Lactate production by white adipocytes in relation to insulin sensitivity

Higher lactate production from glucose in cultured adipose nucleated stromal cells than for rat adipocytes

White adipose tissue (WAT) nucleated stromal cells (NSC) play important roles in regulation, defense, regeneration and metabolic control. In WAT sites, the proportions and functions of NSC change under diverse physiological or pathologic conditions. We had previously observed the massive anaerobic wasting of glucose to lactate and glycerol in rat epididymal adipocytes. To test site variability, and whether the adipocyte extensive anaerobic metabolism of glucose was found in NSC, we analyzed, in parallel, subcutaneous, mesenteric and epididymal WAT of male adult Wistar rats. Adipocytes and NSC fractions, were isolated, counted and incubated (as well as red blood cells: RBC) with glucose, and their ability to use glucose and produce lactate, glycerol, and free fatty acids was measured. Results were computed taking into account the number of cells present in WAT samples. Cell numbers were found in proportions close to 1:13:100 (respectively, for adipocytes, NSC and RBC) but their volumes followed a reversed pattern: 7,500:10:1. When counting only non-fat cell volumes, the ratios changed dramatically to 100:10:1. RBC contribution to lactate production was practically insignificant. In most samples, NSC produced more lactate than adipocytes did, but only adipocytes secreted glycerol (and fatty acids in smaller amounts). Glucose consumption was also highest in NSC, especially in mesenteric WAT. The heterogeneous NSC showed a practically anaerobic metabolism (like that already observed in adipocytes). Thus, NSC quantitative production of lactate markedly contributed (i.e. more than adipocytes) to WAT global use (wasting) of glucose. We also confirmed that glucose-derived glycerol is exclusively produced by adipocytes.

Utilization of dietary glucose in the metabolic syndrome

This review is focused on the fate of dietary glucose under conditions of chronically high energy (largely fat) intake, evolving into the metabolic syndrome. We are adapted to carbohydrate-rich diets similar to those of our ancestors. Glucose is the main energy staple, but fats are our main energy reserves. Starvation drastically reduces glucose availability, forcing the body to shift to fatty acids as main energy substrate, sparing glucose and amino acids. We are not prepared for excess dietary energy, our main defenses being decreased food intake and increased energy expenditure, largely enhanced metabolic activity and thermogenesis. High lipid availability is a powerful factor decreasing glucose and amino acid oxidation. Present-day diets are often hyperenergetic, high on lipids, with abundant protein and limited amounts of starchy carbohydrates. Dietary lipids favor their metabolic processing, saving glucose, which additionally spares amino acids. The glucose excess elicits hyperinsulinemia, which may derive, in the end, into insulin resistance. The available systems of energy disposal could not cope with the excess of substrates, since they are geared for saving not for spendthrift, which results in an unbearable overload of the storage mechanisms. Adipose tissue is the last energy sink, it has to store the energy that cannot be used otherwise. However, adipose tissue growth also has limits, and the excess of energy induces inflammation, helped by the ineffective intervention of the immune system. However, even under this acute situation, the excess of glucose remains, favoring its final conversion to fat. The sum of inflammatory signals and deranged substrate handling induce most of the metabolic syndrome traits: insulin resistance, obesity, diabetes, liver steatosis, hyperlipidemia and their compounded combined effects. Thus, a maintained excess of energy in the diet may result in difficulties in the disposal of glucose, eliciting inflammation and the development of the metabolic syndrome.

Adipose tissue lipolysis revisited (again!): lactate involvement in insulin antilipolytic action

Lactate is produced from glucose by adipose tissue. In this issue of Cell Metabolism, Ahmed et al. (2010) show that the metabolite is involved in the antilipolytic effect of insulin, providing a new link between fat and carbohydrate metabolism.

In vivo alpha(1)-adrenergic lipolytic activity in subcutaneous adipose tissue of obese subjects

The role of alpha(1)-adrenoceptors in lipid mobilization and blood flow was investigated in situ using microdialysis of subcutaneous adipose tissue in severely obese subjects. The lipolysis rate was assessed by determination of interstitial glycerol concentration. The alpha(1)-adrenoceptor agonist norfenefrine caused an increase in glycerol level in adipose tissue that was similar to that observed with the physiologic alpha(1,2)-beta(1)-adrenoceptor agonist norepinephrine, whereas the alpha(1)-adrenoceptor antagonist urapidil showed no effect on basal lipolysis rate. However, the enhanced glycerol concentration due to norfenefrine and norepinephrine was suppressed in the presence of urapidil. The beta-adrenoceptor antagonist propranolol showed no effect on norfenefrine-stimulated glycerol outflow. Blood flow was assessed using the ethanol escape technique. Perfusion with norfenefrine decreased blood flow, whereas urapidil enhanced blood flow significantly. Despite the increase in blood flow, the basal interstitial glycerol concentration remained unchanged. Although norfenefrine at high concentrations could inhibit the urapidil-induced increase in blood flow, the norfenefrine-induced glycerol output was not affected. These results demonstrate that alpha(1)-adrenoceptors are involved in regulation of lipolysis rate and microcirculation of adipose tissue. However, the observed changes in local blood flow were not related to glycerol output.

Effects of chronic treatment with noradrenaline or a specific beta3-adrenergic agonist, CL 316 243, on energy expenditure and epididymal adipocyte lipolytic activity in rat

1. The effects of 7 days exposure to a specific beta3-adrenergic agonist, CL 316 243 (1 mg/kg x 24 hr), or to the physiological hormone, noradrenaline (5 mg/kg x 24 hr), were tested on energy expenditure and on in vitro lipolysis in male Sprague-Dawley rats. 2. At the second day of treatment, the total energy expenditure and the resting metabolic rate were increased by 20 and 30%, respectively, in the CL-treated group. Under the same conditions, a dose five times higher of NA increased the resting metabolic rate by 11% without any significant change in the total daily energy expenditure. 3. The CL-treated group showed a lower weight gain, correlated with a significant reduction in retroperitoneal adipose tissue weight. Both treatments resulted in a marked desensitization (increased EC50 values) of the NA stimulated lipolysis of epididymal adipocytes. The effects of both treatments on maximal lipolysis were opposite. Indeed, chronic NA-treatment decreased the responsiveness of lipolysis while chronic treatment with CL increased the maximal stimulation of lipolysis to NA. Furthermore, dose-response curve for CL on lipolysis showed a marked functional desensitization of beta3-adrenergic response. 4. Our results demonstrate the high selectivity of beta3-adrenergic agonists to stimulate whole body energy expenditure and lipid mobilization in rodents. The present results point out for the first time an adrenergic desensitization of the lipolytic response after chronic administration of a beta3-agonist.

Differential acute effects of oxovanadiums and insulin on glucose and lactate metabolism under in vivo and in vitro conditions

Oxovanadium compounds such as vanadate and peroxovanadiums have been shown to have insulin-mimetic effects on various metabolic pathways, including glucose metabolism. A differential effect of various oxovanadium species on glucose metabolism in different tissues has been reported. The results from our present in vivo studies using rats show that peroxovanadiums and insulin have similar acute effects on decreasing blood glucose levels, but dissimilar effects on blood lactate levels. Furthermore, when bisperoxovanadate (BPV) was administered acutely to intact animals immediately before a bolus insulin challenge, it blunted the effectiveness of insulin in decreasing the blood lactate level, but at the same time demonstrated a synergistic effect on the hypoglycemic action of insulin. It was also observed in in vitro studies using normal 3T3-L1 adipocytes (not serum-deprived) that 1,10-phenanthroline bisperoxovanadate (PHEN-BPV) attenuates the incorporation of carbon from lactate but not from glucose, into lipid in both the absence and presence of insulin. Additionally, it was observed that PHEN-BPV had no effect on lactate dehydrogenase (LDH) activity. Thus, one may speculate that PHEN-BPV interferes with carrier-mediated lactate transport. These observations demonstrate that insulin and oxovanadiums differ in the handling of different metabolic substrates. Thus, even though oxovanadiums mimic many of the metabolic actions of insulin, their metabolic effects are by no means identical. Moreover, since vanadate had no acute effect on glucose metabolism under in vivo conditions, this may suggest that to be effective as a hypoglycemic agent vanadate needs to be converted to some other biologically active oxovanadium species. Finally, the observed interference by PHEN-BPV in the metabolism of lactate may predispose subjects using oral vanadate, as a part of the therapeutic regimen for management of diabetic hyperglycemia, to lactic acidosis.

Effect of aging on norepinephrine and phenylephrine stimulated lactate production by white adipocytes

We have recently demonstrated that adipose tissue can produce lactate independently of lipolysis in insulin-resistant rats and that lactate production depends on alpha 1-adrenergic stimulation. In this study, we have investigated the influence of aging on norepinephrine-and-phenylephrine-stimulated lactate production and glycerol production. We showed that basal and norepinephrine stimulated lactate production were significantly increased in adipocytes isolated from old vs. young rats (0.165 +/- 0.006 vs. 0.055 +/- 0.008 for basal and 0.567 +/- 0.026 vs. 0.277 +/- 0.019 mumol lactate/10(6) cells/15 minutes for norepinephrine-stimulated lactate production, respectively, p < 0.05). The sensitivity of lactate production to norepinephrine stimulation in adipocytes isolated from old rats was significantly decreased (EC50 = 523 +/- 63.7 vs. 46.7 +/- 6.34 nM, respectively, p < 0.05). Maximal lactate production obtained with norepinephrine and phenylephrine was not significantly different in either group (0.567 +/- 0.026 vs. 0.520 +/- 0.036 in old and 0.277 +/9 0.019 vs. 0 275 +/- 0.017 mumol/10(6) cell/15 minutes in young rats, respectively, ns). Lactate production by adipocytes isolated from old rats were significantly less sensitive to phenylephrine stimulation compared with young (EC50 = 3.67 +/- 1.16 vs. 0.07 +/- 0.01 nM, respectively, p < 0.05) indicating that the effects of aging on norepinephrine and phenylephrine stimulation were probably induced by a decreased number of alpha 1-adrenoceptors. The mechanism by which aging increases adipocyte responsiveness of lactate production has not yet been elucidated.