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The aim of the study was to investigate the regulation affecting the recently described atrial natriuretic peptide (ANP)-dependent lipolytic pathway in comparison with the adrenergic lipolytic cascade. We studied in vivo the effect of a euglycemic-hyperinsulinemic clamp on the changes occurring in the extracellular glycerol concentration (EGC) of subcutaneous adipose tissue (SCAT) during ANP or epinephrine perfusion in a microdialysis probe. Homologous desensitization and the incidence of hyperinsulinemia on the ANP- and catecholaminergic-dependent control of lipolysis were also investigated in vitro on fat cells from SCAT. When perfused in SCAT, epinephrine and ANP promoted an increase in EGC; the EGC increase was significantly lower during the clamp. The reduction of epinephrine-induced lipolysis was limited (18%) when phentolamine (an alpha(2)-adrenergic receptor [AR] antagonist) was perfused together with epinephrine. Unlike the effect of epinephrine, the response to ANP observed during the second perfusion was reduced by 32%. The increase in extracellular guanosine 3',5' -cyclic monophosphate concentration, which reflects ANP activity, was also reduced during the second perfusion. Desensitization of the lipolytic effects of ANP was observed in vitro after a 2-hour period of recovery, while the effects of alpha(2)-AR agonist or of epinephrine were unchanged. Insulin was without any effect on ANP-induced lipolysis and alpha(2)-AR-mediated antilipolysis, while it reduced beta-AR-induced lipolysis. The ANP-dependent lipolytic pathway undergoes desensitization in vitro and in situ. Insulin had no inhibitory effect on either ANP- or alpha(2)-AR-dependent pathways, while it counteracted the beta-AR pathway.

In man, the major hormones controlling the lipolytic function are insulin (inhibition of lipolysis) and catecholamines (stimulation of lipolysis). Catecholamines are of major importance for the regulation of lipid mobilization in human adipose tissue and for the increase of non-esterified fatty acid supply to the working muscle. In vitro studies have shown that there are differences in the catecholaminergic control of fat cells from various fat deposits and a number of physiological and pathological alterations of catecholamine-induced lipolysis have been reported. Lipolytic resistance to catecholamines has been reported in subcutaneous adipose tissue, the major fat depot in obese subjects. Multiple alterations in catecholamine signal transduction pathways have been reported. In situ microdialysis allows a physiological exploration of adipose tissue biology. Recent data obtained on the catecholaminergic regulation of lipolysis and lipid mobilization, using microdialysis in humans, will be analysed. A potent lipolytic and lipomobilizing effect of atrial natriuretic peptide has recently been discovered; the mechanisms of action and physiological relevance will also be discussed.

Five adrenoceptor (AR) subtypes (beta 1, beta 2, beta 3, alpha 2 and alpha 1), are involved in the control of white and brown fat cell function. A number of metabolic events are controlled by the adrenergic system in fat cells. The stimulatory effect of catecholamines on lipolysis and metabolism is mainly connected to increments in cAMP levels, cAMP protein kinase activation and phosphorylation of various target proteins. Norepinephrine and epinephrine operate through differential recruitment of alpha 2- and beta-AR subtypes on the basis of their relative affinity for the different subtypes (the relative order of affinity is alpha 2 > beta 1 > or = beta 2 > beta 3 for norepinephrine). Antagonistic actions at the level of cAMP production exist between alpha 2- and beta 1-, beta 2- and beta 3-AR-mediated lipolytic effects in human white fat cells. The role of fat cell alpha 2-ARs, which largely outnumber beta-ARs in fat cells of certain fat deposits, in human and primate has never been clearly understood. The other AR type which is not linked to lipolysis regulation, the alpha 1-AR, is involved in the control of glycogenolysis and lactate production. Pharmacological approaches using in-situ microdialysis and selective alpha 2- and beta-AR agonists and antagonists have revealed sex- and tissue-specific differences in the adrenergic control of fat cell function and nutritive blood flow in the tissue surrounding the microdialysis probe.

beta 3-Adrenoceptor agonists are potent lipolytic activators in rats, but they are only weak stimulators in human adipocytes, indicating interspecies differences in the adrenergic regulation of lipid mobilization. Like human but not rat adipocytes, guinea pig fat cells were poorly responsive to the beta 3-agonists BRL-37344, CGP-12177, SR-58611, and ICI-215001, acid metabolite of ICI-D7114. In guinea pigs, the beta 1-agonist dobutamine was more lipolytic than the beta 2-agonist procaterol. Anatomic location of fat deposits was without major influence on the beta-adrenergic responsiveness. Weak responses to beta 3-agonists were found whatever the sex or the age (from 2 days to 16 mo) of the animals. Even in the interscapular brown adipose tissue, which is well known in rats for its beta 3-adrenergic responsiveness, a blunted response to BRL-37344 was observed. The alpha 2-adrenergic antilipolytic effect and receptor number were smaller in guinea pig than in human adipocytes, but the beta-adrenergic receptor number was similar in the two species. Thus guinea pig adipocytes resemble human fat cells when their weak beta 3-adrenergic responsiveness is considered.

Five adrenoceptor subtypes are involved in the adrenergic regulation of white and brown fat cell function. The effects on cAMP production and cAMP-related cellular responses are mediated through the control of adenylyl cyclase activity by the stimulatory beta 1-, beta 2-, and beta 3-adrenergic receptors and the inhibitory alpha 2-adrenoceptors. Activation of alpha 1-adrenoceptors stimulates phosphoinositidase C activity leading to inositol 1,4,5-triphosphate and diacylglycerol formation with a consequent mobilization of intracellular Ca2+ stores and protein kinase C activation which trigger cell responsiveness. The balance between the various adrenoceptor subtypes is the point of regulation that determines the final effect of physiological amines on adipocytes in vitro and in vivo. Large species-specific differences exist in brown and white fat cell adrenoceptor distribution and in their relative importance in the control of the fat cell. Functional beta 3-adrenoceptors coexist with beta 1- and beta 2-adrenoceptors in a number of fat cells; they are weakly active in guinea pig, primate, and human fat cells. Physiological hormones and transmitters operate, in fact, through differential recruitment of all these multiple alpha- and beta-adrenoceptors on the basis of their relative affinity for the different subtypes. The affinity of the beta 3-adrenoceptor for catecholamines is less than that of the classical beta 1- and beta 2-adrenoceptors. Conversely, epinephrine and norepinephrine have a higher affinity for the alpha 2-adrenoceptors than for beta 1-, 2-, or 3-adrenoceptors. Antagonistic actions exist between alpha 2- and beta-adrenoceptor-mediated effects in white fat cells while positive cooperation has been revealed between alpha 1- and beta-adrenoceptors in brown fat cells. Homologous down-regulation of beta 1- and beta 2-adrenoceptors is observed after administration of physiological amines and beta-agonists. Conversely, beta 3- and alpha 2-adrenoceptors are much more resistant to agonist-induced desensitization and down-regulation. Heterologous regulation of beta-adrenoceptors was reported with glucocorticoids while sex-steroid hormones were shown to regulate alpha 2-adrenoceptor expression (androgens) and to alter adenylyl cyclase activity (estrogens).

[3H]Idazoxan binding to myometrial membranes was investigated in four groups of ewes under different steroid hormone status: control, estradiol-treated and progesterone plus estradiol-treated ovariectomized ewes and pregnant ewes. [3H]Idazoxan binding to myometrial membrane fractions was saturable, reversible, specific and of high affinity. The affinity did not vary significantly between the four groups of ewes (2.8 less than KD less than 4.7 nM). Maximal binding capacity varied significantly among groups: binding of [3H]idazoxan was lower in control ovariectomized ewes than in either estradiol or progestagen plus estrogen-treated ewes (maximal binding capacity, 73 +/- 11 fmol/mg of protein vs. 108 +/- 16 and 318 +/- 65, respectively). The highest [3H]idazoxan binding was measured in pregnant ewes (maximal binding capacity, 1302 +/- 256 fmol/mg of protein). Based on the saturation studies with accurate nonspecific binding definition (phentolamine vs. epinephrine), and on the relative order of potency for selected adrenergic drugs, it could be stated that the binding sites labeled by [3H]idazoxan in our study exhibited most of the alpha-2 adrenoceptor properties. Nevertheless, these alpha-2 adrenoceptors obviously differed from the standard alpha-2A-subtype based on Ki values obtained with yohimbine and prazosin in competition studies of [3H]idazoxan binding. The increase in the number of alpha-2 adrenoceptors under progesterone domination, and especially during gestation, supported the hypothesis that this adrenoceptor subtype could play a major role in the control of the motility pattern of the ovine pregnant uterus.

3H-clonidine bound to membrane sites of human fat cells, which have the characteristics of alpha 2-adrenoceptors. Specific binding was rapid, reversible and saturable. [3H]Clonidine binding was of high affinity with a KD of 3.9 nM and with a maximal occupancy of 348 fmol/mg protein. The correlation between alpha-adrenergic agonist or antagonist affinities for the membrane [3H]clonidine binding site with their physiological potencies demonstrates the usefulness of the human fat cell as a model for investigating postsynaptic alpha 2-adrenoceptor properties and regulation.

The present review summarizes recent data on fat cell adrenoceptors with the aim of clarifying the role played by catecholamines in the regulation of adipocyte metabolism. Part of the review is focused on the possible interest of animal models for the study of catecholamine-mediated effects in human fat cells. It is now clearly demonstrated that human, hamster, dog and rabbit fat cells possess three basic types of adrenoceptor: the beta 1-, alpha 2- and alpha 1-adrenoceptors identified in biological assays or binding studies with selected radioligands. The rat is an exception in the species commonly studied as catecholamines exert an exclusive lipolytic effect through beta-adrenoceptor stimulation, there are no alpha 2-adrenoceptors in rat white fat cells although an alpha 1-adrenoceptor does exist. In human fat cells, physiological amines are lipolytic or antilipolytic. Binding studies have revealed that alpha 2-adrenoceptors are three to four times more numerous than beta 1-adrenoceptors. Moreover physiological amines, in particular epinephrine, have a higher affinity for alpha 2-sites than for beta 1-sites. Dose-response studies of the effect of epinephrine on adenosine-deaminase or isoproterenol-stimulated fat-cells demonstrate an inhibitory effect of epinephrine on lipolysis promoted by stimulation of alpha 2-adrenoceptors which occurs before the commonly described beta 1-adrenergic effect which promotes stimulation of lipolysis. This aspect and its putative physiological interest is described and discussed. Intraspecific variations in adrenergic responses of adipocytes have been briefly analysed. The appearance and disappearance of alpha 2-adrenoceptors according to the extent of adipose tissue and increment of fat cell size are discussed. Variations of adrenergic responsiveness during fasting, calorie restriction or chronic stimulation of the adipocytes by physiological amines are also discussed.

Lipolytic activity of human isolated fat cells from different fat deposits was studied. The purpose of the present investigations was to determine the epinephrine responsiveness, with regard to alpha- and beta-adrenergic receptor site activity, of omental and subcutaneous adipocytes (abdominal or from the lateral part of the thigh). Adipocytes were obtained from normal subjects or from obese subjects on iso- or hypocaloric diets. The lipolytic effect of epinephrine varied according to the fat deposits, while the beta-lipolytic effect of isoproterenol was more stable (Fig. 1). We explored the possible involvement of adrenergic alpha-receptors, in order to explain these results. The potentiating action of phentolamine on epinephrine-induced lipolysis, and the antilipolytic effect of alpha-agonists on basal or theophylline--induced lipolysis, were found to be a good indication of alpha-adrenergic activity. The alpha-adrenergic antilipolytic effect was most prominent in adipose tissue from the lateral part of the thigh, and less noticeable in omental adipocytes. In conclusion, the inability of epinephrine to induce lipolysis, and the epinephrine-induced inhibition of lipolysis observed when the basal rate of FFA release was spontaneously increased in subcutaneous fat-cells of the thigh, could be explained by an increased alpha adrenergic responsiveness (Fig. 2). Moreover, various alpha-adrenergic agonists (phenylephrine, noradrenaline and adrenaline) showed a clear inhibiting effect on theophylline-stimulated adipocytes from the thigh. The pharmacological study of the antilipolytic effect of epinephrine on theophylline-induced lipolysis showed that the inhibition was linked to a specific stimulation of the alpha-receptors of the subcutaneous adipocytes (Fig. 4). From the different sets of experiments, it is shown that the modifications in the lipolytic effect of epinephrine on adipocytes of different areas could be explained by the occurrence of a variable alpha-adrenergic effect initiated by catecholamine. Furthermore, theophylline stimulation of lipolysis provides an accurate system to investigate the alpha-inhibiting effect of catecholamines. Our study was completed by the investigation of the lipolytic activity of subcutaneous fat cells from obese subjects submitted to a hypocaloric diet (800-1 000 Cal/day). An increased alpha-inhibitory effect of epinephrine was shown on the increased basal lipolytic activity observed in the fat cells of obese subjects on a hypocaloric diet (Fig. 5); a similar effect was observed when these adipocytes were stimulated by theophylline. To conclude, these investigations allow the alpha-adrenergic effect to be considered as a regulator mechanism of the in vitro lipolytic activity in human adipose tissue, since the antilipolytic effect is operative whenever the basal rate of lipolysis is increased (spontaneously, after caloric restriction, or with a lipolytic agent such as theophylline).

Spontaneous and stimulated lipolytic activity of subcutaneous abdominal adipocytes from obese subjects treated by a hypocaloric diet (800-1000 kcal/d) for 13-15 d were studied. A strong increase in basal lipolytic activity of the adipocytes occurred after the restrictive diet treatment. Linked to this phenomenon, an important alpha-adrenergic antilipolytic effect of adrenaline appeared whereas, the beta-stimulating effect of isoproterenol (beta-adrenomimetic drug) was not affected. The increased alpha-inhibitory effect of adrenaline was shown to theophylline-stimulated adipocytes of energy-restricted subjects. It is concluded that after a period of hypocaloric diet, the adrenaline-dependent lipolysis of subcutaneous abdominal fat cells of obese subjects is reversed. The lipolytic response induced by adrenaline, described in the adipocytes of obese controls, develop towards an alpha-adrenergic antilipolytic response after the hypocaloric treatment. Our data suggest that, in states with increased rate of lipolysis, there is an increased readiness for alpha-adrenergic response.

Rabbit white adipose tissue, in vitro, may not respond to epinephrine by an increase in the rate of lipolysis although beta adrenergic receptors may be postulated from our experiments with isoproterenol. When basal rate of lipolysis is high, an antilipolytic effect may be shown for low concentrations of epinephrine (0.01 and 0.1 mug/ml). However, epinephrine and an alpha adrenergic blocker, phentolamine (5 mug/ml), acting together, abolish the antilipolytic effect and better still a strong lipolytic effect of epinephrine is unmasked in such a case. The best evidence of these results is that inhibitory alpha, adrenergic receptors are involved in Rabbit white fat cells. These facts partially explain the absence of lipolytic response with epinephrine previously described.

Adrenergic control of human fat cell lipolysis is mediated by two kinds of receptor sites that are simultaneously stimulated by physiological amines. To establish a correlation between the binding characteristics of the receptor and biological functions, the ability of physiological amines to stimulate or inhibit isolated fat cell lipolysis in vitro was compared to the beta- and alpha 2-adrenoceptor properties of the same fat cell batch. The beta-selective antagonist (-)[3H]dihydroalprenolol ([3H]DHA) and the alpha 2-selective antagonists [3H]yohimbine ([3H]YOH) and [3H]rauwolscine ([3H]RAU) were used to identify and characterize the two receptor sites. Binding of each ligand was rapid, saturable, and specific. The results demonstrate 1) the weaker lipolytic effect of epinephrine compared with norepinephrine. This can be explained by the equipotency of the amines at the beta 1-sites and the higher affinity of epinephrine for alpha 2-adrenergic receptors. 2) The preponderance of alpha 2-adrenergic receptor sites labeled by [3H]YOH (Bmax, 586 +/- 95 fmol/mg protein; KD, 2.7 +/- 0.2 nM) or [3H]RAU (Bmax, 580 +/- 100 fmol/mg protein; KD, 3.7 +/- 0.1 nM). These two ligands can be successfully used to label alpha 2-adrenergic receptor sites. 3) The beta 1-adrenergic receptor population labeled by [3H]DHA(Bmax, 234 +/- 37 fmol/mg protein; KD, 1.8 +/- 0.4 nM), although a third as numerous as the alpha 2-adrenergic population, is responsible for the lipolytic effect of physiological amines and is weakly counteracted by simultaneous alpha 2-adrenergic receptor stimulation under our experimental conditions. It is concluded that, in human fat cells, the characterization of beta 1- and alpha 2-adrenergic receptors by saturation studies or kinetic analysis to determine affinity (KD) and maximal number of binding sites (Bmax) is not sufficient for an accurate characterization of the functional adrenergic receptors involved in the observed biological effect.

1. The small population of [3H]clonidine binding sites in rat fat cell membranes do not have the characteristics of typical alpha 2-adrenoceptors. 2. Clonidine (an alpha 2-adrenoceptor agonist) has no antilipolytic effect on rat fat cells stimulated by theophylline. 3. In contrast to the rat, [3H]clonidine labels an alpha 2-adrenoceptor in hamster fat cell membranes and clonidine exerts a strong antilipolytic effect on theophylline-stimulated lipolysis.

Theophylline-stimulated human subcutaneous adipocytes were incubatged in vitro in the presence of selected alpha-adrenergic agents in order to characterize the alpha-adrenoceptor of human fat cells. Inhibition of theophylline-induced lipolysis occurred with the agents tested; clonidine was the most potent agonist while methoxamine had no effect. The relative order of potency of the various agonists was: clonidine > adrenaline > phenylephrine > methoxamine; this order is consistent with the classification of agonists described for the presynaptic alpha 2-receptor. Moreover, selected antagonists were used in order to antagonise clonidine inhibition of theophylline-induced lipolysis. The order of potency of the alpha-antagonists for the human alpha-adrenoceptor of adipocytes was: yohimbine > piperoxane > phenoxybenzamine > prazosin. This order is consistent with an alpha 2 type receptor. In conclusion, the results demonstrate that the alpha-adrenergic receptor which inhibits lypolysis in human fat cells is of the alpha 2 type. it is noteworthy that although localized postsynaptically this alpha-receptor can be classified as alpha 2 like the commonly known presynaptic alpha-adrenergic receptor which inhibits noradrenaline release from sympathetic neurons.

The main intention of this study was to characterize the alpha-adrenoceptor responsible for the inhibition of lipolysis in dog fat cell and to define circumstances that may be associated to a modification of the alpha-mediated antilipolytic effect. Isolated fat cells from omental and subcutaneous adipose tissue from normal and obese dogs were used. Basal and theophylline stimulated lipolysis was studied in the presence of selected alpha-adrenergic agonists and antagonists. The antilipolytic effect of catecholamines is mediated by alpha 2-type adrenoceptors in dog fat cell. The alpha-adrenergic responsiveness is enhanced (or unmasked) in large fat cells of obese dogs and depends on the site from which the adipose tissue sample is taken. The alpha-response is stronger in subcutaneous than in omental adipocytes. In conclusion, the weakened lipolytic responsiveness to epinephrine of obese dog fat cells seems related to an increased alpha-adrenergic response rather than a decreased beta-lipolytic effect. Obesity is a circumstance characterized in the dog fat cell by a modification of the balance between alpha-2 and beta receptors.

The effects of selected alpha-agonists and alpha-antagonists on theophylline-induces lipolysis were investigate in isolated hamster white fat cells, alpha 2-Agonists (tramazoline, clonidine) inhibited theophylline-induced lipolysis while in alpha 1-agonist (methoxamine) was without any effect. The inhibitory effect of alpha 2-agonists was suppressed by yohimbine (alpha 2-antagonist), whereas alpha 1-antagonists were inefficient. This result implies that the alpha-adrenergic receptor of hamster fat cells is of the alpha 2-type, although postsynaptically.

The relative proportion of antilipolytic alpha-2 and lipolytic beta adrenoceptors and the adrenoceptors was studied in adipocytes from lean and obese dogs. The modification of the adrenergic status in the adipose tissue from obese dogs consists of an increase in alpha-2 adrenoceptor number (identified by [3H]yohimbine) and a decrease in beta adrenoceptor number (identified by [3H]dihydroalprenolol). Neither the number of beta adrenoceptors in the leukocytes nor the number of alpha-2 adrenoceptors in the platelets were altered in obesity. This predominance of alpha-2 adrenoceptors in adipocytes from obese dogs induced a reduction of the lipolytic efficacy of epinephrine (i.e., increase in the concentration able to induce half-maximal stimulation of lipolysis). Moreover, the number of beta adrenoceptors in the high-affinity state was increased in adipose tissue from obese dogs. It is concluded first that the striking modifications in the adrenergic status of the adipose tissue in obesity is specific to this tissue and secondly that the rise of the beta adrenoceptor in the high-affinity state could explain the fact that catecholamines remain lipolytic agents and that weight loss is increased by starvation in the obese dog.

alpha-Adrenergic receptor which inhibit lipolysis in human and hamster fat cells are known to be of the alpha 2-type. By measuring the lipolytic activity on isolated fat cells and by studying the [3H]-yohimbine binding on the fat cell membranes, we tried to determine the alpha-adrenergic potency of the alpha-adrenolytic drug nicergoline. 1. Clonidine inhibits, in a concentration-dependent manner, the theophylline-stimulated lipolysis. This antilipolytic effect is suppressed by various alpha 2-adrenolytic agents but not by alpha 1-adrenolytic agents. Nicergoline is found to partially and competitively inhibit the antilipolytic effect of clonidine. 2. Specific binding of [3H]-yohimbine on fat cell membranes was a rapid and saturable process and was of high affinity (KD = 3.1 +/- 0.3 nM, Bmax = 615 +/- 90 f.mol./mg protein). 3. At low concentration, nicergoline displaced the specific binding of [3H]-yohimbine (EC50 = 0.440 microM). Compared to other alpha-adrenolytic agents, the affinity decreased in the following order: yohimbine greater than or equal to phentolamine greater than piperoxane greater than nicergoline much greater than prazosin. 4. The results show that the alpha-adrenergic antagonists capacity to suppress the antilipolytic effect of clonidine on isolated adipocytes is directly correlated with the ability to inhibit [3H]-yohimbine binding on fat cell membranes. Moreover we conclude that, on this model, nicergoline exhibits alpha 2-adrenolytic properties.

Investigations were carried out to demonstrate the function and the possible advantage of the interplay between beta 1 and alpha 2 adrenoceptor sites in the regulation of human subcutaneous fat-cell lipolysis. alpha 2 and beta adrenoceptor binding studies were conducted with antagonist radioligands and revealed that alpha 2-adrenoceptors ([3H]yohimbine and [3H]rauwolscine binding sites) are more numerous than beta 1-adrenoceptors ([3H]dihydroalprenolol and [3H]CGP-12177 binding sites) in human fat-cell membranes. Physiological agonists epinephrine and norepinephrine competed with [3H]-ligand sites with a higher affinity for alpha 2 sites than for beta 1 sites. Epinephrine exhibited a higher affinity than norepinephrine for the alpha 2 sites; the two amines had the same affinity for beta 1 sites. In lipolysis studies conducted in the absence of adenosine deaminase the beta lipolytic action of the biological amines predominated; after alpha 2-adrenoceptor blockade by yohimbine or idazoxan, the amines exhibited an intrinsic activity similar to that of isoproterenol. When adenosine was prevented from accumulating in the incubation medium by inclusion of adenosine deaminase, low concentrations of epinephrine and norepinephrine preferentially exerted an antilipolytic action. We conclude that: he lipolytic response in abdominal human subcutaneous fat cells to physiological amines results from the interplay between beta 1-and alpha 2-adrenoceptor stimulation; alpha 2 adrenoceptors, with their higher number and higher affinity for the physiological amines, and the adrenoceptor population involved at the lowest (i.e. physiological) concentrations of the amines.(ABSTRACT TRUNCATED AT 250 WORDS)

Guanine nucleotides and sodium were found to reduce the affinity of adrenaline for the alpha 2-adrenoceptor sites labelled by [3H]yohimbine in human fat cell membranes. Surprisingly, the negative regulation by these agents was far weaker when clonidine, the alpha 2-agonist prototype was used. The results are discussed in relation to the weaker or the lack of adenylate cyclase inhibition promoted by clonidine and indicate that the formation of the lower affinity state of the alpha-receptor is essential to the alpha 2-adrenoceptor-mediated adenylate cyclase inhibition.

The response of the hamster adipocyte to various lipolytic (beta-adrenergic) and antilipolytic (alpha(2)-adrenergic and adenosine-dependent) stimuli was studied during the development and after cold-induced regression of fat stores. Alpha(2)-adrenergic binding ([(3)H]clonidine binding sites) was also investigated. Adipocytes came from young animals (4-5 weeks), adults (20-25 weeks), and adults submitted to a 6-week cold exposure (6 degrees C) that promoted a large decrease in fat stores and in fat cell size. The lipolytic response induced by isoproterenol (beta-agonist) was equivalent in the different groups. Adenosine and alpha(2)-adrenergic antilipolytic effects were estimated through the inhibition of theophylline-induced lipolysis by phenylisopropyladenosine and clonidine, respectively. The adenosine effect was unchanged in all the groups. In contrast, the alpha(2)-adrenergic effect, which was not present in young hamsters, increased simultaneously with fat cell size, was fully effective in adult hamsters, and had completely disappeared in small adipocytes from cold-exposed hamsters. In fat cell ghosts, alpha(2)-adrenoceptors ([(3)H]clonidine binding sites), followed similar modifications: they increased with fat cell enlargement and disappeared after cell size reduction following cold exposure. These results suggest that: 1) the increased alpha(2)-adrenergic antilipolytic response which is concomitant with fat cell enlargement could partly explain the growth-related decrease in the previously reported lipolytic effect of epinephrine; 2) the alpha(2)-receptivity of the adipocyte seems to be strictly fat cell size-dependent while the beta-adrenergic and adenosine responses are unaffected; and 3) the regulation in the adipocytes of the adenosine, alpha(2)- and beta-receptors seems to be unrelated.-Carpene, C., M. Berlan, and M. Lafontan. Influence of development and reduction of fat stores on the antilipolytic alpha(2)-adrenoceptor in hamster adipocytes: comparison with adenosine and beta-adrenergic lipolytic responses.