Sensitivity of adipocyte lipolysis to stimulatory and inhibitory agonists in hypothyroidism and starvation.

The responsiveness of lipolysis to the stimulatory agonists noradrenaline, corticotropin and glucagon and to the inhibitory agonists N6-phenylisopropyladenosine, prostaglandin E1 and nicotinic acid was investigated with rat white adipocytes incubated with a high concentration of adenosine deaminase (1 unit/ml). The cells were obtained from fed or 48 h-starved euthyroid animals or from fed or starved animals rendered hypothyroid by 4 weeks of treatment with low-iodine diet and propylthiouracil. Hypothyroidism increased sensitivity to and efficacy of all three inhibitory agonists in their opposition of noradrenaline-stimulated lipolysis. Starvation decreased sensitivity to all three inhibitory agonists when opposing basal lipolysis. Hypothyroidism decreased sensitivity to noradrenaline, glucagon and corticotropin by 37-, 4- and 4-fold respectively and decreased the maximum response to these agonists by approx. 50%, 50% and 75% respectively. Starvation reversed decreases in maximum response to these agonists in hypothyroidism. Starvation in the euthyroid state increased sensitivity to glucagon and noradrenaline, but did not alter sensitivity to corticotropin. Cells from hypothyroid rats were relatively insensitive to Bordetella pertussis toxin, which substantially increased basal lipolysis in the euthyroid state.     

Glucagon inhibition of insulin-stimulated 2-deoxyglucose uptake by rat adipocytes in the presence of adenosine deaminase.

Effects of adenosine deaminase and glucagon on insulin-stimulated 2-deoxyglucose uptake by rat adipocytes are reported. (1) Adenosine deaminase (10 micrograms/ml) caused a rightward shift in the dose-response curve for the stimulation by insulin of 2-deoxyglucose uptake, but the enzyme did not alter either the basal or the maximally insulin-stimulated uptake rate. (2) In adipocytes obtained from 24 h-starved rats, glucagon inhibited the effect of insulin on 2-deoxyglucose uptake in the presence (but not in the absence) of adenosine deaminase. Basal uptake rates were unaffected. (3) Glucagon inhibited insulin-stimulated 2-deoxyglucose uptake to a greater extent in cells isolated from starved rats than in cells from fed rats. (4) Adipocytes isolated from fed and from starved rats did not differ in their capacity for degradation of 125I-labelled glucagon. The results suggest that adenosine and glucagon are regulators of insulin action in adipose tissue.     

Nutritional and hormonal control of lipolysis in isolated gilthead seabream (Sparus aurata) adipocytes

We examined the effects of diet composition and fasting on lipolysis of freshly isolated adipocytes from gilthead seabream (Sparus aurata). We also analyzed the effects of insulin, glucagon, and growth hormone (GH) in adipocytes isolated from fish fed with different diets. Basal lipolysis, measured as glycerol release, increased proportionally with cell concentration and time of incubation, which validates the suitability of these cell preparations for the study of hormonal regulation of this metabolic process. Gilthead seabream were fed two different diets, FM (100% of fish meal) and PP (100% of plant protein supplied by plant sources) for 6 wk. After this period, each diet group was divided into two groups: fed and fasted (for 11 days). Lipolysis was significantly higher in adipocytes from PP-fed fish than in adipocytes from FM-fed fish. Fasting provoked a significant increase in the lipolytic rate, about threefold in isolated adipocytes regardless of nutritional history. Hormone effects were similar in the different groups: glucagon increased the lipolytic rate, whereas insulin had almost no effect. GH was clearly lipolytic, although the relative increase in glycerol over control was lower in isolated adipocytes from fasted fish compared with fed fish. Together, we demonstrate for the first time that lipolysis, measured in isolated seabream adipocytes, is affected by the nutritional state of the fish. Furthermore, our data suggest that glucagon and especially GH play a major role in the control of adipocyte lipolysis.     

Regulation of lipolysis in isolated adipocytes of rainbow trout (Oncorhynchus mykiss): the role of insulin and glucagon

In the present study, we have examined the effects of insulin and glucagon on the lipolysis of rainbow trout (Oncorhynchus mykiss). To this end, adipocytes were isolated from mesenteric fat and incubated in the absence (basal lipolysis) or presence of different concentrations of insulin and glucagon. In addition, to further elucidate the effects of these hormones in vivo on adipocyte lipolysis, both fasting and intraperitoneal glucagon injection experiments were performed. Basal lipolysis, measured as the glycerol released in the adipocyte medium, increased proportionally with cell concentration and incubation time. Cell viability was verified by measuring the release of lactate dehydrogenase (LDH) activity in the medium. Insulin (at doses of 35 and 350 nM) decreased lipolysis in isolated adipocytes of rainbow trout in vitro, while glucagon was clearly lipolytic at concentrations of 10 and 100 nM. Furthermore, hypoinsulinemia induced by fasting, as well as glucagon injection, significantly increased lipolysis in isolated adipocytes approximately 1.5- and 1.4-fold, respectively, when compared with adipocytes from control fish. Our data demonstrate that lipolysis, as measured in isolated adipocytes of rainbow trout, can be regulated by both insulin and glucagon. These results not only indicate that insulin is an important hormone in lipid deposition via its anti-lipolytic effects on rainbow trout adipocytes, but also reveal glucagon as a lipolytic hormone, as shown by both in vitro and in vivo experiments.     

Altered lipolytic response to glucagon and adenosine deaminase in adipocytes from starved rats.

1. Adipocytes isolated from epididymal adipose tissue of fed or 24 h-starved rats were incubated with a range of glucagon concentrations in the presence and absence of adenosine deaminase (4 munits/ml). 2. With adenosine deaminase present, the lipolytic response to low concentrations of glucagon (1-6 ng/ml) was considerably enhanced in cells from starved rats. 3. The effect of adenosine deaminase on basal lipolysis was altered after starvation. 4. D-3-Hydroxybutyrate (5 mM) decreased the sensitivity of lipolysis to glucagon. 5. The possible involvement of glucagon-stimulated lipolysis in the regulation of ketogenesis is briefly discussed.     

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.     

Some aspects of metabolic adaptations in lipid metabolism during starvation are mimicked by epinephrine in rat adipocytes

1. The effects of fasting on the neutral lipid synthesis to insulin and/or epinephrine in isolated fat cells have been examined using [1-14C]glucose. 2. The ability of adipocytes from starved rats to synthesize fatty acids from both labeled substrates was markedly diminished compared to adipocytes from control rats. 3. The response of lipogenic stimulation to insulin at all concentrations tested was greatly diminished in adipocytes from 24 hr starved rats. 4. [1-14C]glucose utilization rates in the absence or in the presence of insulin were not significantly different in adipocytes from 24 hr starved rats as compared with control adipocytes, although basal and insulin stimulated glyceride-glycerol synthesis were significantly higher in starved adipocytes. 5. Epinephrine acutely inhibited [1-14C]acetate incorporation into fatty acids for insulin-stimulated lipogenesis in control adipocytes, in contrast, this lipolytic agent strongly increased [1-14C]glucose conversion to triacylglycerols. 6. In both cases, the differences in lipid synthesis capacities found in both nutritional states were abolished by epinephrine.     

The role of glucagon in the regulation of myocardial lipoprotein lipase activity

The role of glucagon in regulating the lipoprotein lipase activities of rat heart and adipose tissue was examined. When starved rats were fed glucose, heart lipoprotein lipase activity decreased while that of adipose tissue increased. Glucagon administration to these animals at the time of glucose feeding prevented the decline in heart lipoprotein lipase activity, but had no effect on the adipose tissue enzyme. When glucagon was administered to fed rats, heart lipoprotein lipase activity increased to levels found in starved animals but there was no change in the adipose tissue enzyme. It is suggested that the reciprocal lipoprotein lipase activities in heart and adipose tissue of fed and starved animals may be regulated by the circulating plasma insulin and glucagon concentrations.     

Insulin sensitivity and responsiveness of epitrochlearis and soleus muscles from fed and starved rats. Recognition of differential changes in insulin sensitivities of protein synthesis and glucose incorporation into glycogen.

The insulin sensitivity of protein synthesis and glucose incorporation into glycogen by the soleus and epitrochlearis muscles from fed rats and 24 h-starved rats was determined in vitro during the first and second hours of incubation after isolation of the muscles. Rates of protein synthesis by both muscles from fed rats in the first hour of incubation were 2-fold higher than in the second hour and were not increased by insulin. Rates of protein synthesis during the first hour in the presence of 6000 microunits of insulin/ml were increased in soleus, but not in epitrochlearis, muscles from starved rats. Rates of protein synthesis in both muscles from fed and starved rats were increased significantly by insulin during the second hour. High concentrations of insulin caused a marked stimulation of the rates of glucose incorporation by both muscles from fed and starved rats in both the first and second hours of incubation. The insulin sensitivity of glucose incorporation during the second hour, defined as the concentration of insulin causing half-maximal stimulation, was increased 10-fold for both muscle types from starved rats (soleus, 65 microunits/ml; epitrochlearis, 45 microunits/ml) relative to muscles from fed rats (soleus, 600 microunits/ml; epitrochlearis, 500 microunits/m). The insulin sensitivity of protein synthesis in the second hour was greater for soleus muscles from starved rats (65 microunits/ml) than from fed rats (500 microunits/ml). In contrast, the insulin sensitivity of protein synthesis in epitrochlearis muscles from starved rats was significantly decreased (225 microunits/ml) compared with fed rats (25 microunits/ml Maximal rates achieved by high concentrations of insulin were not different from those in the same muscle from fed rats. It is suggested that protein synthesis, in distinction to glucose utilization, may be resistant to insulin stimulation during periods of acute starvation in muscles with fibre compositions similar to the epitrochlearis, but not in muscles with fibre compositions similar to the soleus. Partial reversal of the resistance observed in vitro for epitrochlearis muscles from starved rats may be due to the loss of factors which suppress the effect of insulin in vivo.     

Fat accumulation in the rat during early pregnancy is modulated by enhanced insulin responsiveness

Insulin sensitivity has been implicated in the variation of fat accumulation in early gestation by as-yet-unknown mechanisms. In the present study, we analyzed the insulin sensitivity of lipolysis and lipogenesis in lumbar adipocytes from rats at 0, 7, 14, and 20 days of gestation. In adipocytes of 7-day pregnant rats, we found a twofold decrease in both beta-agonist (isoproterenol and BRL-37344)-stimulated lipolysis and beta3-adrenoceptor protein but not in lipolysis initiated by forskolin or isobutylmethylxanthine, suggesting a modification of the lipolytic pathway at the receptor level. Whereas adipocytes from 7-day pregnant rats showed a twofold increase in fatty acid synthesis from glucose, those from 20-day pregnant animals displayed a decreased lipogenic activity. Insulin responsiveness of the lipolytic and lipogenic pathways was analyzed by dose-response experiments, giving evidence for the involvement of improved insulin responsiveness in the enhanced lipogenic and reduced lipolytic activities of adipocytes in early pregnancy. In contrast, insulin resistance is responsible for lower antilipolytic and lipogenic actions of insulin in late pregnant animals. In conclusion, the present study shows that enhanced adipose tissue insulin responsiveness during early pregnancy contributes to maternal fat accumulation, whereas decreased insulin responsiveness during late gestation modulates fat breakdown.     

Effects of hormones and N6O2''-dibutyryl-adenosine 3'' :5''-cyclic monophosphate, administered in vivo, on phosphate transport and metabolism in isolated rat liver mitochondria.

1. The administration of glucagon or N6O2'-dibutyryl cyclic AMP to fed rats by intraperitoneal injection was associated with a 2-fold increase in the amounts of endogenous Pi and ATP, and an increase in the rate and extent of transport of exogenous Pi (measured in either the presence or the absence of Ca2+) in mitochondria subsequently isolated from the liver. No change was observed in either the maximum rate of transport of exogenous Pi or in the rate of 32Pi exchange. 2. The changes induced by glucagon and dibutyryl cyclic AMP were markedly decreased by the co-administration of cycloheximide. 3. The administration of insulin to rats resulted in an increase of about 1.3-fold in the concentration of endogenous mitochondrial Pi 4. The amounts of endogenous Pi in mitochondrial isolated from the livers of starved rats were 3 times those in mitochondria isolated from fed animals. 5. It is concluded that the liver mitochondrial phosphatetransport system may be an important site of hormone action. 6. In the course of these experiments, it was shown that Ca2+ markedly stimulates mitochondrial phosphate transports.     

Impaired insulin-mediated inhibition of lipolysis and glucose transport with aging

Regulation of hormone action with aging has been extensively studied; adipocytes provide an interesting model for some of these questions. We have compared the ability of insulin to stimulate glucose uptake and suppress lipolysis in adipocytes isolated from two month and twelve month-old rats. The ability of insulin to stimulate maximal glucose transport was decreased in adipocytes from the older rats (P less than 0.001); as well, insulin's EC50 was also higher (P less than 0.01) in these cells. Furthermore, these defects were present when insulin-stimulated glucose transport was measured in the presence or absence of adenosine deaminase which metabolizes endogenously released adenosine. Endogenously released adenosine is a stimulator of glucose transport and an inhibitor of lipolysis. Maximal suppression of isoproterenol-induced lipolysis by insulin was similar when adipocytes isolated from the two age groups were incubated in the absence of adenosine deaminase. However, maximal insulin-mediated suppression of lipolysis was found to be significantly decreased (P less than 0.001) in adipocytes isolated from older rats when the experiments were done in the presence of adenosine deaminase; also, insulin's EC50 was increased in these cells under these conditions (P less than 0.001). These results emphasize the importance of the adenosine receptor in modulating the response of isolated adipocytes to insulin, particularly for lipolysis, and document the presence of age-associated defects in insulin regulation of both glucose transport and lipolysis.     

Regulation of fructose 2,6-bisphosphate concentration in white adipose tissue.

Injection of insulin to fed rats diminished the concentration of fructose 2,6-bisphosphate in white adipose tissue. Incubation of epididymal fat-pads or adipocytes with insulin stimulated lactate release and sugar detritiation and also decreased fructose 2,6-bisphosphate concentration. Such a decrease was, however, not observed in fat-pads from starved or alloxan-diabetic rats. Incubation of adipocytes from fed rats with various concentrations of glucose or fructose led to a dose-dependent rise in fructose 2,6-bisphosphate which correlated with lactate output and detritiation of 3-3H-labelled sugar. In adipocytes from fed rats, palmitate stimulated the detritiation of [3-3H]glucose without affecting lactate production and fructose 2,6-bisphosphate concentration. Incubation of epididymal fat-pads from fed rats in the presence of antimycin stimulated lactate output but decreased fructose 2,6-bisphosphate concentration. Changes in lipolytic rates brought about by noradrenaline, insulin, adenosine and corticotropin in adipocytes from fed rats were not related to changes in fructose 2,6-bisphosphate or to rates of lactate output. In fed rats, the activity of 6-phosphofructo-2-kinase was not changed after treatment of adipocytes with insulin, noradrenaline or adenosine. It is suggested that the decrease in fructose 2,6-bisphosphate concentration observed after insulin treatment can be explained by the increase in sn-glycerol 3-phosphate, an inhibitor of 6-phosphofructo-2-kinase.     

Alterations in response of rat white adipocytes to insulin, noradrenaline, corticotropin and glucagon after adrenalectomy. Correction of these changes by adenosine deaminase.

1. Adipocytes isolated from rats 6--9 days after adrenalectomy had significantly increased sensitivity to insulin action against noradrenaline-stimulated lipolysis. In the presence of adenosine deaminase there was no significant difference in insulin sensitivity between cells from adrenalectomized and sham-operated rats. 2. Adipocytes from adrenalectomized rats had decreased lipolytic responses to all concentrations of noradrenaline and glucagon tested and a decreased lipolytic response to low but not high concentrations of corticotropin. There was no difference in lipolytic response to theophylline after adrenalectomy. Adenosine deaminase corrected the differences in response to noradrenaline and glucagon resulting from adrenalectomy. 3. In the presence of adenosine deaminase rates of lipolysis, after stimulation by high concentrations of noradrenaline, glucagon, corticotropin or theophylline, were the same in cells from adrenalectomized or sham-operated rats. 4. These findings and previously reported effects of adenosine and adrenalectomy on adipocyte function are discussed. It is proposed that changes in adipocyte hormone responsiveness after adrenalectomy may result from changes in adenosine metabolism or release.     

Factors influencing the altered thermogenic response of rat brown adipose tissue in streptozotocin-diabetes.

1. Adipocytes were isolated from the interscapular brown fat of male rats maintained at 21 degrees C. These animals were controls, streptozotocin-diabetics or 2-day insulin-treated diabetics. 2. With adipocytes from diabetic animals, maximum rates of noradrenaline-stimulated O2 uptake were decreased by 58%, and the Bmax. of [3H]GDP binding to mitochondria was decreased by 55%. Insulin administration reversed both of these changes. 3. Streptozotocin-diabetes increased basal lipolysis in adipocytes incubated with adenosine deaminase (1 unit/ml), decreased the EC50 (concn. giving 50% of maximum effect) for noradrenaline, but did not change the maximum rate of noradrenaline-stimulated lipolysis. Except for some small differences at very low concentrations (10-100 pM), diabetes or insulin treatment did not alter the sensitivity of noradrenaline-stimulated lipolysis or O2 uptake to the inhibitory effect of N6-phenylisopropyladenosine. It is therefore concluded that the lesion(s) in thermogenesis in diabetes are not attributable to any changes in lipolysis. 4. Blood flow through interscapular brown fat, measured by accumulation of [14C]DDT [14C-labelled 1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane] was increased by 2.3-fold 70 min after a single administration of insulin to diabetic rats. This treatment decreased blood flow through epididymal white fat by 58%. 5. Propranolol treatment of diabetic rats muted the ability of insulin treatment to increase the maximum rate of noradrenaline-stimulated O2 uptake, suggesting that this action of insulin may be a secondary one rather than a direct effect of the hormone on the adipocytes.     

Insulin action on adipocytes. Evidence that the anti-lipolytic and lipogenic effects of insulin are mediated by the same receptor.

1. The dose-response relationships of insulin stimulation of lipogenesis and inhibition of lipolysis were studied simultaneously by using rat adipocytes to determine whether these different effects of insulin are mediated through the same or different sets of receptors. 2. The sensitivity (defined as the concentration of insulin required to produce a half-maximal effect) of the stimulated lipogenic response to insulin was not significantly different from the sensitivity of the anti-lipolytic response to insulin. The addition of different adrenaline and glucose concentrations did not alter the half-maximal concentration of insulin required to inhibit lipolysis. 3. The specificities of the lipogenic and antilipolytic responses were studied by using insulin analogues. The sensitivities of the lipogenic and anti-lipolytic responses were the same for five chemically modified insulins and hagfish insulin, which have potencies compared with bovine insulin of between 3 and 90%. 4. Starving rats for 48h significantly increased the sensitivities of both the antilipolytic and lipogenic responses to insulin, but the changes in the sensitivities of both lipogenesis and anti-lipolysis returned to that of fed rats. 5. We conclude that insulin stimulates lipogenesis and inhibits lipolysis over the same concentration range. These observations provide powerful evidence that the different effects of insulin are mediated through the same set of receptors.     

Effects of glucagon and N6O2''-dibutyryladenosine 3'':5''-cyclic monophosphate on calcium transport in isolated rat liver mitochondria.

1. The administration of glucagon to fed rats by intraperitoneal injection, or the perfusion of livers from fed rats with glucagon by the method of Mortimore [Mortimore (1963) Am.J. Physiol. 204, 699--704] was associated with increases of 15- and 5-fold respectively, in the time for which a given load of exogenous Ca2+ is retained by mitochondria subsequently isolated from the liver. This effect of glucagon was (a) also induced by N6O2'-dibutyryl cyclic AMP, (b) completely blocked by cycloheximide, (c) relatively slow in onset (15--60 min) and (d) associated with a stimulation of about 20% in the rates of ADP-stimulated oxygen utilization and Ca2+ transport measured in the presence of succinate. 2. Perfusion of livers with glucagon resulted in the isolation of mitochandria which showed a 50% increase, no significant change and a 40% increase in the concentrations of endogenous Ca, Mg and Pi respectively, when compared with mitochondria isolated from control perfused livers. 3. The administration of insulin or adrenaline to fed rats induced increases of 10- and 8-fold respectively, in the time for which Ca2+ is retained by isolated liver mitochondria. Perfusion of livers with insulin had no effect on mitochondrial Ca2+ retention time. 4. The perfusion of livers from starved rats with glucagon, or the administration of either glucagon or insulin to starved rats, increased by about 2.5- and 15-fold respectively, the time for which isolated mitochondria retain Ca2+. 5. Mechanisms which may be responsible for the observed alterations in Ca2+-retention time are discussed.     

The maximal activity of phosphate-dependent glutaminase and glutamine metabolism in late-pregnant and peak-lactating rats.

The maximal activity of phosphate-dependent glutaminase was increased in the small intestine, decreased in the liver and unchanged in the kidney of late-pregnant rats. This was accompanied by increases in the size of both the small intestine and the liver. The maximal activity of phosphate-dependent glutaminase was increased in both the small intestine and liver but unchanged in the kidney of peak-lactating rats. Enterocytes isolated from late-pregnant or peak-lactating rats exhibited an enhanced rate of utilization of glutamine and production of glutamate, alanine and ammonia. Arteriovenous-difference measurements across the gut showed an increase in the net glutamine removed from the circulation in late-pregnant and peak-lactating rats, which was accompanied by enhanced rates of release of glutamate, alanine and ammonia. Arteriovenous-difference measurements for glutamine showed that both renal uptake and skeletal-muscle release of glutamine were not markedly changed during late pregnancy or peak lactation; but pregnant rats showed a hepatic release of the amino acid. It is concluded that, during late pregnancy and peak lactation, the adaptive changes in glutamine metabolism by the small intestine, kidneys and skeletal muscle of hindlimb are similar; however, the liver appears to release glutamine during late pregnancy, but to utilize glutamine during peak lactation.     

Augmentation of lipolysis in adipocytes from fed rats, but not from starved rats, by inhibition of rolipram-sensitive phosphodiesterase 4

The sensitivity of adipocytes to lipolytic agents is increased after starvation. In this study, we found that LY294002, an inhibitor of phosphatidylinositol-3 kinase (PI3K), in the concentration of more than 50 microM potentiates lipolysis induced by adenosine deaminase in adipocytes from fed rats (f-adipocytes), but not from starved rats (s-adipocytes). It also enhanced the sensitivity to lipolytic action of isoproterenol in f-adipocytes much more than s-adipocytes. The target of LY294002 may be an anti-lipolytic regulator expressed in response to food intake. Since another PI3K inhibitor, wortmannin, or a phosphodiesterase 3 (PDE3) inhibitor, cilostamide, failed to cause any specific effect to f-adipocytes, the PI3K-PDE3B pathway cannot be a target of LY294002. We found that LY294002 inhibits efficiently the cytoplasmic PDE activity of adipocytes. Rolipram, a specific inhibitor of PDE4, also inhibited the cytoplasmic PDE and caused a preferential increase of lipolysis in f-adipocytes. LY294002 blunted the actions of rolipram on lipolysis and the PDE activity. LY294002 accelerated protein kinase A activation. These data suggest that the rolipram-sensitive PDE4 is an anti-lipolytic enzyme expressed according to food intake. LY294002 may potentiate lipolysis through inhibition of the PDE4.     

Effects of starvation, refeeding, and fat feeding on adipocyte ghost adenyl cyclase activity

Basal adenyl cyclase activity and its response to epinephrine and glucagon were studied in isolated adipocyte ghosts obtained from fed, starved, refed, and fat-diet-adapted rats. Epinephrine stimulation of adenyl cyclase was significantly increased in fasted rats, but the glucagon response did not change. Rats fasted for 48 hr and refed a high carbohydrate, low fat diet for 48 or 96 hr showed no differences from chow-fed animals in either basal or hormone-stimulated adenyl cyclase activity. Rats adapted to a high fat, low carbohydrate diet showed an initial and transitory increase in basal activity but a progressive loss of epinephrine- and glucagon-stimulated enzyme activities. The loss in hormone responsiveness correlated well with a decrease in hormone-stimulated lipolysis of fat pads and was associated with a significant increase in fat cell diameter.