Influence of development and reduction of fat stores on the antilipolytic a,-adrenoceptor in hamster adipocytes: comparison with adenosine and ,&adrenergic lipolytic responses'

The response of the hamster adipocyte to various lipolytic (@-adrenergic) and antilipolytic (an-adrenergic and adenosine-dependent) stimuli was studied during the development and after cold-induced regression of fat stores. Alphanadrenergic binding (['Hlclonidine 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 (6OC) that promoted a large decrease in fat stores and in fat cell size. The lipolytic response induced by isoproterenol (@-agonist) was equivalent in the different groups. Adenosine and a2-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 an-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, an-adrenoceptors (['H]clonidine binding sites), followed similar modifications: they increased with fat cell enlargement and disappeared after cell size reduction following cold exp0sure.l These results suggest that: I) the increased a2-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 cu2-receptivity of the adipocyte seems to be strictly fat cell size-dependent while the 0-adrenergic and adenosine responses are unaffected; and 3) the regulation in the adipocytes of the adenosine, cyzand @-receptors seems to be unrelated.-Carpene, C., M. Berlan, and M. Lafontan. Influence of development and reduction of fat stores on the antilipolytic a'-adrenoceptor in hamster adipocytes: comparison with adenosine and @-adrenergic lipolytic responses. J . Lipid Res. 1983. 24: 766-774. Supplementary key words fat cell size ap-receptor sites @-receptor sites adenosine receptor sites ['H]clonidine isoproterenol phenylisopropyladenosine clonidine A great number of studies have shown that both aand @-adrenoceptors are present in adipocyte membranes of various species (1-6). The stimulation of lipolysis is mediated by the 0-adrenoceptor while the a*-adrenoceptor subtype is involved in the inhibition of lipolysis. So, it is now well accepted that the lipolytic differences in the responsiveness of the adipose tissue between various species are mainly dependent on the balance between aand 8-adrenergic receptors (6). Most of the studies on the effect of aging on hormone responsiveness and sensitivity of adipose tissue have been carried out in the rat. Concerning the adrenergic agents, the most striking result is that aging is associated with a decrease in the lipolytic response to catecholamines (715) and that this effect is related to a decrease in P-adrenoceptor number (1 4) and adenylate cyclase activity (16, 17). Although the rat has been the most commonly used species for investigations into the agedependent effect of catecholamines on adipose tissue, its adipocytes lack the a*-adrenergic receptors controlling lipolytic processes (1 8, 19). Little is known concerning changes of the a*-antilipolytic adrenoceptor during aging. We have shown that in rabbit (20) and in dog adipocytes (21), the absence (22) or the loss (23) of the lipolytic response to epinephrine in the aging animal is linked to the involvement of an increased a-adrenergic response. Moreover, Pecquery and Giudicelli (24) reported that the a-adrenergic responsiveness increases with age in hamster adipocytes concomitantly with [3H]dihydroergocryptine Abbreviations: KRBA, Krebs-Ringer bicarbonate buffer containing albumin; PIA, phenylisopropyladenosine. ' A preliminary report of a portion of these data has been published in abstract form at the International Conference on the Adipocyte and Obesity: Cellular and Molecular Mechanisms, held in Toronto, June 1982. ' C. Carpene and M. Lafontan. '' M. Berlan. 766 Journal of Lipid Research Volume 24, 1983 at P E N N S T A T E U N IV E R S IT Y , on F ebuary 3, 2013 w w w .j.org D ow nladed fom binding site number. In the hamster, as in the dog and the rabbit, aging is associated with an extension of the adipose tissue as demonstrated by the development of large fat pads with bigger adipocytes. It is of interest to distinguish the relative importance of aging and obesity in the appearance and the increment of the a-adrenergic antilipolytic effect in fat cells. In the present report, a description of the antilipolytic a-adrenergic response in adipocytes of young and mature hamsters is associated with a study of a2-adrenergic receptor binding sites. Moreover, w e also present evidence suggesting that the main factor involved in the genesis of a2-adrenergic responsiveness is the increase of the fat cell size rather than aging as previously supposed. In this study we also compare the antilipolytic effect initiated by cy2-adrenoceptor stimulation to that of phenylisopropyladenosine, a well known antilipolytic agent acting on inhibitory adenosine receptors of the fat cell membrane (25). MATERIALS AND METHODS