Roles of alpha and beta adrenergic receptors in control of glucose oxidation in hamster epididymal adipocytes

Oxidation of [¹⁴C]glucose in isolated epididymal adipocytes from Golden hamsters was stimulated by isoproterenol and norepinephrine, which all interact with β-adrenergic receptors and by adrenorticotrophic hormone. In contrast α-receptor agonists, such as phenylephrine, methoxamine or clonidine did not increase basal glucose oxidation. The β-adrenergic blocking drug propranolol inhibited both lipolysis and glucose oxidation when these had been stimulated by isoproterenol, ephinephrine and phenoxybenzamine did not the α-adrenergic blocking drugs phentolamine and phenoxybenzamine did not influence lipolysis or glucose oxidation when isoproterenol provided the stimulus and increased both liposlysis and glucose metabolism in the presence of either epinephrine or norepinephrine. All α-adrenergic agonists tested (phenylephrine, methoxamine and clonidine) lowered liposlysis and glucose oxidation in isolated adipocytes exposed to isoproterenol. However, when adrenorcortropin provided the stimulus for glucose oxidation and lipolysis, only clonidine produced a significant reduction in lipolysis and glucose oxidation. None of the α-agonists influenced glucose metabolism which had been increased by insulin. These data confirm the presence of both α and β adrenergic receptors on hamster epididymal adipocytes and suggests that they exert antagonistic influences on lipolysis and glucose oxidation. These data are also consistent with the view that adrenergic stimulation of glucose oxidation and lipolysis in adipocytes are both mediated through β receptors.     

Roles of alpha and beta adrenergic receptors in control of glucose oxidation in hamster epididymal adipocytes.

Oxidation of [14C] glucose in isolated epididymal adipocytes from Golden hamsters was stimulated by isoproterenol, epinephrine and norepinephrine, which all interact with beta-adrenergic receptors and by adrenocorticotrophic hormone. In contrast alpha-receptor agonists, such as phenylephrine, methoxamine or clonidine did not increase basal glucose oxidation. The beta-adrenergic blocking drug propranolol inhibited both lipolysis and glucose oxidation when these had been stimulated by isoproterenol, epinephrine or norepinephrine. Conversely, the alpha-adrenergic blocking drugs phentolamine and phenoxybenzamine did not influence lipolysis or glucose oxidation when isoproterenol provided the stimulus and increased both lipolysis and glucose metabolism in the present of either epinephrine or norepinephrine. All alpha-adrenergic agonists tested (phenylephrine, methoxamine and clonidine) lowered lipolysis and glucose oxidation isolated adipocytes exposed to isoproterenol. However, when adrenocorticotropin provided the stimulus for glucose oxidation and lipolysis, only clonidine produced a significant reduction in lipolysis and glucose oxidation. None of the alpha-agonists influenced glucose metabolism which had been increased by insulin. These data confirm the presence of both alpha and beta adrenergic receptors on hamster epididymal adipocytes and suggest that they exert antagonistic influences on lipolysis and glucose oxidation. These data are also consistent with the view that adrenergic stimulation of glucose oxidation and lipolysis in adipocytes are both mediated through beta receptors.     

Regulation of lipolysis and cyclic AMP synthesis through energy supply in isolated human fat cells.

The effects of glucose and of various inhibitors of glycolysis or of oxidative phosphorylation on stimulated lipolysis and on intracellular cyclic AMP and ATP levels were investigated in isolated human fat cells. The glycolysis inhibitors, NaF and monoiodoacetate, inhibited epinephrine or theophylline-stimulated lipolysis and parallely reduced the intracellular cyclic AMP and ATP levels; however, neither NaF nor monoidoacetate significantly affected dibutyryl cyclic AMP-induced lipolysis. Removal of glucose from the medium also reduced the rate of epinephrine-stimulated lipolysis and the intracellular cyclic AMP and ATP levels but failed to modify the lipolytic activity of dibutyryl cyclic AMP. The oxidative phosphorylation inhibitors, antimycin A and, under fixed conditions, 2,4-dinitrophenol also strongly decreased the adipocyte cyclic AMP and ATP levels but inhibited as well the rate of epinephrine- and of dibutyryl cyclic AMP-induced lipolysis. N-Ethylmaleimide, a mixed glycolysis and oxidative phosphorylation inhibitor, not only reduced the intracellular cyclic AMP and ATP levels and epinephrine- or theophylline-induced lipolysis, but also that stimulated by dibutyryl cyclic AMP. When glycolysis was almost fully inhibited, human fat cells were insensitive to epinephrine but remained fully responsive to dibutyryl cyclic AMP. These results, showing a relationship between ATP availability, cyclic AMP synthesis and lipolysis, suggest a different ATP requirement for cyclic AMP synthesis and triacylglycerol lipase activation, a difference which could explain why ATP issued from glucose breakdown appears to be a determinant factor for cyclic AMP synthesis, but not for triacylglycerol lipase activation in human fat cells.     

The Adipose Tissue Phenotype of Hormone‐Sensitive Lipase Deficiency in Mice

Objective: To directly ascertain the physiological roles in adipocytes of hormone‐sensitive lipase (HSL; E.C. 3.1.1.3), a multifunctional hydrolase that can mediate triacylglycerol cleavage in adipocytes. Research Methods and Procedures: We performed constitutive gene targeting of the mouse HSL gene (Lipe), subsequently studied the adipose tissue phenotype clinically and histologically, and measured lipolysis in isolated adipocytes. Results: Homozygous HSL^?/? mice have no detectable HSL peptide or cholesteryl esterase activity in adipose tissue, and heterozygous mice have intermediate levels with respect to wild‐type and deficient littermates. HSL‐deficient mice have normal body weight but reduced abdominal fat mass compared with normal littermates. Histologically, both white and brown adipose tissues in HSL^?/? mice show marked heterogeneity in cell size, with markedly enlarged adipocytes juxtaposed to cells of normal morphology. In isolated HSL^?/? adipocytes, lipolysis is not significantly increased by β₃‐adrenergic stimulation, but under basal conditions in the absence of added catecholamines, the lipolytic rate of isolated HSL^?/? adipocytes is at least as high as that of cells from normal controls. Cold tolerance during a 48‐hour period at 4 °C was similar in HSL^?/? mice and controls. Overnight fasting was well‐tolerated clinically by HSL^?/? mice, but after fasting, liver triglyceride content was significantly lower in HSL^?/? mice compared with wild‐type controls. Conclusions: In isolated fat cells, the lipolytic rate after β‐adrenergic stimulation is mainly dependent on HSL. However, the observation of a normal rate of lipolysis in unstimulated HSL^?/? adipocytes suggests that HSL‐independent lipolytic pathway(s) exist in fat. Physiologically, HSL deficiency in mice has a modest effect under normal fed conditions and is compatible with normal maintenance of core body temperature during cold stress. However, the lipolytic response to overnight fasting is subnormal.     

Growth Hormone Treatment of Hypophysectomized Rats Increases Catecholamine-induced Lipolysis and the Number of β-Adrenergic Receptors in Adipocytes: No Differences in the Effects of Growth Hormone on Different Fat Depots

Growth hormone (GH) has a lipolytic effect in adipose tissue but this effect may differ in adipose tissue from various fat depots. This latter possibility was investigated in the present study, in which the effects of GH in vivo on catecholamine-induced lipolysis and the number of β-adrenergic receptors in isolated adipocytes from different fat depots of hypophysectomized rats were investigated. Female and male Sprague-Dawley rats were hypophysectomized or sham-operated at 45 days of age. One week after the operation, hormonal replacement therapy with L-thyroxine and hydrocortisone acetate was given. In addition, groups of rats were treated with GH (1.33 mg/kg per day, given as two daily subcutaneous injections). After 1 week of hormonal treatment, adipocytes were isolated from the parametrial, epididymal and inguinal fat pads, and glycerol release after catecholamine-stimulation and ¹²⁵I-cyanopindolol binding were measured. Hypophysectomy resulted in a marked decrease in the lipolytic response to catecholamines. GH treatment significantly increased catecholamine-induced lipolysis with similar effects in adipocytes from parametrial or epididymal and inguinal fat depots in both female and male rats. There were no differences between norepinephrine compared with isoproterenol-induced responses. ¹²⁵I-cyanopindolol binding was reduced after hypophysectomy and normalized by GH treatment, without differences between parametrial and inguinal adipose tissue regions. We conclude that the lipolytic effects of GH in the rat may partly be mediated by a stimulatory effect on β-adrenergic receptors in adipocytes. In addition, GH exerted similar effect on catecholamine-induced lipolysis and β-adrenergic receptors in adipocytes from parametrial, epididymal and inguinal fat depots.     

Modulation of lipolysis by endotoxin and indomethacin in fat cells of germfree and conventional dogs.

Germfree fat cells released significantly less FFA and glycerol under basal conditions (i.e. in the absence of hormonal stimulation) than conventional cells. The lipolytic response to norepinephrine stimulation (0.2 μg/ml) was not different in the two cell populations.$\text{E. coli}$ endotoxin increased basal lipolysis and norepinephrine stimulated (0.2 μg/ml) FFA release in adipocytes from conventional dogs, while having no consistent influence on lipolysis of adipocytes from germfree dogs. The endotoxin effect was not dose dependent (0.2–2.0 μg/0.5 ml cell suspension).Indomethacin (5.0 μg/ml) significantly increased basal FFA and glycerol release from cells of germfree origin, and FFA efflux from cells of conventional dogs. Endotoxin obviated the influence of indomethacin on basal lipolysis of germfree cells.Endotoxin by itself did not alter cAMP concentrations in adipocytes from germfree dogs. The combination of indomethacin and endotoxin, however, significantly increased intracellular cyclic nucleotide concentrations.Compared to conventional fat cells, germfree fat cells are characterized by significantly reduced basal lipolysis, lack of a consistent lipolytic response to endotoxin stimulation and dissociation of the lipolytic response and cAMP levels by the combined influence of endotoxin and indomethacin.     

Natriuretic peptide-dependent lipolysis in fat cells is a primate specificity

We have recently demonstrated that natriuretic peptides (NPs), which are known for regulation of blood pressure via membrane guanylyl cyclase (GC) receptors, are lipolytic in human adipose tissue. In this study, we compared the NP control of lipolysis in adipocytes from humans, nonhuman primates (macaques), rodents (rats, mice, hamsters), and nonrodent mammals (rabbits, dogs). Isolated adipocytes from these species were exposed to increasing concentrations of atrial NP (ANP) or isoproterenol (beta-adrenergic agonist). Although isoproterenol was lipolytic in all of the species, ANP only enhanced lipolysis in human and macaque adipocytes. In primate fat cells, NP-induced lipolysis involved a cGMP-dependent pathway. Binding studies and real-time quantitative PCR assays revealed that rat adipocytes expressed a higher density of NP receptors compared with humans but with a different subtype pattern of expression; type-A GC receptors predominate in human fat cells. This was also confirmed by the weak GC-activity stimulation and the reduced cGMP formation under ANP exposure in rat adipocytes compared with human fat cells. In conclusion, NP-induced lipolysis is a primate specificity, and adipocytes from ANP-nonresponsive species present a predominance of "clearance" receptors and very low expression of "biologically active" receptors.     

A Comparison of the Lipolytic Activity of Different Natriuretic Peptides on Human Adipocytes

Atrial natriuretic peptide (ANP) was recently shown to promote triacylglycerol hydrolysis in human white adipocytes both in vitro and in vivo through a cGMP-dependent pathway. The ANP-stimulated lipolytic effect is known to be specific to primates. In this study, we compared the lipolytic effect of different natriuretic peptides obtained from several species, including ANP from human, rat, chicken, frog, and eel, brain natriuretic peptide (BNP) from porcine and rat, C-type natriuretic peptide (CNP) from human, chicken, and frog, Dendroaspis natriuretic peptide (DNP), urodilatin, and des-[Gln¹⁸, Ser¹⁹, Gly²⁰, Leu²¹, Gly²²]-ANP (C-ANP), on human and rat adipocytes. We also compared the amount of intracellular cGMP produced in both human and rat adipocytes that were treated with natriuretic peptides. Among these NPs, rat ANP, as well as porcine and rat BNP, DNP and urodilatin showed the ability to elevate intracellular cGMP and to stimulate lipolysis as human ANP. No natriuretic peptide showed the ability to stimulate lipolysis in rat adipocytes, though some of them induced significant elevation of intracelluar cGMP concentrations. These results suggest that ANP and BNP from species close to human have the ability to induce lipolysis in human adipocytes. Jiahua Yu and Yeon Jun Jeong contributed equally.     

Adrenergic receptors are not mediators in the lipolytic effect of somatostatin in rat adipocytes

Beta-adrenergic receptors have been purported to act as possible mediators in the lipolytic effect of somatostatin in vivo. Investigations with isolated rat adipocytes studying the lipolytic activity of somatostatin (1.7 x 10(-7) M), glucagon (8.1 x 10(-8 M) and norepinephrine (10(-6) M), have shown that the lipolytic effect stimulated by somatostatin is not altered by 10(-5) M propranolol (beta-antagonist); is significantly enhanced by 10(-5) M isoproterenol (beta-agonist) and is not altered by the addition of 10(-6) M phenoxybenzamine (alpha-antagonist) or 10(-6) M phenylephrine (alpha-agonist). Similar results were found when lipolysis was stimulated by glucagon, whereas the lipolytic effect stimulated by norepinephrine was blocked by propranolol. These results indicate that the direct lipolytic effect of somatostatin on isolated rat adipocytes does not seem to be mediated through mechanisms involved with adrenergic receptors.     

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.     

Inhibition of protein synthesis on homologous and heterologous desensitization in the rat adipocyte

Desensitization of lipolysis was induced in isolated rat adipocytes by incubation with isoproterenol 10^?5M or ACTH 250 mU/ml for two and three hours, respectively. Those cells desensitized with isoproterenol were restimulated with either isoproterenol 10^?7M or ACTH 6 mU/ml and those cells desensitized with ACTH were restimulated with isoproterenol 10^?7M. Lipolysis was quantitated by the release of cyclic AMP and glycerol. No effect on either homologous or heterologous desensitization was observed when either cycloheximide 2 μg/ml or puromycin 10^?4M was included in the incubation media during the induction of desensitization. These findings support the conclusion that protein synthesis plays no role in the desensitization of lipolysis in the isolated rat adipocyte.     

Comparison of the Lipolytic Effects of Norepinephrine and BRL 37344 in Rat Brown and White Adipocytes

The lipolytic effects of norepinephrine (a non-selective β-agonist) and BRL 37344 (a selective β₃-agonist) were compared in isolated rat brown and white adipocytes. Norepinephrine and BRL 37344 maximally stimulated lipolysis in brown and white adipocytes, approximately 10 times above basal values. However, adipocyte sensitivity for BRL 37344 was greater than that for norepinephrine, particularly in brown adipocytes [the EC₅₀ values (nM) for BRL 37344 and norepinephrine were 5 ± 1 and 103 ± 31 in brown adipocytes (P <0.01) versus 56 ± 9 and 124 ± 17 in white adipocytes (P <0.05), respectively]. On the other hand, the lipolytic effects of norepinephrine were totally blocked by 20–40 times superior concentrations of propranolol or bupranolol in brown as well as in white adipocytes. In contrast, the lipolytic effects of BRL 37344 were fully inhibited by concentrations of propranolol or bupranolol that were 200–1000 superior to the β₃ agonist concentration. The results demonstrate that: (1) the (β₃-agonist BRL 37344 is as effective as norepinephrine for maximally stimulating lipolysis in rat brown and white adipocytes, (2) both adipocyte types are more sensitive to the lipolytic effects of BRL 37344 than to those of norepinephrine, (3) although bupranolol is a better antagonist than propranolol on BRL 37344-stimulated lipolysis, it cannot be considered as a specific β₃-antagonist, (4) brown adipocytes are 10 times more sensitive than white adipocytes to the lipolytic effects of BRL 37344, suggesting an important role of β₃-receptors in brown adipose tissue.     

Mice Deficient in Phosphofructokinase‐M Have Greatly Decreased Fat Stores

Synthesis of triacylglycerol requires the glucose‐derived glycerol component, and glucose uptake has been viewed as the rate‐limiting step in glucose metabolism in adipocytes. Furthermore, adipose tissue contains all three isoforms of the glycolytic enzyme phosphofructokinase (PFK). We here report that mice deficient in the muscle isoform PFK‐M have greatly reduced fat stores. Mice with disrupted activity of the PFK‐M distal promoter were obtained from Lexicon Pharmaceuticals, developed from OmniBank OST#56064. Intra‐abdominal fat was measured by magnetic resonance imaging of the methylene proton signal. Lipogenesis from labeled glucose was measured in isolated adipocytes. Lipolysis (glycerol and free fatty acid release) was measured in perifused adipocytes. Intra‐abdominal fat in PFK‐M–deficient female mice (5–10 months old) was 17 ± 3% of that of wild‐type littermates (n = 4; P < 0.02). Epididymal fat weight in 15 animals (7–9.5 months) was 34 ± 4% of control littermate (P < 0.002), with 10–30% lower body weight. Basal and insulin‐stimulated lipogenesis in PFK‐M–deficient epididymal adipocytes was 40% of the rates in cells from heterozygous littermates (n = 3; P < 0.05). The rate of isoproterenol‐stimulated lipolysis in wild‐type adipocytes declined ～10% after 1 h and 50% after 2 h; in PFK‐M–deficient cells it declined much more rapidly, 50% in 1 h and 90% in 2 h, and lipolytic oscillations appeared to be damped (n = 4). These results indicate an important role for PFK‐M in adipose metabolism. This may be related to the ability of this isoform to generate glycolytic oscillations, because such oscillations may enhance the production of the triacylglycerol precursor α‐glycerophosphate.     

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.     

Lipolytic effect of BRL 35 135, a beta3 agonist, and its interaction with dietary lipids on the accumulation of fats in rat body

The type of intaked fat and fat uptake mechanisms such as adrenergic-induced lipolysis affect patterns of fat accumulation in animal body. In this study, in vitro lipolytic effect of BRL 35135, a selectivebeta3 agonist, and its interaction with different dietary fats on fat accumulation in animal body (in vivo) were studied. For in vitro study, adipocytes isolated from epididymal fat were incubated with 10(-5) M -10(-9) M of either BRL 35135 or isoproterenol, a non-selectivebeta-agonist. In animal study, two groups of SD-rats, i.e., BRL35135-intaked (dosed at 0.5 mg/kg/day in diet) and control, were divided into 4 sub-groups and fed diets containing 12% of either beef tallow (BT), canola oil (CO), olive oil (OO) or safflower oil(SO) for 6 weeks. In vitro study showed that BRL 35135 was 10 times more potent than isoproterenol in increasing the lipolysis in rat adipocytes. In animal study, inclusion of BRL35135 reduced daily weight gain in CO and SO groups (P < 0.05). Abdominal fat weight in BRL35135-intaked group was significantly lower than control in all dietary sub-groups (CO, OO and SO) except BT (P < 0.05). In BT group, abdominal fat contained significantly higher amount of total saturated fatty acids (SFAs) compared to CO, OO or SO. It was concluded that, although BRL 35135 was very potent in increasing lipolysis in the isolated adipocytes of rat, its preventive effect on lipid accumulation in animal body through the lipolysis could be affected by the type of dietary fat and was lesser when rats fed fats rich in SFAs.     

Effect of aging and cellularity on lipolysis in isolated mouse fat cells

The effects of age and cellularity on lipolysis have been investigated in isolated epididymal fat cells from both Swiss albino mice and Sprague-Dawley rats. No significant lipolytic response to glucagon could be demonstrated with adipocytes from either young or old mice, while glycerol output was increased by this hormone with fat cells from young rats. Larger adipocytes from older mice showed significantly greater isoproterenol-stimulated lipolysis than those from younger animals if the glycerol output was expressed on a per cell basis. However, the lipolytic response per cell appeared to be equivalent in young and old rat adipocytes with either isoproterenol or ACTH-(1-24). In a complete aging study, relationships between body weight, epididymal fat pad weight and cellularity were examined covering the life span of the mouse. ACTH-(1-24)- and dibutyryl cyclic AMP-stimulated lipolysis increased with age and cell size but fell at senescence when adipocyte size diminished. Although an effect of aging per se cannot be ruled out with the experimental techniques used in the present study, a dominant influence of adipocyte size on the lipolytic process was demonstrated.     

Divergent effects of adrenocorticotropin and melanotropin on isolated rat and rabbit adipocytes

The stimulation of lipolysis in isolated rat and rabbit fat cells by adrenocorticotropin (ACTH) and α-melanotropin has been studied. The concentration of α-melanotropin required for half maximal stimulation is 0.23 times that of ACTH in rabbit adipocytes but as high as 1140 times that of ACTH in rat fat cells. Chemical modification of the tryptophan residue in ACTH and melanotropin resulted in a loss of lipolytic activity in rat adipocytes and an increase in lipolytic potency in rabbit fat cells. These differences between rat and rabbit fat cells were evident when stimulation of cyclic AMP synthesis was measured in isolated cells or ghosts. The results are discussed in terms of the difference in the hormone receptors of the fat cells of the two species.     

Divergent effects of adrenocorticotropin and melanotropin on isolated rat and rabbit adipocytes.

The stimulation of lipolysis in isolated rat and rabbit fat cells by adrenocorticotropin (ACTH) and alpha-melanotropin has been studied. The concentration of alpha-melanotropin required for half maximal stimulation is 0.23 times that of ACTH in rabbit adipocytes but as high as 1140 times that of ACTH in rat fat cells. Chemical modification of the tryptophan residue in ACTH and melanotropin resulted in a loss of lipolytic activity in rat adipocytes and an increase in lipolytic potency in rabbit fat cells. These differences between rat and rabbit fat cells were evident when stimulation of cyclic AMP synthesis was measured in isolated cells or ghosts. The results are discussed in terms of the difference in the hormone receptors of the fat cells of the two species.     

Regulation of hormone stimulation of adipose tissue lipolysis by guanosine triphosphate

Guanosine triphosphate (GTP) enhanced the rate of mobilization of free fatty acids from isolated rat epididymal fat cells and potentiated the lipolytic response to norepinephrine, adrenocorticotropic hormone, glucagon, and theophylline. ITP, CTP, UTP, and TTP also increased basal and norepinephrine-stimulated lipolysis but to a lesser extent than GTP. ATP differed from the other nucleotides by inhibiting norepinephrine-stimulated lipolysis. The degree of phosphorylation of the guanine was important for activity since GTP was more active than GDP which, in turn, was more active than GMP in potentiating hormone-sensitized free fatty acid mobilization. Cyclic 3′, 5′-GMP, guanine, and guanosine were inactive in this regard. Activation of lipolysis by GTP occurred immediately upon addition of the nucleotide. The lipolytic response to GTP alone or in combination with norepinephrine or theophylline was exquisitely sensitive to inhibition by prostaglandin E₂. Nicotinic acid also inhibited the GTP response but to a lesser extent than prostaglandin E₂ and the β-blocker, propranolol, had no effect. Lipolytic concentrations of GTP in combination with norepinephrine increased intracellular levels of cAMP. By some as yet unknown mechanism GTP and GDP sensitized the adenylate cyclase of adipocytes to the actions of both agonists and antagonists.     

High lipolytic activity and dyslipidemia in a Spontaneous Hypertensive/NIH Corpulent (SHR/N-cp) rat: a genetic model of obesity and type 2 diabetes mellitus

In order to better understand the link between obesity and type 2 diabetes, lipolysis and its adrenergic regulation was investigated in various adipose depots of obese adult females SHR/N-cp rats. Serum insulin, glucose, free fatty acids (FFA), triglycerides (TG) and glycerol were measured. Adipocytes were isolated from subcutaneous (SC), parametrial (PM) and retroperitoneal (RP) fat pads. Total cell number and size, basal lipolysis or stimulated by norepinephrine (NE) and BRL 37344 were measured in each depot. Obese rats were hyperinsulinemic and hyperglycemic, suggesting high insulin resistance. They presented a marked dyslipidemia, attested by increased serum FFA and TG levels. High serum glycerol levels also suggest a strong lipolytic rate. Obese rats showed an excessive development of all fat pads although a more pronounced effect was observed in the SC one. The cellularity of this depot was increased 8 fold when compared to lean rats, but these fat cells were only 1.5 to 2-fold larger. SC adipocytes showed a marked increase in their basal lipolytic activity but a lack of change in responsiveness to NE or BRL 37344. The association between high basal lipolysis and increased cellularity yields to a marked adipose cell lipolytic rate, especially from the SC region. SHR/N-cp rats were characterized by a hyperplasic type of obesity with an excessive development of the SC depot. The dyslipidemia, attested by an altered serum lipid profile could be attributed to excessive lipolysis that contributes to increased FFA levels, and to early development of insulin resistance through a lipotoxicity effect.