The influence of sodium, potassium and lithium on the response of glycogen synthetase I to insulin and epinephrine in the isolated rat diaphragm

Exercise-induced increase in muscle insulin sensitivity

Exercise/muscle contraction activates glucose transport. The increase in muscle glucose transport induced by exercise is independent of insulin. As the acute effect of exercise on glucose transport wears off, it is replaced by an increase in insulin sensitivity. An increase in insulin sensitivity results in a shift in the insulin dose-response curve to the left, with a decrease in the concentration of insulin needed to induce 50% of the maximal response. This phenomenon, which plays a major role in rapid muscle glycogen accumulation after exercise, is not mediated by amplification of the insulin signal. Development of the increase in insulin sensitivity after contractions does not require protein synthesis or activation of p38 MAPK. It does require the presence of a serum protein during the period of contractile activity. The effect of exercise on muscle insulin sensitivity is mimicked by hypoxia and by treatment of muscles with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside to activate AMP-activated protein kinase. The postexercise increase in sensitivity of muscle glucose transport to activation is not specific for insulin but also involves an increased susceptibility to activation by a submaximal contraction/hypoxia stimulus. The increase in insulin sensitivity is mediated by translocation of more GLUT4 glucose transporters to the cell surface in response to a submaximal insulin stimulus. Although the postexercise increase in muscle insulin sensitivity has been characterized in considerable detail, the basic mechanisms underlying this phenomenon remain a mystery.

Glucose transport rate and glycogen synthase activity both limit skeletal muscle glycogen accumulation

We varied rates of glucose transport and glycogen synthase I (GS-I) activity (%GS-I) in isolated rat epitrochlearis muscle to examine the role of each process in determining the rate of glycogen accumulation. %GS-I was maintained at or above the fasting basal range during 3 h of incubation with 36 mM glucose and 60 microU/ml insulin. Lithium (2 mM LiCl) added to insulin increased glucose transport rate and muscle glycogen content compared with insulin alone. The glycogen synthase kinase-3beta inhibitor GF-109203 x (GF; 10 microM) maintained %GS-I about twofold higher than insulin with or without lithium but did not increase glycogen accumulation. When %GS-I was lowered below the fasting range by prolonged incubation with 36 mM glucose and 2 mU/ml insulin, raising rates of glucose transport with bpV(phen) or of %GS-I with GF produced additive increases in glycogen concentration. Phosphorylase activity was unaffected by GF or bpV(phen). In muscles of fed animals, %GS-I was approximately 30% lower than in those of fasted rats, and insulin-stimulated glycogen accumulation did not occur unless %GS-I was raised with GF. We conclude that the rate of glucose transport is rate limiting for glycogen accumulation unless %GS-I is below the fasting range, in which case both glucose transport rate and GS activity can limit glycogen accumulation.

The effect of short term lithium treatment on the leukocyte, liver and muscle carbohydrate metabolism of guinea-pigs

1. The effect of lithium treatment on the leukocyte, liver and muscle glycogen levels of guinea-pigs has been studied. 2. A 4-week treatment with lithium chloride (5 mmol/l i.p. in saline) increased leukocyte, liver and muscle glycogen and decreased blood glucose levels. 3. These results strongly suggest that a lithium-induced insulin-like effect may be related to the long term effect.

Renal pathology associated with lithium therapy

Renal biopsies have been examined in patients either currently (50 patients) or previously (5 patients) on maintenance lithium therapy for severe affective disorders. In patients currently taking lithium, a distinctive lesion of the distal convoluted tubules and collecting ducts was identified. This lesion consisted of vacuolation and swelling of the cytoplasm with accumulations of PAS positive material; this material was confirmed to be particulate glycogen by light and ultrastructural techniques. This 'lithium-associated' lesion was not present in patients who had discontinued lithium therapy. Prospective studies showed that the accumulation of glycogen occurs in distal tubules and collecting ducts within days of commencing lithium treatment. The relationship of this distinctive lesion to the patchy focal cortical interstitial fibrosis seen in these biopsies remains to be elucidated.

'Insulin-like' effects of lithium ion on isolated rat adipocytes. II. Specific activation of glycogen synthase

Lithium ion, like insulin, activated adipocyte glycogen synthase with or without glucose in the medium. However, the effect of lithium ion was much greater than that of insulin under both conditions. The lithium-activated glycogen synthase was stable to both Sephadex chromatography and ethanol precipitation of the enzyme, indicating that the effect of lithium ion on glycogen synthase was through covalent modification of the enzyme. Glycogen synthase was significantly activated by lithium ion under conditions where concentrations of cellular ATP were unaffected. The effect of lithium ion on glycogen synthase was rapid and observed at concentrations as low as 1 to 3 mM, reaching a maximum at the concentration of 40 mM. It was thus the most sensitive of all the effects studied (see previous paper). Insulin further stimulated glycogen synthase at low concentrations but not at maximal concentration of lithium ion. Lithium-activated glycogen synthase was inhibited by both epinephrine and dibutyryl cyclic AMP, but was not affected by the removal of extracellular Ca++. Interestingly, lithium ion had no detectable effect on basal pyruvate dehydrogenase as well as on epinephrine-stimulated phosphorylase. The failure of lithium ion to thus mimic insulin actions on pyruvate dehydrogenase and on phosphorylase suggests that the action of lithium ion on glycogen synthase is quite specific and may be mediated by stimulating a phosphatase or by inhibiting a protein kinase acting specifically on glycogen synthase.

Short-term stimulation of net glycogen production by insulin in rat hepatocytes

Isolated liver cells from 24 h starved rats were incubated in Krebs-Ringer buffer containing 4% albumin. In the presence of 10, 20 and 30 mM glucose, addition of insulin stimulated net glycogen production by 52, 39 and 20%, respectively. 2 . 10(-9) M insulin was required for half-maximal stimulation. Increases of glycogen production and of glycogen synthase a activity were observed after 15-30 min of incubation with insulin. The stimulatory effect of insulin was additive to that of lithium. In agreement with the literature, insulin antagonized the inhibitory action of suboptimal doses of glucagon on glycogen deposition whereby a decrease of glucagon-elevated cyclic AMP levels was observed. In addition, we found that insulin also decreased the basal cyclic AMP levels in the absence of added glucagon by 22%. It is concluded that physiological concentrations of insulin stimulate net glycogen deposition in hepatocytes from fasted rats; the decrease of basal cyclic AMP levels upon insulin addition may play a role in the mechanism of the hormone action.

The effect of the combination of lithium and haloperidol on brain intermediary metabolism in vivo

The effect of the chronic administration of the combination of lithium and haloperidol has been studied in rat brain in vivo. Lithium was administered in food in amounts sufficient to maintain serum lithium levels of 1.0 +/- 0.1 mEq/l; haloperidol (1.5 mg/kg) was given i.p. once daily. Control animals pair-fed with the lithium/haloperidol alone, or neither drug. Fifteen days after the beginning of the experiments the brains were instantaneously frozen with a rapid brain-freezing device and multiple metabolites were measured in the perchloric acid extract of the tissue. Intermediates examined included selected metabolites of the glycolytic pathway and the tricarboxylic acid cycle, N-acetylaspartate and cofactors such as ATP, CoA, and acetyl-CoA. Estimates of the effects of the treatments on cytoplasmic and mitochondrial redox states were also made. The results showed only minor effects of any of the treatments on any of the parameters studied and little or nothing to distinguish the combination of lithium and haloperidol from either treatment alone.

The influence of insulin on various enzyme activities in human and rat hepatoma cells

1. Incubation of human and rat hepatoma cells with insulin (1 mU/10(6) cells) decreases their content of adenosine 3':5'-monophosphate by more than half after 1 h and by about a quarter after 4 h. 2. The activities of the ATP-metabolising enzymes, adenylate kinase and Mg2+-adenosine triphosphatase are significantly increased by insulin within 1 h and after 4 h. Activity of succinate dehydrogenase and lactic dehydrogenase showed no change at either time interval. 3. Insulin markedly stimulated glucose 6-phosphate dehydrogenase activity within 1 h but by 4 h the increase was less apparent. Glutamate dehydrogenase activity by contrast was not increased by 1 h but was elevated at 4 h.