T. Norman Palmer

Alcoholic myopathy: biochemical mechanisms.

Between one- and two-thirds of all alcohol abusers have impairment of muscle function that may be accompanied by biochemical lesions and/or the presence of a defined myopathy characterised by selective atrophy of Type II fibres. Perturbations in protein metabolism are central to the effects on muscle and account for the reductions in muscle mass and fibre diameter. Ethanol abuse is also associated with abnormalities in carbohydrate (as well as lipid) metabolism in skeletal muscle. Ethanol-mediated insulin resistance is allied with the inhibitory effects of ethanol on insulin-stimulated carbohydrate metabolism. It acutely impairs insulin-stimulated glucose and lipid metabolism, although it is not known whether it has an analogous effect on insulin-stimulated protein synthesis. In alcoholic cirrhosis, insulin resistance occurs with respect to carbohydrate metabolism, although the actions of insulin to suppress protein degradation and stimulate amino acid uptake are unimpaired. In acute alcohol-dosing studies defective rates of protein synthesis occur, particularly in Type II fibre-predominant muscles. The relative amounts of mRNA-encoding contractile proteins do not appear to be adversely affected by chronic alcohol feeding, although subtle changes in muscle protein isoforms may occur. There are also rapid and sustained reductions in total (largely ribosomal) RNA in chronic studies. Loss of RNA appears to be related to increases in the activities of specific muscle RNases in these long-term studies. However, in acute dosing studies (less than 1 day), the reductions in muscle protein synthesis are not due to overt loss of total RNA. These data implicate a role for translational modifications in the initial stages of the myopathy, although changes in transcription and/or protein degradation may also be superimposed. These events have important implications for whole-body metabolism.

Neuroprotective peptides fused to arginine-rich cell penetrating peptides: Neuroprotective mechanism likely mediated by peptide endocytic properties

Several recent studies have demonstrated that TAT and other arginine-rich cell penetrating peptides (CPPs) have intrinsic neuroprotective properties in their own right. Examples, we have demonstrated that in addition to TAT, poly-arginine peptides (R8 to R18; containing 8-18 arginine residues) as well as some other arginine-rich peptides are neuroprotective in vitro (in neurons exposed to glutamic acid excitotoxicity and oxygen glucose deprivation) and in the case of R9 in vivo (after permanent middle cerebral artery occlusion in the rat). Based on several lines of evidence, we propose that this neuroprotection is related to the peptides endocytosis-inducing properties, with peptide charge and arginine residues being critical factors. Specifically, we propose that during peptide endocytosis neuronal cell surface structures such as ion channels and transporters are internalised, thereby reducing calcium influx associated with excitotoxicity and other receptor-mediated neurodamaging signalling pathways. We also hypothesise that a peptide cargo can act synergistically with TAT and other arginine-rich CPPs due to potentiation of the CPPs endocytic traits rather than by the cargo-peptide acting directly on its supposedly intended intracellular target. In this review, we systematically consider a number of studies that have used CPPs to deliver neuroprotective peptides to the central nervous system (CNS) following stroke and other neurological disorders. Consequently, we critically review evidence that supports our hypothesis that neuroprotection is mediated by carrier peptide endocytosis. In conclusion, we believe that there are strong grounds to regard arginine-rich peptides as a new class of neuroprotective molecules for the treatment of a range of neurological disorders.

Alcohol and glucose metabolism in skeletal muscles in the rat

Ethanol is known to cause an acute and profound insulin resistance in man and the rat primarily via effects on glucose utilization. This paper examines the nature of these inhibitory effects on whole‐body glucose utilization using the euglycaemic hyperinsulinaemic clamp in the conscious unrestrained rat. We confirm that ethanol infusion causes an acute insulin resistance, the rate of glucose infusion required to maintain euglycaemia (GIR) being decreased markedly by ethanol. To ensure that the GIR is a measure of whole‐body glucose disposal, glucose turnover and hepatic glycogen levels were measured. These studies showed that ethanol totally suppressed hepatic glucose production. The reduction in GIR is associated with marked decreases in glucose uptake and glycogen synthesis in most skeletal muscles. In oxidative but not in non‐oxidative muscles, the activation of glycogen synthase in response to insulin was decreased by ethanol, suggesting that a defect in glycogen synthase activation may be responsible for the decrease in glycogen synthesis. The basis of the inhibitory effects of ethanol on insulin‐stimulated glucose metabolism in muscle is unknown, but may involve membrane‐associated impairments in insulin signalling and/or the glucose transport system.

Total contractile protein contents and gene expression in skeletal muscle in response to chronic ethanol consumption in the rat

An investigation was carried out to determine changes in the contents of skeletal muscle myofibrillary proteins (i.e., the contractile fraction composed principally of actin and myosin) and gene expression in skeletal muscle in response to ethanol feeding. Male Wistar rats were fed a nutritionally complete liquid diet, which contained 35% of total calories as ethanol. Controls were pair-fed isocaloric amounts of the same diet, in which ethanol was replaced by isocaloric glucose. Total mixed and contractile protein contents of the gastrocnemius in ethanol-fed rats were rapidly reduced by ethanol feeding: a response was discernible as early as 1 week after the commencement of the ethanol feeding regimen (approx. -10%, p < 0.025 and p = 0.05 for mixed and myofibrillary proteins, respectively). At 2, 4, and 6 weeks, mixed and myofibrillary protein contents were further reduced in alcohol-fed rats, by between 12% and 22%, compared to pair-fed controls. Similar changes occurred in the soluble (i.e., sarcoplasmic) protein fractions of skeletal muscle. At 2 weeks the composition of total messenger RNA and individual messenger RNA species was measured. Total messenger RNA content per muscle was reduced by 35% (p < 0.05). Messenger RNA levels for alpha-actin, beta-myosin heavy chain, and carbonic anhydrase III were not significantly altered. In conclusion, skeletal muscle protein contents are rapidly reduced by ethanol feeding, compared to pair-fed controls, though mRNA species encoding specific isoforms of myosin and actin are not affected. It is possible that chronic ethanol feeding may significantly alter the stability of mRNAs encoding other contractile proteins, or alternatively, defects in translation may predominate.

Proglycogen and macroglycogen: artifacts of glycogen extraction?

Most recent studies on the physiology of proglycogen and macroglycogen in skeletal muscles have adopted a homogenization-free acid extraction protocol to separate these 2 pools of glycogen. The purposes of this study were to determine (a) whether this protocol is suitable; (b) if the acid-insoluble glycogen fraction corresponds to proglycogen; and (c) if this fraction accounts for most of the changes in muscle glycogen content, irrespective of muscle fiber types. Using the rat as our experimental model, this study shows that when the conditions of acid extraction are optimized, 52% to 64% of glycogen in rat muscles is found as acid-soluble glycogen as opposed to approximately 16% when glycogen is extracted using a homogenization-free extraction protocol. Moreover, there is no evidence that the acid-insoluble glycogen corresponds to proglycogen because gel chromatography of the acid-insoluble and acid-soluble glycogen fractions shows similar elution profiles of high-molecular weight glycogen. Finally, irrespective of muscle fiber types, the acid-soluble glycogen accounts for most of the changes in total muscle glycogen levels during the fasting-to-fed transition, whereas the levels of the acid-insoluble glycogen remain stable or increase marginally. In conclusion, this study shows that the homogenization-free acid extraction of muscle glycogen underestimates the proportion of acid-soluble glycogen and that the findings of the studies that have adopted such an extraction protocol to examine the physiology of acid-insoluble and acid-soluble glycogens require reexamination.

Ethanol acutely impairs glycogen repletion in skeletal muscle following high intensity short duration exercise in the rat

Ethanol is recognized to affect adversely carbohydrate metabolism in skeletal muscle. This paper seeks to establish whether ethanol acutely impairs glycogen repletion during recovery from high intensity short duration exercise in the rat. High intensity exercise caused the massive mobilization of glycogen stores in muscles rich in type IIa and IIb fibres and marked increases in plasma and muscle lactate levels. During the 30‐minute recovery period, there was substantial glycogen repletion in these muscles in both the ethanol‐treated and control rats. Ethanol, however, was associated with reduced glycogen resynthesis in both the tibialis anterior (by 22%) and red gastrocnemius (by 31%) muscles but not in the white gastrocnemius muscle. This reduction in post‐exercise glycogen deposition was accompanied by decreased lactate disposal and elevated plasma glucose levels. These effects of ethanol on glycogen repletion may involve interactions with hepatic gluconeogenesis, glucose uptake and utilization in muscle, muscle glycogen synthesis and lactate glyconeogenesis. The ethanol‐mediated impairment in post‐exercise glycogen repletion may have important implications for the pathogenesis of chronic alcoholic skeletal myopathy.

Repeated bouts of high-intensity exercise and muscle glycogen sparing in the rat

SUMMARY Even in the absence of food intake, several animal species recovering from physical activity of high intensity can replenish completely their muscle glycogen stores. In some species of mammals, such as in rats and humans, glycogen repletion is only partial, thus suggesting that a few consecutive bouts of high-intensity exercise might eventually lead to the sustained depletion of their muscle glycogen. In order to test this prediction, groups of rats with a lead weight of 10% body mass attached to their tails were subjected to either one, two or three bouts of high-intensity swimming, each bout being separated from the next by a 1 h re covery period. Although glycogen repletion after the first bout of exercise was only partial, all the glycogen mobilised in subsequent bouts was completely replenished during the corresponding recovery periods and irrespective of muscle fibre compositions. The impact of repeated bouts of high-intensity exercise on plasma levels of fatty acids, acetoacetate and β-hydroxybutyrate suggests that the metabolic state of the rat prior to the second and third bouts of exercise was different from that before the first bout. In conclusion, rats resemble other vertebrate species in that without food intake there are conditions under which they can replenish completely their muscle glycogen stores from endogenous carbon sources when recovering from high-intensity exercise. It remains to be established, however, whether this capacity is typical of mammals in general.

Metabolic effects of alcohol on skeletal muscle

The purpose of this review is to summarize our current understanding of the acute and chronic interactions between alcohol and nutrient metabolism in skeletal muscle. Insulin is well known to play an important regulatory role in nutrient, especially glucose, uptake and utilization in skeletal muscle. Several studies have shown that alcohol can acutely reduce the normal metabolic responses of skeletal muscle to the action of insulin. The most obvious of these is an acute impairment in glucose metabolism associated with alcohol consumption. While the exact mechanism(s) underlying this acute insulin resistance is presently unclear, several possible factors are discussed in this review. In contrast to these short‐term effects, the effects of alcohol on skeletal muscle insulin sensitivity in chronic alcohol abusers are not as well established. Chronic alcohol abuse is known to be associated with skeletal myopathies, believed to result from alcohol induced abnormalities in muscle protein synthesis. Finally, the alcohol‐mediated impairments of many aspects of skeletal muscle metabolism are discussed in relation to the insulin resistance associated broad spectrum of common lifestyle‐related disorders, including non‐insulin dependent diabetes mellitus and obesity, the consequences of which may be important to the pathogenesis of alcohol‐related diseases.

Alcohol and Protein Turnover

Ethanol administration induces a variety of pathological responses which in many tissues are manifested as reductions in tissue protein content. These alterations must be due to changes in protein turnover. This review is, therefore, primarily concerned with investigating the mechanisms whereby tissue protein synthesis or degradation is altered as a consequence of ethanol toxicity. Attention will be focused on the various muscle types (i.e. skeletal, cardiac and smooth muscle) and bone.

Activation of erythrocyte aldose reductase in man in response to glycaemic challenge

Flux via the polyol pathway, which comprises the enzymes aldose reductase (AR) and sorbitol dehydrogenase (SDH), has been implicated in the debilitating complications of diabetes. Previous studies in this laboratory have indicated that erythrocyte AR activities are increased (by 72%) in insulin-dependent diabetic patients. To investigate the mechanism underlying this activation, the response of AR activity to oral glucose challenge was investigated in eight overnight-fasted human volunteers. Glucose consumption led to a transient activation (by 76%: P less than 0.01) of erythrocyte AR, which paralleled the rise and subsequent fall in blood glucose concentrations. It is concluded that erythrocyte AR activity is acutely modulated in response to hyperglycaemia by an as yet unknown mechanism.