphofructokinase (PFK; E.C. 2.7.1.11) in the yeast Kluyveromyces lactis. Like other eukaryotic PFKs, the K. lactis enzyme is activated by the allosteric effectors AMP and fructose-2,6-bisphosphate. PFK activity is induced in cells grown on glucose as compared to ethanol-grown cells, in contrast to the constitutive expression of PFK in Saccharomyces cerevisiae. We show here that phosphofructokinase of the yeast K. lactis is composed of two non-identical types of subunits, encoded by the genes KIPFK1 and KIPFK2. We have cloned and sequenced both genes. KIPFK1 and KIPFK2 encode the alpha- and the beta-PFK subunits with deduced molecular weights of 109.336 Da and 104.074 Da, respectively. Sequence analysis indicates that the genes evolved from a double duplication event. Null mutants in either of the genes lack detectable PFK activity in vitro and the respective subunits cannot be detected on Western blots. In contrast to the situation in S. cerevisiae, Klpfk1 Klpfk2 double mutants retain the ability to grow on glucose. However, Klpfk2 mutants and the double mutants do not grow on glucose, when respiration is blocked. These data suggest that the pentose phosphate pathway and respiration play a substantial role in glucose utilization by K. lactis. The K. lactis PFK genes can be expressed independently in S. cerevisiae and each of them complements the glucose-negative phenotype of pfk1 pfk2 double deletion mutants in this yeast. Expression of both K. lactis PFK genes simultaneously in S. cerevisiae pfk double deletion mutants complements for PFK activity. However, expression of a combination of PFK genes from K. lactis and S. cerevisiae does not lead to the production of a functional enzyme.

Vascular cambial sucrose metabolism and growth in loblolly pine (Pinus taeda L.) in relation to transplanting stress.

Sucrose synthase (SS) was the dominant enzyme of sucrose metabolism in both stem and root vascular cambial zone tissues of nursery-grown loblolly pine (Pinus taeda L.) seedlings. Acid invertase (AI) and neutral invertase (NT) activities were generally less than 10% of the SS activity in both tissues. In both cambial tissues, seasonal patterns of enzyme activity were observed for SS but not for AI or NI. The seasonal patterns of SS activity in stem and root cambia paralleled the periodic growth of stems and roots. Stems had high SS activity and growth during summer and early fall. Roots had substantial SS activity and growth during summer and fall, but SS activity and growth were even higher in winter. When seedlings were transplanted, about eight months elapsed before stem and root cambia resumed rates of growth and sucrose metabolism similar to those in control nontransplanted seedlings. Two months after transplanting, root SS was at its lowest, whereas AI activity in transplants was 50% higher than in control nontransplanted seedlings. In stems, SS activity decreased in response to transplanting, whereas AI and NI activities did not change appreciably. In loblolly pine tissues, SS was specific for uridylates, whereas the nucleotide triphosphate-dependent phosphofructokinase (NTP-PFK) had similar activity with either UTP or ATP. Except in winter, the NTP-PFK was less active than the pyrophosphate-dependent phosphofructokinase (PPi-PFK) during all seasons. The PPi-dependent PFK activity in nontransplanted seedlings followed similar seasonal and spatial patterns to those of SS activity. In actively growing tissues, such as stem cambial tissues in summer and root cambial tissues in winter, the measured total PFK to SS ratio ranged between 1.5/1 and 3/1. In contrast, in less actively growing tissues or transplanted seedlings, a greater decrease occurred in SS than in PFK activity, hence the ratio rose to as high as 12/1. It was concluded that: (1) SS was the dominant enzyme for sucrose metabolism in root and stem cambial tissues of loblolly pine seedlings; (2) both SS and PPi-PFK in the cambial tissues can be used as biochemical indicators of growth sink strength in stems and roots; and (3) both enzymes can be used as indicators of seedling stress caused by events such as transplanting and winter freezing.

Phosphofructokinase from the Host Fraction of Soybean Nodules

Summary The predominant form of phosphofructokinase (EC 2.7.1.11; PFK) in the host fraction of soybean nodules was isolated as a large aggregated protein with apparent molecular mass near 2,000kDa. SDS-PAGE indicated that the enzyme was made up of three polypeptides of molecular mass of 55, 59 and 62 kDa. The soybean nodule PFK displayed typical Michaelis-Menten kinetics with respect to fructose-6-phosphate and the nucleoside triphosphate substrate at the pH optimum (PH 8.6) and at pH 7. MgATP was the most effective phosphoryl donor. Phosphoenolpyruvate (PEP) was a potent inhibitor of PFK activity, and the enzyme was also strongly inhibited by 3-phosphoglycerate, 2-phosphoglycerate, and to a lesser extent, PPi. PFK was activated by KCI, NaCI and Pi, and Pi relieved the inhibition caused by PEP. A minor form of PFK, with apparent native molecular mass of 200 kDa, was also present in extracts from the host fraction of soybean nodules. The 200-kDa form of PFK was only weakly activated by KCI and Pi and was much less sensitive to inhibition by PEP than the predominant form of PFK in the nodules.

Glutathione Depletion Alters Hepatocellular High-Energy Phosphate Metabolism

Oxygen free radicals have recently been implicated as a major cause of tissue injury in critically ill patients. Glutathione (GSH) is a potent endogenous antioxidant that may be important in minimizing oxidant-induced organ damage. However, this tripeptide is depleted during severe illness. In order to determine the effect of GSH depletion on hepatic high-energy phosphate metabolism, in vivo31P magnetic resonance spectroscopy was used to measure phosphate ratios in male Wistar rats given 1 ml/kg of diethylmaleate (DEM), an agent that binds and thus depletes tissue GSH, or corn oil vehicle intraperitoneally. Spectra of the liver were obtained in noninjected animals (baseline, n = 15) and in rats 2 and 24 hr after the intraperitoneal injection of DEM (n = 20) or corn oil (control, n = 20). These spectra were used to measure hepatocellular pH, phosphomonoester to ATP (PME/ATP), and phosphodiester to ATP ratios, measures of hepatocellu]ar damage; and the inorganic phosphate (Pi)/ATP ratio, a measure of energy status. In addition, tissue GSH, phosphofructokinase, citrate synthase, and β-OH-acyl-Co-A dehydrogenase activities as well as hepatocellular ATP were measured in vitro in representative liver samples. Hepatic GSH levels were maximally depressed by 85% 2 hr after the injection of DEM (6.94 ± 0.34 vs 0.94 ± 0.22 μM/g wet wt, baseline vs 2° DEM). This was associated with a marked increase in the PME/ATP and Pi/ ATP ratios by 25 and 33%, respectively, and both ratios were significantly correlated with the severity of hepatic GSH depletion (r = 0.63, P < 0.001 and r = 0.42, P < 0.01, respectively). In addition, the intracellular pH decreased from 7.41 ± 0.02 to 7.33 ± 0.02 (P < 0.05) 2 hr after the injection of DEM. At this time citrate synthase and β-OH-acyl-Co-A dehydrogenase activities were significantly elevated in the GSH-depieted animals. In contrast, ATP levels did not change significantly (2.92 ± 0.07 vs 3.20 ± 0.28 μM/g wet wt, baseline vs 2° DEM) and PFK activity was similar in both groups. We conclude that (1) GSH depletion in rats alters the hepatocellular energy status primarily due to increased ATP turnover; (2) under these conditions, hepatic ATP synthesis via the oxidation of fatty acids accelerates to meet an increased energy demand; and (3) these changes in hepatocellular energetics and PME/ATP ratios may indicate early hepatocyte injury and are likely due to increased oxidative stress unveiled by GSH depletion. The GSH depletion associated with critical illness may contribute to the development of liver dysfunction in severely injured patients.

Regulation of brain 6-phosphofructo-1-kinase: effects of aging, fructose-2,6-bisphosphate, and regional subunit distribution.

Total 6-phosphofructo-1-kinase (PFK) activity, amounts of each type of PFK subunit, and levels of fructose-2,6-P2 in the cerebral cortex, midbrain, pons-medulla, and cerebellum of 3, 12, and 25 month rats were measured. Further, the role of fructose-2,6-P2 in the regulation of brain PFK activity was examined. A positive correlation was found to exist between the reported losses of glucose utilization as measured by 2-deoxy-D-glucose uptake and PFK activity in each region. That is, both parameters decreased to their lowest level by 12 months of age and remained decreased and fairly constant thereafter. Fructose-2,6-P2 levels did not appear to directly correlate with regional changes in glucose utilization. Also, region-specific and age-related alterations of the PFK subunits were found although these changes apparently did not correlate with decreased glucose utilization. Brain PFK is apparently saturated with fructose-2,6-P2 due to the high endogenous levels, and it contains a large proportion of the C-type subunit which dampens catalytic efficiency. Consequently, brain PFK could exist in a conformational state such that it can readily consume fructose-6-P rather than in an inhibited state requiring activation. This may explain, in part, the ability of brain to efficiently but conservatively utilize available glucose in energy production.

Effect of experimental hypothyroidism on the control of 6-phosphofructo-1-kinase activity in rat jejunal mucosa

Changes in the activity of 6-phosphofructo-1-kinase (PFK, EC 2.7.1.11) from the epithelial cells of rat small intestine during experimental hypothyroidism were studied. Hypothyroism resulted in significant decreases in the plasma concentrations of total tri-iodothyronine, free tri-iodothyronine, total thyroxine, free thyroxine and insulin. These changes were associated with a significant increase in the plasma concentration of thyrotropin. The total activity and activity ratios (activity at 0.5 mM fructose 6-phosphate at pH 7.0/activity at pH 8.0 ( v 0.5/V)) of jejunal PFK of hypothyroid rats were significantly diminished as compared to control rats. PFK of hypothyroid rats was more sensitive to inhibition by ATP. The mucosal enzyme of both control and hypothyroid state was sensitive to stimulation by AMP and fructose 2,6-bisphosphate. It is concluded that during hypothyroidism the rate of glycolytic pathway in the small intestine is reduced as a result of a fall in glucose uptake, and the subsequent kinetic changes of PFK are primarily to maintain the concentrations of fructose 6-phosphate (and glucose 6-phosphate) during the reduced glycolytic flux. These changes in PFK activity may be caused by changes in plasma insulin concentrations, glucose utilization and fructose 2,6-biphosphate concentrations.

Pilocarpine-induced increases in the activity of 6-phosphofructo-2-kinase and the fructose-2,6-bisphosphate content of rat salivary glands

The activity of 6-phosphofructo-1-kinase (PFK-1), an important regulatory enzyme of glycolysis, was determined after injection of the sialagogue pilocarpine. The fructose-2,6-bisphosphate content of the glands and 6-phosphofructo-2-kinase (PFK-2) activity were also measured. The increase in PFK-1 activity after pilocarpine treatment was likely to be due to the increase in the content of its potent modulator, fructose-2,6-bisphosphate. This in turn was assumed to be due to the increase in the activity of the active form of PFK-2.

Developmental changes of 6-phosphofructo-1-kinase subunit levels in erythrocytes from normal dogs and dogs affected by glycogen storage disease type VII

1. 1. The subunit proportions (L:M:C) of the PFK isozymes from normal adult erythrocytes were 2:86:12. Affected adult erythrocyte 6-phosphofructo-1-kinase (PFK) isozymes contained normal L-type (31%) and C-type (61%) subunits as well as a small amount (8%) of truncated M-type subunit. 2. 2. When measured within 24 hr of birth, both normal and affected dog erythrocytes contained high PFK activities due to elevated levels of the L-type subunit. As the dogs matured, PFK activity decreased due to a greater than 99% loss of the L-type subunit. 3. 3. By 2 weeks of age, the M-type and C-type subunits in normal dog PFK isozymes increased several-fold and attained near adult levels. 4. 4. During post-natal development, the L-type subunit from affected dog erythrocytes decreased more rapidly than from normal dog erythrocytes; but it was maintained at a higher level in the affected adult erythrocytes. Also, in the affected dog erythrocytes, truncated M-type subunits were detected; and the initially high levels of the C-type subunit decreased approximately 50% after 4 weeks.

6-Phosphofructo-1-kinase from the flight muscle of the grasshopper, Poekilocerus bufonius

1. 1. The ultrastructure of the flight muscle of the adult male grasshopper, Poekilocerus bufonius, showed a variation in the shape and size of each individual myofibril profile which was in almost parallel register. 2. 2. Moderate number of mitochondria are present as well as high numbers of compact cristae. 3. 3. 6-Phosphofructo-1-kinase (PFK) from the flight muscle of P. bufonius was purified more than 1000-fold to homogeneity with a yield of 77%. 4. 4. The sodium dodecyl sulphate-treated purified enzyme migrated as a single band in 7.5% polyacrylamide gel. 5. 5. The enzyme is a tetramer, with a monomer M r 82,000 ± 2000. 6. 6. The regulatory properties of the purified enzyme were studied at pH 7.0 and the affinity of the enzyme for fructose 6-phosphate was slightly increased by fructose 2,6-bisphosphate whereas the enzyme inhibition by high concentrations of ATP was slightly relieved by fructose 2,6-bisphosphate. 7. 7. The activity of flight muscle PFK was markedly inhibited by glucose 1,6-bisphosphate and phosphoarginine, and was weakly activated by ADP, AMP and fructose 2,6-bisphosphate. 8. 8. These data indicate that flight muscle PFK is not strongly affected by activators or by synergism between AMP and fructose 2,6-bisphopshate which is consistent with the fact that this insect is incapable of a long flight and the flight muscles of the adult females are very small and reduced to a thread-like structure.

Tolbutamide inhibits cAMP-dependent phosphorylation of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase.

The activity of a bifunctional enzyme, liver 6-phosphofructo-2-kinase (PFK-2)/fructose-2,6-bisphosphatase (F-2,6-Pase), which regulates the level of liver fructose-2,6-bisphosphate (F-2,6-P2), the most potent activator of PFK, is modulated by its phosphorylation rate mainly catalyzed by cAMP-dependent protein kinase A (PKA). To elucidate the action mechanism of sulfonylurea on liver F-2,6-P2 production, effects of tolbutamide on PKA-dependent phosphorylation of purified liver PFK-2/F-2,6-Phase protein and on kinase and phosphatase activities of the purified enzyme were examined in vitro. The purified enzyme was phosphorylated in the presence of the catalytic subunit of PKA, and tolbutamide inhibited the enzyme phosphorylation catalyzed by PKA in a dose-dependent manner. By adding the same dosages of tolbutamide used in the phosphorylation experiment, reduced activity of PFK-2 and increased activity of F-2,6-Pase in the presence of PKA were restored to the levels observed in the absence of PKA. On the other hand, carboxytolbutamide, an inactive metabolite of tolbutamide, had little effect on enzyme phosphorylation and activity. Our results indicate that tolbutamide inhibits a phosphorylation of the liver PFK-2/F-2,6-Pase catalyzed by PKA along with an activation of PFK-2 and an inactivation of F-2,6-Pase, leading to liver F-2,6-P2 production.

Stimulation of glycolysis as an activation signal in rat peritoneal macrophages. Effect of glucocorticoids on this process.

1. Peritoneal macrophages were prepared from control, Escherichia coli-treated and triamcinolone acetonide-treated rats. Control and E. coli-treated rats produced resident and activated macrophages respectively. Glycolysis in these cells was studied by the fructose 2,6-bisphosphate (Fru-2,6-P2) content, lactate release and 6-phosphofructo-1-kinase (PFK-1) and 6-phosphofructo-2-kinase (PFK-2) activities. 2. In activated macrophages, lactate release and Fru-2,6-P2 content were increased several-fold compared with those in resident cells. Moreover, the response of these parameters to phorbol 12-myristate 13-acetate in activated macrophages was greater than for resident cells. 3. PFK-2 activity was moderately increased (about 3-fold), but PFK-1 activity was increased 5-fold in activated macrophages compared with resident cells. Partially purified preparations of PFK-1 were sensitive to Fru-2,6-P2, with K0.5 about 0.25 microM in both control and activated cells. However, the Vmax. of PFK-1 from activated cells was increased. In addition, AMP stimulated PFK-1, but the kinetic pattern was different from that described for Fru-2,6-P2. Moreover there was no difference in the stimulation by AMP of PFK-1 from resident and activated cells. 4. Fru-2,6-P2 content and lactate release in macrophages from triamcinolone acetonide-treated rats were decreased in both resident and activated cells. Also, the glucocorticoid inhibited PFK-1 and PFK-2 activities in both resident and activated macrophages. PFK-1 from triamcinolone acetonide-treated rats was not stimulated by Fru-2,6-P2, whereas the effect of AMP was unchanged. The effects of glucocorticoid seem to be specific for phagocytic cells, since the glucocorticoid treatment increased PFK-1 and PFK-2 activities in liver.

Stimulation of glycolysis by corticotropin and phorbol ester in cultured neurons

Incubation of cultured neurons from chick embryo forebrain with corticotropin (ACTH) or the phorbol ester TPA (12- O-tetradecanoylphorbol 13-acetate) stimulates the production of lactate. The stimulation is seen after 2 h of treatment and is maximal after 12 h. Both ACTH (1–24) and TPA increase the concentration of fructose 2,6-bisphosphate (Fru-2,6-P 2), a metabolic activator of 6-phosphofructo-1-kinase (PFK-1). This effect is concentration-dependent and is maximal after 4 h of treatment. PFK-1 activity is increased in a dose-dependent manner by ACTH (1–24) or TPA. This increase is not visible during the first 6 h and reaches its maximum after 8 h of treatment. The stimulation of PFK-1 activity is not due the increase of Fru-2,6-P 2 by ACTH (1–24) or TPA, since saturating concentrations of Fru-2,6-P 2 are present in the PFK-1 assay medium. Thus, it appears that ACTh (1–24) and TPA regulate glycolysis through two modes with different time responses: increase in Fru-2,6-P 2 is the main mechanism operating during the first 6 h following the treatments and increase in the amount, or stable increase in activity of PFK-1, takes place during the later phase. It is suggested that the action of corticotropin on glycolysis is part of the mechanism of the neurotrophic activity of this hormone.

Rat brain glycolysis regulation by estradiol-17β

The effect of estradiol-17β on the activities of glycolytic enzymes from female rat brain was studied. The following enzymes were examined: hexokinase (HK, EC 2.7.1.1), phosphofructokinase (PFK, EC 2.7.1.11), aldolase (EC 4.1.2.13), glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.12), phosphoglycerate kinase (EC 2.7.2.3), phosphoglycerate mutase (EC 2.7.5.3), enolase (EC 4.2.1.11) and pyruvate kinase (PK, EC 2.7.1.40). The activities of HK (soluble and membrane-bound), PFK and PK were increased after 4 h of hormone treatment, while the others remained constant. The changes in activity were not seen in the presence of actinomycin D. The significant rise of the activities of the key glycolytic enzymes was also observed in the cell culture of mouse neuroblastoma C1300 treated with hormone. Only three of the studied isozymes, namely, HK11, B 4 and K 4 were found to be estradiol-sensitive for HK, PFK and PK, respectively. The results obtained suggest that rat brain glycolysis regulation by estradiol is carried out in neurons due to definite isozymes induction.

The subunit proportions and kinetic properties of 6-phosphofructo-1-kinase isozymes from rat heart atria and ventricle progressively change during aging

Relative to 2-3 month rats, total 6-phosphofructo-1-kinase (PFK) activity in heart atria from 12 month rats declined 31%; but, by 24 months it was decreased by only 13%. PFK activities from 12 and 24 month ventricles relative to the 2-3 month rats were decreased by 40% and 30%, respectively. This change in PFK activity in each heart region was associated with alterations of subunit composition. In heart atria from 12 and 24 month rats when compared to 3 month rats, the levels of L-type subunit were not significantly different; but the levels of the M-type subunit were decreased by 43% and 38%, respectively. With respect to levels in 2-3 month atria, the C-type subunit in 12 month atria decreased by 27%; and at 24 months it increased by 31%. Making the same comparison for the heart ventricle at 12 and 24 months, L-type subunit decreased by 30% and 24% respectively; M-type subunit decreased by approximately 47%; and the C-type subunit increased 1.9 and 4.7 fold, respectively. These age-related changes of subunit composition in atrial and ventricular PFK isozyme pools led to changes in their kinetic and regulatory properties suggesting that the aged rat could exhibit a diminished capacity to produce ATP from glucose.

Regulation of 6-phosphofructo-1-kinase from the epithelial cells of rat small intestine during pregnancy and lactation

The regulation of 6-phosphofructo-1-kinase (PFK) in the epithelial cells of rat small intestine was studied during pregnancy and lactation. The total activities and activity ratios (activity at 0.5 mM fructose 6-phosphate at pH 7.0/activity at pH 8.0 ( ν 0.5/ V) of the partially purified mucosal PFK were found to increase initially in early pregnant rats (11–12 days of gestation) and to fall back to normal in late pregnant rats (19–20 days of gestation). These changes in enzyme activity during pregnancy were associated with similar changes in the circulating levels of progesterone. The maximal activity and activity ratio ( ν 0.5/ V) were increased in male and female rats injected with progesterone. An increase in the total activity and activity ratio of mucosal PFK was also obtained in lactating rats. However, the enzyme was not strongly activated by inorganic phosphate, fructose 2,6-bisphosphate or glucose 1,6-bisphosphate either in early pregnant or lactating rats. These results indicate that mucosal PFK is already present as an active form during early pregnancy and lactation. Therefore, it is suggested that female sex hormones increase the circulating levels of insulin during early pregnancy which, in turn, positively affect the activity of mucosal PFK which could be also stimulated by the increased levels of fructose 2,6-bisphosphate. The increased activity of PFK in the peak lactating rats could be possible because of an increased demand for lactate production from glucose together with the stimulation of PFK by the increased concentrations of fructose 2,6-bisphosphate which therefore increases the rate of glycolysis.

Complementation of soluble phosphofructokinase activity in yeast mutants

We describe here the genetic and biochemical analyses of two classes of mutations in the soluble phosphofructokinase (PFK I) of Saccharomyces cerevisiae: those leading to the loss of activity and those giving rise to a kinetically altered enzyme. Complementation and allele-testing between these two classes of mutants show that loss of enzyme activity in vitro can come about not only by mutations in the catalytic subunit but also in the regulatory subunit. Also, a mutation in the catalytic subunit can give rise to an enzyme altered in its kinetic properties in a manner phenomenologically similar to that caused by a mutation in the regulatory subunit. The results of the complementation studies in diploids suggest that, in spite of their distinct functions, both the subunits are essential for activity to be detected in vitro. This is confirmed by the reconstitution of an active PFK I enzyme by mixing cell-free extracts of two complementing parents, each of which lacks the enzyme activity. PFK activity appears in the mixture, reaching a maximum value of 60-100% of that of the diploid in 15-30 min at 24 degrees C. Unlike the catalytic subunit which exists in various multimeric states in cell-free extracts of the mutant bearing only this subunit, the regulatory subunit exists largely as a monomer in a mutant devoid of the catalytic subunit. The reconstituted enzyme, however, is indistinguishable from that of the wild type, as analysed by sedimentation studies and Western blot analysis, demonstrating that only the heteromeric complex of the two subunits is active, while neither of the individual subunits displays activity in vitro.

Nerve conduction velocity and axonal transport of 6-phosphofructokinase activity in galactose-fed rats

This study examined sciatic motor nerve conduction velocity (MNCV) and the accumulation of 6-phosphofructokinase (PFK) activity proximal and distal to a sciatic nerve ligature in rats fed a diet containing 20% galactose for 4 weeks. MNCV was reduced in the galactose-fed rats to 94% of controls, ( P < 0.05) but PFK activity accumulations were not different from those in controls. Daily administration of the aldose reductase inhibitor ponalrestat throughout the study to another group of galactose-fed rats prevented dulcitol accumulation, myo-inositol depletion and increased water content of the sciatic nerve seen in galactose-fed rats. This effective aldose reductase inhibition also prevented the reduced MNCV and had no effect on accumulations of PFK activity. These data support earlier work suggesting that deficits in the axonal transport of PFK activity in diabetic rats are unrelated to either exaggerated flux through the polyol pathway, polyol accumulation or ischemic hypoxia and indicate the possible need for the elucidation of other pathogenic mechanisms which may contribute to the development of diabetic neuropathy.

Anatomic and enzymatic responses of the three‐spined stickleback, Gasterosteus aculeatus to thermal acclimation and acclimatization

AbstractThe three‐spined stickleback (Gasterosteus aculeatus) from the St. Lawrence Estuary prefers temperatures between 9 and 12°C, but encounters temperatures ranging from 0 to 30°C. To determine whether despite this preference for cool temperatures this species retains a capacity for metabolic compensation during thermal acclimation, adult sticklebacks were acclimated to 4°C and 20°C for 6 weeks at various ration levels. When food rations were maximal, PFK, CPT, and CS activities were twofold higher in cold‐acclimated sticklebacks, particularly in the pectoral muscle. This pattern of thermal compensation is similar to that shown by eurythermal fish with considerably higher thermal optima. However, at sub‐maximal rations, warm‐acclimated sticklebacks can undergo a decline in condition which confounds the compensatory response. In the field, even though food is abundant, the condition of male sticklebacks also declined as temperatures rose. When the condition factor declined by 30%, the pectoral and axial muscle indices decreased by 40% and 55%, respectively, and the activities of glycolytic and oxidative enzymes in the pectoral and axial muscles dropped by up to 60%. Clearly, a temperature‐induced increase in metabolic rate can confound a compensatory response to temperature.

Cortisol-induced changes in some aspects of the intermediary metabolism of Salvelinus fontinalis

Cortisol was administered to brook charr ( Salvelinus fontinalis) in the form of slow-release intraperitoneal implants (a) to investigate the effect of chronic cortisol stimulation (up to 60 days) on various aspects of intermediary metabolism, and (b) to determine whether such cortisol-induced changes were comparable to those seen in chronically fasted charr. Except for fish sampled at 1 and 6 h after implantation, there was no consistent increase in the plasma cortisol levels of the cortisol-implanted animals. Nevertheless, there were significant treatment effects (particularly after 60 days) on certain metabolite levels, and key hepatic enzymes, including a lowering of plasma glucose and hepatic glycogen concentrations, increased activities of FBPase, G6PDH, GK, and G3PDH, and reduced activities of PFK. When taken together these changes are indicative of a direct or indirect gluconeogenic action of cortisol, in which metabolites other than amino acids (possibly glycerol) are utilized as substrates. These metabolic changes differed from those found in food-deprived brook charr which appeared to use proteins, as well as lipids, as energy sources.