Effects of epinephrine, corticotropin, and thyrotropin on lipolysis and glucose oxidation in rat adipose tissue

(Log dose)-response curves have been determined for lipolysis and for the conversion of glucose-(14)C to (14)CO(2) by adipose tissue from rats in the presence of epinephrine, corticotropin, and thyrotropin. The stimulatory effect of epinephrine on lipolysis was greater than that of corticotropin or thyrotropin. Lipolysis induced by epinephrine was inhibited by propranolol but only slightly by phenoxybenzamine, whereas lipolysis induced by corticotropin was inhibited by phenoxybenzamine to a much greater extent than by propranolol. Neither blocking drug had a pronounced effect on the response to thyrotropin. Epinephrine stimulated the oxidation of glucose-(14)C to CO(2) more than did either thyrotropin or corticotropin. Moreover, epinephrine stimulated the conversion of glucose-(14)C to CO(2) and fatty acids even when lipolysis was not increased. These studies indicate that epinephrine can affect glucose utilization independently of its effect on lipolysis.     

Leptin-induced lipolysis opposes the tonic inhibition of endogenous adenosine in white adipocytes.

The aim of the present study was to gain insight into the signaling pathway used by leptin to stimulate lipolysis. The lipolytic rate of white adipocytes from sex- and age-matched lean (+/+) and fa/fa rats was determined in the absence or presence of leptin together with a number of agents acting at different levels of the signaling cascade. Leptin did not modify FSK-, dbcAMP-, and IBMX-stimulated lipolysis. Lipolysis can also be maximally stimulated by lowering media adenosine levels with adenosine deaminase (ADA), i.e., in the ligand-free state. Although ADA produced near maximal lipolysis in adipocytes of lean animals, only half of the maximal lipolytic rate (50.9+/-3.2%) was achieved in fat cells from fa/fa rats (P=0.0034). In adipocytes from lean animals preincubated with ADA, leptin caused a concentration-related stimulation of lipolysis (P=0.0001). However, leptin had no effect on the lipolytic activity of adipocytes in the ligand-free state from fa/fa rats. The adenosine A1 receptor agonist CPA effectively inhibited basal lipolysis in both lean and obese adipocytes (P=0.0001 and P=0.0090, respectively). Leptin had no effect on the lipolytic rate of adipocytes isolated from fa/fa rats and preincubated with CPA. When adipocytes were incubated with the A1 receptor antagonist DPCPX, a significant increase in glycerol release was observed in fa/fa fat cells (P=0.009), whereas cells isolated from lean rats showed no differences to ADA-stimulated lipolysis. After pretreatment with PTX, which inactivates receptor-mediated Gi function, adipocytes of obese rats became as responsive to the stimulatory actions of ISO as cells from lean rats (P=0.0090 vs. ISO in fa/fa rats; P=0.2416 vs. lean rats, respectively). PTX treatment of lean cells, however, did not alter their response to this lipolytic agent. It can be concluded that the lipolytic effect of leptin is located at the adenylate cyclase/Gi proteins level and that leptin-induced lipolysis opposes the tonic inhibition of endogenous adenosine in white adipocytes.     

Cyclic nucleotides and lipolysis in rat fat cells. Interaction between calcium ionophore A23187 and FCCP,uncoupler of oxidative phosphorylation

The interaction between calcium ionophore A23187 and carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) has been studied at the level of cyclic-AMP (cAMP), cyclic-GMP (cGMP) and lipolysis in isolated rat fat cells. Ionophore A23187 (1–10 μM) stimulated cGMP accumulation and glycerol release without affecting cAMP level. FCCP (1–100 μM) inhibited the effect of A23187 on cGMP level and glycerol release, but did not affect or increase cAMP. Thus a correlation exists between the changes of cGMP levels and lipolysis and a dissociation of lipolysis from cAMP.     

Effect of tolbutamide on lipolytic processes in blood and fat tissue of rats maintained in normothermic and hypothermic conditions]

The effect of tolbutamide administered in vivo or added to the incubation medium on lipolysis in adipose tissue and in blood has been studied in rats. Tissue lipolysis was lowered by 118 to 156% in groups of rats receiving tolbutamide in vivo. The addition of tolbutamide directly to the incubation medium caused an increase in the lipid mobilizing activity, while the addition of insulin inhibited lipolysis in each case. The effect of tolbutamide administered in vivo may be indirect through an increase in insulin binding and/or the drug may influence directly the permeability of cell membrane for ions and glucose. An increase in the lipid mobilizing activity observed in experiments consisting in the addition of tolbutamide to the incubation medium may be linked to the low glucose level and greater demand for free fatty acids or to the low level of insulin in the incubation medium.     

Inhibitory effect and mode of action of somatostatin on lipolysis in chicken adipocytes

The effect of somatostatin on lipolysis was investigated utilizing isolated chicken adipocytes. Somatostatin-14 and -28 inhibited basal lipolysis. This ability to suppress glycerol release (used as an index of lipolysis) was emphasized in presence of stimulated lipolysis. Concentration of 1 ng/ml somatostatin-14 (0.625 nM) and somatostatin-28 (0.312 nM) was found to inhibit completely the glycerol release induced by concentrations of glucagon up to 2 ng/ml (0.58 nM). The percentage of inhibition was dose-dependent. The antilipolytic effect of somatostatin-14 was also observed during ACTH and aminophylline-stimulated lipolysis. Among the mechanisms which could account for the inhibition, a possible competitive effect of somatostatin-14 with 125I-labelled glucagon binding to adipocyte membranes was excluded. The small inhibiting effect of somatostatin-14 on glycerol release prompted by dibutyryl cyclic AMP, together with the significant inhibiting effect on aminophylline-stimulated lipolysis argued for a reduction of cyclic AMP accumulation. The increase of cyclic AMP levels induced by glucagon was substantially reduced in presence of somatostatin-14. It was concluded that in chicken adipocytes somatostatin inhibited the rate of lipolysis and that reduction on cyclic AMP could be responsible, at least in part, for the antilipolytic effect.     

Adrenaline responsiveness of glucose metabolism in insulin-resistant adipose tissue of rats fed a high-fat diet.

The effects of adrenaline (0.5 microM) and the combination of adrenaline and insulin (1.7nM) on [6-14C]glucose metabolism were assessed in epididymal fat-pads from rats fed either a low- or high-fat diet. The response of lipolysis to adrenaline was clearly diminished in fat-fed rats. Insulin added to adrenaline inhibited the lipolysis by 50% regardless of the diet. Glucose utilization in adipose tissue of fat-fed rats was markedly stimulated by adrenaline (glucose uptake was increased 3-fold and the production of CO2 and the glycerol moiety of acylglycerol was increased 4-fold). However, adipose tissue from fat-fed rats was resistant to the effect of insulin to produce a further increase in adrenaline-stimulated glucose uptake. The intracellular capacity of lipogenesis on the one hand, and the production of CO2 and the glycerol moiety of acylglycerol on the other, are of prime importance in the action of insulin and adrenaline on glucose utilization in this model.     

Inhibition of compensatory renal growth by the N-terminus of a sheep-derived peptide

The N-terminal sequence of a novel sheep-derived peptide with growth inhibitory activity has been obtained. The N-terminal fragment was chemically synthesised and designated EPL001. The kidney was chosen as the first mammalian system in which to study EPL001 since kidney growth can be accurately quantified following a surgical reduction in renal mass. Cell proliferation was measured in mouse collecting duct kidney (MCDK) cells stimulated with insulin-like growth factor I (IGF-I). Compensatory renal growth (CRG) was induced in Wistar rats and either EPL001 or an EPL001 antibody delivered by continuous renal tissue infusion. Mouse monoclonal antibodies to EPL001 were generated for immunoneutralisation, rabbit polyclonal antibodies were generated for immunohistochemistry. EPL001 had no apparent effect on IGF-I stimulated cell proliferation in MCDK cells in vitro, yet provoked a dose-dependent inhibition of CRG in vivo. An EPL001 antibody potentiated CRG, in the absence of exogenous EPL001, consistent with an inhibitory role in kidney growth for an endogenous peptide containing the EPL001 sequence. Tubular staining for epitopes to the EPL001 sequence was detected in normal human kidney sections and enhanced in renal cell carcinoma. Results support the presence of growth inhibitory activity in the N-terminus of a sheep-derived peptide with evidence for both its presence and endogenous activity in the kidney. Attempts to further characterise its structure and activity are ongoing.     

Parathyroid hormone-induced lipolysis in human adipose tissue

Relative lipolytic activity of human parathyroid hormone-(1-34) (hPTH-(1-34], hPTH-(3-34), desamino-Ser1-hPTH-(1-34), and rat PTH-(1-34) was compared in human subcutaneous adipose tissues in vitro. Human PTH-(1-34), rat PTH-(1-34), and desamino-Ser1-hPTH-(1-34) stimulated in vitro lipolysis significantly above basal level at the concentration of 10(-6) M. Average increments of lipolytic rate were 2.39, 1.82, and 0.87 mumol/g per 2 hr, respectively, being significantly different among the three groups. On the other hand, hPTH-(3-34)-induced lipolytic rate was 0.83 +/- 0.18 mumol/g per 2 hr, not significantly different from the basal level (0.71 +/- 0.20 mumol/g per 2 hr). The effect of hPTH-(3-34) on glycerol release stimulated by hPTH-(1-34), isoproterenol, or forskolin was subsequently investigated. Human PTH-(3-34) produced a dose-dependent inhibition of hPTH-(1-34)-stimulated lipolysis. In contrast, isoproterenol- and forskolin-induced lipolytic rates were not influenced by hPTH-(3-34). The effect of propranolol on hPTH-(1-34)- or isoproterenol-induced lipolysis was also studied. Propranolol dose-dependently inhibited isoproterenol-induced lipolysis but had no effect on lipolysis stimulated by hPTH-(1-34). These results suggest that the amino acids at positions 1 (serine) and 2 (valine) of PTH are critical for the stimulation of lipolysis in human adipose tissue. Human PTH-(1-34) causes lipolysis after binding to receptors distinct from beta-adrenergic receptors of fat cells and possibly hPTH-(3-34) inhibits hPTH-(1-34)-stimulated lipolysis by competing at the level of PTH receptor.     

Biological properties of griseolic acid, a cyclic AMP phosphodiesterase inhibitor with an adenine group

Griseolic acid inhibited cAMP phosphodiesterase (PDE) at low concentrations, the I50 being of the order of 0.01-0.1 microM. Administration of griseolic acid to rats increased the cAMP level in liver and plasma several-fold. It increased glycogen degradation in mouse liver and stimulated lipolysis in isolated rat fat cells. Griseolic acid did not block the adenosine-elicited accumulation of cAMP in guinea pig brain slices. It had no effect on cAMP-dependent protein kinase from rat liver nor on the adenyl cyclase from rat brain.     

The effect of myo-inositol deficiency on lipid metabolism in rats. III. The mechanism of an enhancement in lipolysis due to myo-inositol deficiency in rats.

myo-Inositol deficiency in rats produced an overaccumulation of triacylglycerols in the liver due to stimulated lipolysis in the adipose tissue (Hayashi, E., Maeda, T. and Tomita, T. (1974) Biochim. Biophys. Acta 360, 134--155). The mechanism of the enhancement in lipolysis has now been investigated. The lipolytic response to adrenalin, corticotropin and insulin of the epididymal adipose tissue did not change due to the deficiency, but hormone-sensitive lipase activity, plasma adrenalin level and blood pressure were higher in the deficient rats. Adrenalectomy had no influence, but administration of sympathetic nervous blockers (reserpine, hexamethonium and bupranolol) inhibited the liver lipid deposition and an increase of serum free fatty acids in the deficient rats. These results indicate that the enhancement in lipolysis is mediated by an excitation of sympathetic nerve terminals innervating in the adipose tissues.     

Factors regulating the production of prostaglandin E2 and prostacyclin (prostaglandin I2) in rat and human adipocytes.

The regulation of PGE2 (prostaglandin E2) and PGI2 (prostaglandin I2; prostacyclin) formation was investigated in isolated adipocytes. The formation of both PGs was stimulated by various lipolytic agents such as isoproterenol, adrenaline and dibutyryl cyclic AMP. During maximal stimulation the production of PGE2 and PGI2 (measured as 6-oxo-PGF1 alpha) was 0.51 +/- 0.04 and 1.21 +/- 0.09 ng/2 h per 10(6) cells respectively. Thus PGI2 was produced in excess of PGE2 in rat adipocytes. The production of the PGs was inhibited by indomethacin and acetylsalicylic acid in a concentration-dependent manner. The half-maximal effective concentration of indomethacin was 328 +/- 38 nM and that of acetylsalicylic acid was 38.5 +/- 5.3 microM. The PGs were maximally inhibited by 70-75% after incubation for 2 h. In contrast with their effect on PG production, the two agents had a small potentiating effect on the stimulated lipolysis (P less than 0.05). The phospholipase inhibitors mepacrine and chloroquine inhibited both PG production and triacylglycerol lipolysis and were therefore unable to indicate whether the PG precursor, arachidonic acid, originates from phospholipids or triacylglycerols in adipocytes. Angiotensin II significantly (P less than 0.05) stimulated both PGE2 and PGI2 production in rat adipocytes without affecting triacylglycerol lipolysis. Finally, it was shown that PGE2 and PGI2 were also produced in human adipocytes, although in smaller quantities than in rat adipocytes. It is concluded that the production of PGs in isolated adipocytes is regulated by various hormones. Moreover, at least two separate mechanisms for PG production may exist in adipocytes: (1) a mechanism that is activated concomitantly with triacylglycerol lipolysis (and cyclic AMP) and (2) an angiotensin II-sensitive, but lipolysis (and cyclic AMP)-independent mechanism.     

Studies of phosphodiesterase effects on adipose tissue metabolism in obese subjects by the microdialysis technique.

The effect of non-selective (theophylline) inhibition of cyclic AMP breakdown on norepinephrine stimulated lipolysis rate was investigated in subcutaneous adipose tissue of obese subjects. In addition, changes in interstitial glucose and lactate concentration were assessed by means of the microdialysis technique. The interaction of endogenous released insulin and theophylline on adipocyte metabolism was determined. Theophylline and norepinephrine alone increased glycerol outflow significantly. When both agents were perfused in combination, interstitial glycerol concentration increased further. The enhanced glycerol level due to theophylline application was slightly decreased by insulin. In the presence of theophylline, extracellular glucose concentration increased, in contrast to the catecholamine. Norepinephrine decreased interstitial glucose level. When both drugs were added in combination, the level of interstitial glucose increased to about 1 mM, greater than with theophylline alone. With each intervention, lactate was synthesized. Local adipose tissue blood flow was increased by theophylline and theophylline plus norepinephrine. In conclusion, post-receptor mechanisms increased norepinephrine maximal stimulated lipolysis rate in subcutaneous adipose tissue. Glucose uptake was inhibited by the non-specific inhibitor of phosphodiesterase. The effect of insulin on inhibition of lipolysis was modest but sustained in the presence of high theophylline (10(-4) M) concentration. Phosphodiesterase activity may be relatively low in obese subjects in comparison with lean subjects. In lean subjects theophylline caused a transient reversal of the antilipolytic effect of insulin.     

Effect of some phytoestrogens on metabolism of rat adipocytes.

The aim of this experiment was to evaluate the direct effect of genistein, daidzein and zearalenone on basal and hormone-induced lipogenesis and lipolysis in isolated rat adipocytes. In lipogenesis, daidzein and zearalenone were used at concentrations of 0.01, 0.1 and 1 mmol x L(-1) and genistein at concentrations of 0.01, 0.3, 0.6 and 1 mmol x L(-1). In lipolysis, concentrations of tested compounds were 0.01, 0.1 and 1 mmol x L(-1). All tested compounds clearly inhibited basal and insulin (1 nmol x L(-1)) stimulated lipogenesis. Basal lipolysis was particularly enhanced by genistein and daidzein at its higher concentrations. The ability of zearalenone to potentiate of basal lipolysis was less marked. Epinephrine (1 micromol x L(-1))-stimulated lipolysis was inhibited by genistein at 1 mmol x L(-1). At a concentration of 0.1 mmol x L(-1) daidzein also augmented epinephrine-stimulated lipolysis and at its highest concentration exhibited an inhibitory effect, similar to genistein. Zearalenone reduced stimulated lipolysis, particularly at the highest concentration.     

Effect of epinephrine and other lipolytic agents on intracellular lipolysis and lipoprotein lipase activity in 3T3-L1 adipocytes

3T3-L1 adipocytes were used to test the hypothesis that hormone-sensitive lipolysis and lipoprotein lipase activity might be regulated in a reciprocal manner. Intracellular lipolysis was stimulated by catecholamine, dibutyryl cAMP, and ACTH, but not by glucagon. The effects of epinephrine on lipolysis were blocked by the beta-antagonist propanolol but not by the alpha-antagonist phentolamine. Hormone-stimulated lipolysis was not changed by acute (45 min) or chronic (2 days) treatment of the cells with insulin whereas the latter treatment augmented lipoprotein lipase activity about fivefold. Epinephrine did not affect the lipoprotein lipase activity of insulin-stimulated cells. Withdrawal of glucose from the medium decreased lipoprotein lipase activity and the effect of epinephrine on lipolysis. Effects of lipolytic agents on activity of lipoprotein lipase were variable and concentration-dependent. Lipoprotein lipase activity was decreased only by concentrations of epinephrine greater than those inducing maximal intracellular lipolysis, and the decrease in activity occurred about 30 min after the increase in glycerol release. There seems to be no relationship between the level of activity of lipoprotein lipase and the maximal rate of hormone-stimulated lipolysis in 3T3-L1 cells. Unlike in adipose tissue and adipocytes of rats, hormone-stimulated lipolysis and lipoprotein lipase activity in murine 3T3-L1 adipocytes appear to be regulated independently.     

Short-term fasting and lipolytic activity in rat adipocytes.

The aim of this experiment was to study the influence of 18-hour food deprivation on basal and stimulated lipolysis in adipocytes obtained from young male Wistar rats. Fat cells from fed and fasted rats were isolated from the epididymal adipose tissue by collagenase digestion. Adipocytes were incubated in Krebs-Ringer buffer (pH 7.4, 37 degrees C) without agents affecting lipolysis and with different lipolytic stimulators (epinephrine, forskolin, dibutyryl-cAMP, theophylline, DPCPX, amrinone) or inhibitors (PIA, H-89, insulin). After 60 min of incubation, glycerol and, in some cases, also fatty acids released from adipocytes to the incubation medium were determined. Basal lipolysis was substantially potentiated in cells of fasted rats in comparison to adipocytes isolated from fed animals. The inhibition of protein kinase A activity by H-89 partially suppressed lipolysis in both groups of adipocytes, but did not eliminate this difference. The agonist of adenosine A (1) receptor also did not suppress fasting-enhanced basal lipolysis. The epinephrine-induced triglyceride breakdown was also enhanced by fasting. Similarly, the direct activation of adenylyl cyclase by forskolin or protein kinase A by dibutyryl-cAMP resulted in a higher lipolytic response in cells derived from fasted animals. These results indicate that the fasting-induced rise in lipolysis results predominantly from changes in the lipolytic cascade downstream from protein kinase A. The antagonism of the adenosine A (1) receptor and the inhibition of cAMP phosphodiesterase also induced lipolysis, which was potentiated by food deprivation. Moreover, the rise in basal and epinephrine-stimulated lipolysis in adipocytes of fasted rats was shown to be associated with a diminished non-esterified fatty acids/glycerol molar ratio. This effect was presumably due to increased re-esterification of triglyceride-derived fatty acids in cells of fasted rats. Comparing fed and fasted rats for the antilipolytic effect of insulin in adipocytes revealed that short-term food deprivation resulted in a substantial deterioration of the ability of insulin to suppress epinephrine-induced lipolysis.     

Lipolysis in rat adipose tissue in vitro and its alpha 2 adrenergic control

Although the alpha 2-adrenergic inhibitory control on lipolysis in several mammalian species, e.g. in dogs, hamsters, rabbits and also in man has been proved recently, in rats some peculiarities have been described in the alpha-adrenergic regulation of lipid mobilization. In the present study the effect of the specific alpha 2-adrenergic blocking agent--yohimbine (YOH)--on lipid mobilization in young (45-55 days old) and old (6 months) rats of the Wistar strain was followed and also its interaction with the beta-adrenergic blocking agent propranolol and the specific alpha 2-adrenergic agonist--clonidine. Furthermore, the effect of YOH on the isoprenaline (ISO) induced lipolysis was studied. The adipokinetic effect was estimated from the amount of released non-esterified fatty acids from the tissue preparation into the incubation medium. In all experimental series YOH produced a significant response, which was concentration-dependent (pD2 6.2) with the maximum effect corresponding to 40% of the highest adipokinetic action of ISO. Young rats were more sensitive than old animals. In both age groups YOH non-competitively decreased the highest effect of ISO to the level of its own lipid-mobilizing activity. In the presence of YOH, the ISO dose-response curve therefore completely disappears. The adipokinetic effect of YOH can be antagonized competitively by clonidine (CLO) (pA2 6.25 and 6.01 for concentrations of CLO 1 and 10 mumol.1(-1) respectively). The inhibitory effect of propranolol is about two orders lower. It is concluded that in rats the alpha 2-adrenergic control rather concerns basal lipolysis, whereas induced lipid mobilization can be inhibited by depression of the alpha 2-activity.     

Effect of endothelin-1 on lipolysis in rat adipocytes

Objective : To explore the role of endothelin‐1 (ET‐1) on lipid metabolism, we examined the effect of ET‐1 on lipolysis in rat adipocytes. Research Methods and Procedure : Adipocytes isolated from male Sprague‐Dawley rats, weighing 400 to 450 grams, were incubated in Krebs‐Ringer buffer with or without 10^?7 M ET‐1 for various times or with various concentrations of ET‐1 for 4 hours; then glycerol release into the incubation medium was measured. In addition, selective ET_AR and ET_BR blockers were used to identify the ET receptor subtype involved. We also explored the involvement of cyclic adenosine monophosphate (cAMP) in ET‐1‐stimulated lipolysis using an adenylyl cyclase inhibitor and by measuring changes in intracellular cAMP levels in response to ET‐1 treatment. To further explore the underlying mechanism of ET‐1 action, we examined the involvement of the extracellular signal‐regulated kinase (ERK)‐mediated pathways. Results : Our results showed that ET‐1 caused lipolysis in rat adipocytes in a time‐ and dose‐dependent manner. BQ610, a selective ET_AR blocker, blocked this effect. The adenylyl cyclase inhibitor, 2′, 5′‐dideoxyadenosine, had no effect on ET‐1‐stimulated lipolysis. ET‐1 did not induce an increase in intracellular cAMP levels. In addition, ET‐1‐induced lipolysis was blocked by inhibition of ERK activation using PD98059. Coincubation of cells with ET‐1 and insulin suppressed ET‐1‐stimulated lipolysis. Discussion : These findings show that ET‐1 stimulates lipolysis in rat adipocytes through the ET_AR and activation of the ERK pathway. The underlying mechanism is cAMP‐independent. However, this non‐conventional lipolytic effect of ET‐1 is inhibited by the anti‐lipolytic effect of insulin.     

Effect of tyramine, a dietary amine, on glycerol and lactate release by isolated adipocytes from old rats

Amine degradation by adipocyte amine oxidases leads to the production of metabolites that interact with lipid and glucose metabolisms and their hormonal regulations. To further investigate these interactions, we determined the effect of a dietary amine, tyramine (TYR), on glycerol and lactate releases, respectively taken as indices of lipolytic and glycolytic activities of isolated adipocytes. Old male Wistar rats were used to prepare adipocytes by collagenase dissociation of retroperitoneal fat pads. The two tested doses of tyramine (10 microM and 1 mM) had no effect on basal glycerol release. On the other hand, TYR, at the highest dose tested (1 mM), weakly but significantly increased basal lactate release, which was elevated in adipocytes from old rats. Norepinephrine (NE), highly stimulated adipocyte lipolysis with a submaximal effect at 1 microM which was slightly but significantly inhibited by TYR 1 mM. Insulin 1 nM (INS) also poorly inhibited the NE-stimulated lipolysis in adipocytes isolated from old rats. TYR was able to potentiate the poor antilipolytic efficiency of INS. Under similar conditions, a high dose of NE greatly reduced lactate production and TYR (1 mM) reversed this inhibition of lactate release. INS was also able to totally reverse the inhibitory effect of NE on lactate release, but there was no potentiation between insulin and tyramine effects. It can be concluded that high doses of TYR interact with norepinephrine and insulin, at least on the control of glycerol and lactate release, by counteracting catecholamine effects and by mimicking insulin actions.     

Impaired PI 3-kinase activation in adipocytes from early growth-restricted male rats

Epidemiological studies have established a relationship between early growth restriction and subsequent development of type 2 diabetes. Animal studies have shown that offspring of protein-restricted rats undergo a greater age-related loss of glucose tolerance than controls. The aim of this study was to investigate the possibility that this deterioration of glucose tolerance is associated with changes in adipocyte insulin action. Adipocytes from low-protein offspring had higher basal levels of glucose uptake than controls. Insulin stimulated glucose uptake into control adipocytes but had little effect on low-protein adipocytes. Both groups had similar levels of basal and isoproterenol-stimulated lipolysis. Insulin inhibited lipolysis in control adipocytes but had a reduced effect on low-protein adipocytes. These changes in insulin action were not related to altered expression of insulin receptors or insulin receptor tyrosine phosphorylation; however, they were associated with reduced phosphatidylinositol 3-kinase and protein kinase B activation. These results demonstrate that reduced glucose tolerance observed in late adult life after early growth restriction is associated with adipocyte insulin resistance.     

The antilipolytic effect of insulin in human adipocytes requires activation of the phosphodiesterase

Human fat cells were incubated with two different cAMP analogues, 8-bromocAMP and 6N-monobutyrylcAMP. The former analogue is an excellent substrate for the phosphodiesterase while the latter is resistant to hydrolysis. In the presence of adenosine deaminase, isoproterenol (10(-6)M) stimulated lipolysis 8-10 fold which was similar to the effect exerted by the cAMP analogues. Basal lipolysis and lipolysis activated by 6N-monobutyrylcAMP was not inhibited by insulin even at high concentrations, whereas the effect of 8-bromocAMP was virtually completely inhibited. This effect of insulin was completely prevented by the addition of IBMX. Thus, activation of phosphodiesterase by insulin is necessary to elicit the antilipolytic effect in human adipocytes.