Lack of functional antilipolytic alpha 2-adrenoceptor in rat fat cell:comparison with hamster adipocyte

Activation of the sympathetic nervous system suppresses mouse white adipose tissue hyperplasia through the β1 adrenergic receptor

Adipose tissue (AT) expands via both hypertrophy and hyperplasia during the development of obesity. While AT hypertrophy involves the increase in size of existing adipocytes, hyperplasia is the process of creating new adipocytes from the pool of adipocyte precursor cells (APCs), which includes adipocyte progenitor cells and preadipocytes. Prior studies have implicated a role of the sympathetic nervous system (SNS) in regulation of hyperplasia in white adipose tissue (WAT). Here, we aimed to determine the mechanisms underlying SNS regulation of APC proliferation in mouse WAT. Using flow cytometry with antibodies against various cell surface markers, along with an intracellular marker of proliferation (Ki67), we quantitated the percentages and proliferative status of adipocyte progenitor cells and preadipocytes in the stromal vascular fraction (SVF) of WAT. In vivo SNS activation through cold exposure, as well as in vitro adrenergic stimulation via exposure to the canonical SNS neurotransmitter norepinephrine (NE), inhibited preadipocyte proliferation. Pretreatment with propranolol, a β1‐ and β2‐adrenergic receptor (AR) antagonist, trended toward rescuing the inhibitory effects of NE in primary cell culture. The selective β1‐AR agonist dobutamine diminished preadipocyte proliferation both in vivo and in vitro, whereas the selective β2‐AR agonist, salbutamol, promoted proliferation in vitro, suggesting that the β1‐AR may mediate the inhibitory effect of NE on preadipocyte proliferation. Taken together, we conclude that SNS activation suppresses preadipocyte proliferation via activation of the β1 AR in WAT.

Mechanisms of the antilipolytic response of human adipocytes to tyramine, a trace amine present in food

Tyramine is found in foodstuffs, the richest being cheeses, sausages, and wines. Tyramine has been recognized to release catecholamines from nerve endings and to trigger hypertensive reaction. Thereby, tyramine-free diet is recommended for depressed patients treated with irreversible inhibitors of monoamine oxidases (MAO) to limit the risk of hypertension. Tyramine is a substrate of amine oxidases and also an agonist at trace amine-associated receptors. Our aim was to characterize the dose-dependent effects of tyramine on human adipocyte metabolic functions. Lipolytic activity was determined in adipocytes from human subcutaneous abdominal adipose tissue. Glycerol release was increased by a fourfold factor with classical lipolytic agents (1 μM isoprenaline, 1 mM isobutylmethylxanthine) while the amine was ineffective from 0.01 to 100 μM and hardly stimulatory at 1 mM. Tyramine exhibited a partial antilipolytic effect at 100 μM and 1 mM, which was similar to that of insulin but weaker than that obtained with agonists at purinergic A1 receptors, α₂-adrenoceptors, or nicotinic acid receptors. Gi-protein blockade by Pertussis toxin abolished all these antilipolytic responses save that of tyramine. Indeed, tyramine antilipolytic effect was impaired by MAO-A inhibition. Tyramine inhibited protein tyrosine phosphatase activities in a manner sensitive to ascorbic acid and amine oxidase inhibitors. Thus, millimolar tyramine restrained lipolysis via the hydrogen peroxide it generates when oxidized by MAO. Since tyramine plasma levels have been reported to reach 0.2 μM after ingestion of 200 mg tyramine in healthy individuals, the direct effects we observed in vitro on adipocytes could be nutritionally relevant only when the MAO-dependent hepato-intestinal detoxifying system is overpassed.

Serum free fatty acids are not involved in acute exercise-induced reduction of LPL in rat tissues

The present studies were designed to verify whether preventing the rise in serum levels of nonesterified fatty acids (NEFA) by adrenergic blockade would interfere with the decrease in tissue lipoprotein lipase (LPL) activity caused by acute exercise in rats. Ninety minutes before being killed, male rats were injected intraperitoneally with either saline, the beta-adrenergic blocker propranolol (25 mg/kg body weight), or the alpha 2-adrenergic blocker yohimbine (3 mg/kg body weight). Half of each group was killed at rest and the other half immediately after a 1-h run on a treadmill. LPL was determined in white adipose tissue (WAT), heart, and red vastus lateralis muscle (VLM). Exercise enhanced serum levels of NEFA 50% over resting values in saline-injected rats. The latter increase was totally abolished in animals having received propranolol or yohimbine. The activity of LPL in WAT, heart, and red VLM was approximately 35% lower in exercised rats than in resting animals. Serum triacylglycerols were also reduced by the run. Neither propranolol nor yohimbine interfered with any of these reductions. Exercise did not change serum glucose levels in saline-injected rats but decreased it in those injected with propranolol or yohimbine. Serum insulin was unchanged by exercise and by the antagonists. These findings suggest that the beta- and alpha 2-adrenergic pathways, as well as the exercise-induced rise in serum levels of NEFA, are not responsible for the early reducing effect of a 1-h run on tissue LPL activity in untrained rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective reduction of alpha 2-adrenergic responsiveness in hamster adipose tissue during prolonged starvation

The influence of fasting on the dual adrenergic control of adipose tissue lipolysis was investigated in hamsters because in this species the adipocytes exhibit both beta-stimulatory and alpha 2-inhibitory adrenergic responses. In adipocytes from fed animals, the number of alpha 2-receptors (identified with [3H]clonidine and [3H]RX 821002) was greater than that of beta-receptors. As in humans, the alpha 2-adrenoceptor number was greater in adipocyte membranes from subcutaneous (inguinal and popliteal) than from internal (perirenal and epididymal) adipose tissues. Despite this difference in alpha 2-adrenoceptor number, the antilipolytic responses to the alpha 2-agonists clonidine and UK 14304 were similar in the two tissues. Food deprivation for a period of 1-6 days induced a net depletion of both adipose tissues. In 6-day starved animals the number of adipocyte alpha 2-adrenoceptors and the maximal antilipolytic effect of UK 14304 were less than 50% of those in fed controls. In contrast, the antilipolytic responses to phenylisopropyladenosine or prostaglandin E1 remained unchanged. Starvation induced a decrease in alpha 2-adrenoceptor number and an increase in beta-adrenergic sensitivity that were greater in adipocytes from subcutaneous than from internal fad pads. The data suggest that the adipocyte beta- and alpha 2-adrenoceptors are independently regulated during starvation. In the adipocyte, the alpha 2-antilipolytic responses and the alpha 2-adrenoceptor levels are dependent on the extent of the adipose mass; they are particularly reduced in emaciated hamsters.

Evidence of functional alpha 2-adrenergic receptors in adult-rat adipocytes by using the agonist UK 14304

The aim of this study was to re-assess whether alpha 2-adrenergic receptors were present in rat adipocytes, by using UK 14304, a new and very selective alpha 2-agonist. The following observations demonstrate the presence of functional alpha 2-adrenoceptors in rat adipocytes. (1) Adipocyte lipolysis was dose-dependently inhibited by UK 14304 (maximal effect 80% at 1 microM-UK 14304, 45% at 10 microM-UK 14304, under basal or theophylline-stimulated conditions respectively). (2) UK 14304 bound specifically to purified plasma membranes, with Bmax. = 744 fmol/mg of protein and KD = 9 nM. (3) The effect of UK 14304 was suppressed by alpha 2-antagonists. (4) Adrenaline inhibited lipolysis upon beta-adrenergic blockade (propranolol). (5) The anti-lipolytic effect of UK 14304 was modulated by the age of the rats.

Role of alpha 1- and alpha 2-adrenoceptors in catecholamine-induced hyperglycaemia, lipolysis and insulin secretion in conscious fasted rabbits

1. In conscious fasted rabbits an intravenous infusion of clonidine (2 micrograms kg-1 min-1) induced hyperglycaemia. The increase in blood glucose was accompanied by an inhibition of insulin secretion and basal lipolysis. 2. Yohimbine infused at a rate of 20 micrograms kg-1 min-1 suppressed clonidine-induced hyperglycaemia and blocked the inhibitory effect on insulin secretion mediated by the alpha 2-adrenoceptor agonist. 3. The intravenous infusion of amidephrine (10 micrograms kg-1 min-1) induced an increase in insulin secretion in the absence of patent hyperglycaemia. Prazosin, 0.3 mg kg-1 s.c. selectively antagonized the effect of amidephrine on insulin secretion. 4. Isoprenaline infusion (4.4 micrograms kg-1 min-1) evoked a significant increase in blood glycerol and immunoreactive insulin plasma levels. Both responses were clearly attenuated when alpha 2-adrenoceptors were simultaneously stimulated by selective (clonidine) and less selective (phenylephrine, 20 micrograms kg-1 min-1) agonists. 5. Amidephrine infusion did not induce appreciable changes in blood glycerol nor did it modify, isoprenaline-induced lipolytic response. 6. Simultaneous infusion of isoprenaline and amidephrine induced a remarkable increase in insulin secretion. 7. It is concluded that in normal fasted rabbits stimulation of alpha 2-adrenoceptors depresses basal and beta-adrenoceptor mediated lipolysis and insulin secretion. On the other hand, selective stimulation of alpha 1-adrenoceptors does not affect lipolysis but induces insulin release. Simultaneous stimulation of alpha 1- and beta-adrenoceptors potentiates the insulin secretory response.

Modulation of glucose transport in hamster adipocytes by insulin and by beta- and alpha 2-adrenoceptor agonists

Glucose transport in hamster adipocytes and its modulation by insulin and isoprenaline was characterized with the aid of the non-metabolizable hexose 3-0-methylglucose. Insulin stimulated the initial uptake rates by an increase in Vmax of the transport without any detectable change in Km. The hormone concentration producing half maximal stimulation was identical to that required in rat adipocytes. However, hamster adipocytes were much less responsive to insulin (3-fold stimulation as compared to a 12-fold stimulation in rat fat cells), and maximal transport rates were 10-fold lower than that observed in rat adipocytes. Accordingly, the number of glucose transporters, as assessed by glucose-inhibitable cytochalasin-B binding, was considerably lower in plasma membranes of hamster adipocytes. Moreover, no transporters were detected in the low-density microsomes which in insulin-sensitive cell types represent the intracellular pool of recruitable glucose transporters. The relative insulin resistance of the hamster fat cells may therefore be due to a depleted pool of intracellular glucose transporters. In the presence of adenosine, the beta-adrenoceptor agonist isoprenaline produced a moderate stimulation of the basal transport rate which was antagonized by the alpha 2-agonist clonidine. If adenosine deaminase was added in order to remove endogenous adenosine, isoprenaline inhibited the insulin-stimulated transport by 50%. In contrast to the stimulatory effects of insulin and isoproterenol, the inhibitory effect of the catecholamine was reversed by cooling the cells to 22 degrees. Glucagon produced a comparable inhibition, suggesting that the inhibitory effect was mediated by adenylate cyclase or its regulatory subunits.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo evidence for alpha 2-adrenergic-dependent inhibition of lipid mobilization in the rabbit

Both alpha- and beta-agonists were infused i.v. in unanesthetized rabbits. The alpha-adrenergic effect on lipid mobilization was investigated by comparing the modifications of blood glycerol levels induced by clonidine (alpha 2-agonist) and amidephrine (alpha 1-agonist) alone or in combination with isoprenaline (beta-agonist). Clonidine (2 micrograms X kg-1 X min-1) was found to significantly reduce the basal or isoprenaline-stimulated glycerol levels whereas amidephrine (10 micrograms X kg-1 X min-1) was without any effect. Plasma IRI levels were also reduced by clonidine. The results suggest that the reduction of lipid mobilization produced by clonidine could have been the consequence of the stimulation of the alpha 2-adrenergic antilipolytic receptors previously shown to be present in isolated fat cell membranes.