Alpha1-adrenergic activation of phosphatidylinositol labeling in isolated brown fat cells

The Kulinkovich Reaction in the Synthesis of Constrained N,N-Dialkyl Neurotransmitter Analogues

An intermolecular Ti(IV)-mediated cyclopropanation reaction has been used to synthesize substituted 2-phenylcyclopropylamines and constrained analogues of the neurotransmitters histamine and tryptamine. Many hydroxy- and methoxy-substituted phenylcyclopropylamines are known to inhibit monoamine oxidase and have been shown to mimic hallucinogens. These compounds were made in 1 to 5 steps from readily available starting materials.

Alpha-adrenergic receptor-mediated thermogenesis in brown adipose tissue of rat

1. The present studies were undertaken to characterize the thermogenic response to the alpha 1-adrenergic agonist phenylephrine, to compare this response with the beta-adrenergic response and to assess the role of brown adipose tissue (BAT). 2. Phenylephrine and the beta 3-adrenergic agonist CGP 12177A each caused similar increases in O2 consumption. No synergism was observed when combining the two drugs. Phenylephrine stimulated O2 consumption via an alpha-adrenergic mechanism, as indicated by effective blockade with phenoxybenzamine, but not propranolol. 3. Further evidence for the alpha-adrenergic mechanism of phenylephrine action was seen in studies with BAT membranes. Phenylephrine did not stimulate adenylyl cyclase and did not potentiate beta-adrenergic stimulation of adenylyl cyclase. 4. Skeletal muscle was not a major site of phenylephrine-stimulated O2 consumption, as the response was not inhibited by a concentration of dantrolene which inhibited cold-induced muscle shivering. 5. Phenylephrine caused an increase in the density of available 3H-GDP binding sites in BAT mitochondria, indicating an activation of BAT thermogenesis in vivo. This increase was equal in magnitude to what we have reported previously for CGP-12177A. No changes were observed in the affinity for 3H-GDP. 6. We concluded that phenylephrine stimulates O2 consumption by an alpha-adrenergic mechanism that involves activation of BAT thermogenesis. It remains to be determined whether the activation of BAT occurs by a direct or indirect mechanism.

Brown fat thermogenesis and exercise: two examples of physiological oxidative stress?

Both brown fat tissue (BAT) and skeletal muscle experience large increases of oxygen consumption and oxygen radical generation during activation. This, together with the relatively low activities of antioxidant enzymes in these two tissues and the high lipid content and free fatty acid liberation of BAT, can produce a physiological oxidative stress. Increases of in vivo or in vitro (BAT) lipid peroxidation have been described in these tissues after activation. They react to this oxidative stress in an adaptive way after chronic stimulation. Cold acclimation increases antioxidant enzymes, ascorbate, and especially reduced glutathione (GSH) in BAT. There is controversy about the variations of antioxidants in skeletal muscle after acute exercise. Nevertheless, exercise training seems to increase muscle antioxidant enzymes and GSH. Many reports show that vitamin E levels decrease in the muscle and increase in plasma during exercise. Studies of vitamin E deficiency and supplementation strongly suggest that this vitamin is of protective value during exercise.

Membrane responses to norepinephrine in cultured brown fat cells

We used the "perforated-patch" technique (Horn, R., and A. Marty, 1988. Journal of General Physiology. 92:145-159) to examine the effects of adrenergic agonists on the membrane potentials and membrane currents in isolated cultured brown fat cells from neonatal rats. In contrast to our previous results using traditional whole-cell patch clamp, 1-23-d cultured brown fat cells clamped with the perforated patch consistently showed vigorous membrane responses to both alpha- and beta-adrenergic agonists, suggesting that cytoplasmic components essential for the thermogenic response are lost in whole-cell experiments. The membrane responses to adrenergic stimulation varied from cell to cell but were consistent for a given cell. Responses to bath-applied norepinephrine in voltage-clamped cells had three possible components: (a) a fast transient inward current, (b) a slower outward current carried by K+ that often oscillated in amplitude, and (c) a sustained inward current largely by Na+. The fast inward and outward currents were activated by alpha-adrenergic agonists while the slow inward current was mediated by beta-adrenergic agonists. Oscillating outward currents were the most frequently seen response to norepinephrine stimulation. Activation of this current, termed IK,NE, was independent of voltage and seemed to be carried by Ca2(+)-activated K channels since the current oscillated in amplitude at constant membrane potential and gradually decreased when the cells were bathed with calcium-free external solution. IK,NE had a novel pharmacology in that it could be blocked by 4-aminopyridine, tetraethylammonium, apamin, and charybdotoxin. Both IK,NE and the voltage-gated K channels also present in brown fat (Lucero, M. T., and P. A. Pappone, 1989a. Journal of General Physiology. 93:451-472) may play a role in maintaining cellular homeostasis in the face of the high metabolic activity involved in thermogenesis.

Phorbol esters, protein kinase C, and thyroxine 5'-deiodinase in brown adipocytes

Protein kinase C activity has been identified in the rat brown adipocyte. About 60% of this activity is found in the cytosolic fraction under basal conditions, and 12-O-tetradecanoylphorbol 13-acetate (TPA) causes a rapid shift from the cytosol to the particulate fraction. Norepinephrine and phenylephrine cause a similar redistribution that can be blocked by prazosin but not by alprenolol. alpha 1-Adrenergic agonists cause three- to fivefold stimulation of type 2 iodothyronine 5'-deiodinase activity in brown adipocytes. TPA has no effect on basal deiodinase activity and reduces the response of the enzyme to alpha 1-adrenergic agonists. These results suggest that the translocation of protein kinase C from cytosol to particulate fraction is not sufficient to increase deiodinase activity but can modulate the alpha 1-adrenergic agonist-mediated responses in these cells.

Alpha 1-adrenergic inositol trisphosphate production in brown adipocytes is Na+ dependent

In order to investigate the ionic requirements for inositol trisphosphate production, brown adipocytes were prelabelled with myo-[3H]inositol and the formation of inositol trisphosphates and inositol bisphosphates as a consequence of alpha 1-adrenergic stimulation was monitored. Omission of Ca2+ from the incubation medium diminished the norepinephrine-induced increase in inositol trisphosphate levels, but it would seem that this reduction can be fully accounted for by a decreased level of the 'inactive' isomer inositol 1,3,4-trisphosphate. Omission of Na+ fully abolished the norepinephrine-induced inositol trisphosphate response. However, it was observed that the presence of Li+ in the incubation medium could fully reconstitute the ability of the cells to yield the early response of inositol trisphosphate production; Li+ could, however, not substitute for Na+ in the entire alpha 1-adrenergic cellular pathway. It was concluded that the Na+-dependent step is found in the coupling mechanism between the alpha 1-receptor and the activation of the phosphodiesterase responsible for inositol trisphosphate production. Thus, all events in the alpha 1-adrenergic pathway which are consequences of IP3 production should appear to be Na+-dependent in these cells.

Distinction between mechanisms underlying alpha 1- and beta-adrenergic respiratory stimulation in brown fat cells

Experimental conditions are described for selective alpha 1- and beta-adrenergic stimulation of the respiration of brown fat cells. The dual agonist norepinephrine was unsuitable as a selective alpha 1-agonist, since unacceptably high concentrations of propranolol were needed to abolish the beta-response. Phenylephrine at 50 microM, in the presence of 5 microM dl-propranolol, was shown to lead to a maximal, selective alpha 1-stimulation, whereas maximal, selective beta-stimulation was achieved with 1 microM isoproterenol in the presence of 5 microM prazosin. The mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) was able to further increase respiration that was already maximally alpha 1-stimulated, but when added before the alpha 1-stimulation, FCCP totally abolished the response. In contrast, FCCP had no effect on the beta-stimulated response. Similarly, oligomycin (an inhibitor of mitochondrial ATP synthesis) inhibited alpha 1-respiration but had a much smaller effect on beta-respiration. Ouabain (an inhibitor of the Na+-K+-ATPase) halved alpha 1-respiration but only induced a small inhibition of beta-respiration. It is concluded that only a small fraction of thermogenesis from beta-adrenergic processes is due to oxidative phosphorylation, whereas alpha 1-respiration is largely due to the oxygen cost of mitochondrial ATP synthesis, and a large fraction of this ATP is apparently used for the restoration of ion gradients.

Induction of functional uncoupling protein in guinea pigs infused with noradrenaline. Studies with isolated brown adipocytes

Continuous infusion of noradrenaline over the interscapular brown fat of guinea pigs maintained at thermoneutrality (28-32 degrees C) induces changes similar to those after cold-adaptation. (1) Multilocular fat droplets appear within the brown adipocytes. (2) The number of mitochondria per adipocyte and the total number of adipocytes both increase. (3) Noradrenaline addition to isolated adipocytes causes near maximal uncontrolled respiration. (4) The cells become more sensitive to fatty acid-induced uncoupling. (5) The tissue-specific uncoupling protein per mg of mitochondrial protein is increased 5-fold. Specific alpha- and beta-agonists were also chronically infused. (6) Separate infusion of phenylephrine or isoprenaline was not able to stimulate mitochondriogenesis or hyperplasia. (7) Adipocytes from these animals could not be uncoupled by acute noradrenaline. (8) Simultaneous chronic infusion of phenylephrine and isoprenaline reproduced the effects of chronic noradrenaline infusion.

Stimulation of alpha 1-adrenoceptors in rat kidney mediates increased inositol phospholipid hydrolysis

The molecular events which follow activation of alpha 1-adrenoceptors in rat kidney were investigated by measuring inositol phospholipid hydrolysis. Slices were labelled with [3H]-inositol (0.25 microM) and the accumulation of [3H]-inositol phosphates ([3H]-IP's) was measured after stimulation with alpha-adrenoceptor agonists. Phospholipid labelling was both time- and Ca2+-dependent. In kidney, Ca2+ (1 mM) increased the incorporation of [3H]-inositol by 49% and in cerebral cortex reduced it by 46%. Following addition of noradrenaline (NA, 1 mM), accumulation of [3H]-IP's increased linearly for at least 60 min. In Ca2+-free buffers a 2.1 fold increase in [3H]-IP accumulation was observed and further increases in stimulated and control levels were produced in the presence of Ca2+ (2.5 mM). These responses were attenuated by the inclusion of indomethacin (10 microM) and abolished in the presence of EGTA (0.5 mM). Responses to (-)-NA were more than 4 fold higher in the renal cortex than in the medulla. Separation of the IP's which accumulate after alpha-adrenoceptor agonists showed that after 60 min stimulation the major products were glycerophosphoinositol and inositol-phosphate with smaller amounts of inositol-bisphosphate and inositol-trisphosphate. The most effective agonists tested for stimulation of accumulation of [3H]-IP's were (-)-NA greater than phenylephrine greater than methoxamine, (+)-NA. Clonidine and (-)-isoprenaline were ineffective at concentrations up to 100 microM. The order of effectiveness of alpha-adrenoceptor antagonists was prazosin greater than BE2254 greater than phentolamine greater than idazoxan greater than rauwolscine. The results indicate that alpha 1-adrenoceptors in rat kidney are linked to phosphoinositide hydrolysis and that this response is localized mainly to the renal cortex.

Requirement of gene transcription and protein synthesis for cold- and norepinephrine-induced stimulation of thyroxine deiodinase in rat brown adipose tissue

The increase in propylthiouracil-insensitive 'type II' thyroxine 5'-deiodinase activity of brown adipose tissue was investigated in rats exposed to acute cold stress or single-dose norepinephrine injection. The 20-fold cold-induced increase in enzyme activity showed a 2-h lag phase and reached a maximum after only 8 h; reacclimation occurred with a 2-h time lag and a half-life of 2.2 h. 4 h after a single norepinephrine injection, the deiodinase activity was almost identical to that after a 4-h cold stress; norepinephrine could not potentiate the effect of the cold stress. Treatment with the protein synthesis inhibitor cycloheximide before exposure to cold or before norepinephrine injection totally blocked the increase in deiodinase activity, suggesting that the increase is due to de novo protein synthesis. The half-life of the enzyme in vivo was estimated to be 0.7 h. Treatment with the RNA synthesis inhibitor actinomycin D totally abolished the cold- and norepinephrine-induced increases, indicating that the increase requires mRNA synthesis. It was concluded that the dramatic cold-induced increase in thyroxine deiodinase activity in brown adipose tissue was not due to activation of preexisting enzyme but was fully due to a norepinephrine-induced increase in expression of the gene and subsequent synthesis of the protein.

Stimulation of phosphoinositide metabolism in hamster brown adipocytes exposed to alpha 1-adrenergic agents and its inhibition with phorbol esters

The present experiments were undertaken to investigate the role of the phosphoinositides phosphatidylinositol 4-phosphate (PtdIns-4-P) and phosphatidylinositol 4,5-biphosphate (PtdIns-4,5-P2) in the alpha 1-adrenergic stimulation of respiration in isolated hamster brown adipocytes. Exposure of isolated brown adipocytes to the alpha-adrenergic-receptor agonist phenylephrine provoked a breakdown of 30-50% of the PtdIns-4-P and PtdIns-4,5-P2 after prelabelling of the cells with [32P]Pi. Coincident with the breakdown of phosphoinositides was an accumulation of labelled phosphatidic acid, which continued for the duration of the cell incubation. The time course of phosphoinositide breakdown was defined more precisely by pulse-chase experiments. Under these conditions, phenylephrine caused radioactivity in phosphatidylinositol, PtdIns-4-P and PtdIns-4,5-P2 to fall by more than 50% within 30 s and to remain at the depressed value for the duration of the incubation (10 min). This phospholipid response to alpha-adrenergic stimulation was blocked by exposure of the cells to phorbol 12-myristate 13-acetate (PMA); likewise phenylephrine stimulation of respiration was prevented by PMA. beta-Adrenergic stimulation of respiration and inhibition of respiration by 2-chloroadenosine and insulin were, however, unaffected by treatment with PMA. On the assumption that PMA is acting in these cells as an activator of protein kinase C, these results suggest the selective interruption of alpha-adrenergic actions in brown adipocytes by activated protein kinase C. These findings suggest that breakdown of phosphoinositides is an early event in alpha-adrenergic stimulation of brown adipocytes which may be important for the subsequent stimulation of respiration. The results from the pulse-chase studies also suggest, however, that phenylephrine-stimulated breakdown of inositol phospholipids is a short-lived event which does not appear to persist for the entire period of exposure to the alpha 1-adrenergic ligand.

Alpha 1-adrenergic activation of brown adipocytes leads to an increased formation of inositol polyphosphates

alpha 1-Adrenergic activation of isolated brown adipocytes causes a rapid mobilization of intracellular Ca2+. The cells also respond with an increased turnover of inositol lipids. The present work demonstrates that alpha 1-adrenergic stimulation of brown adipocytes results in phospholipase C-mediated breakdown of phosphatidylinositol bisphosphate to form inositol trisphosphate. The rate of appearance of inositol trisphosphate is sufficiently rapid for it to mediate or contribute to Ca2+ mobilization in these cells.

Presence of a Ca2+-dependent K+ channel in brown adipocytes. Possible role in maintenance of alpha 1-adrenergic stimulation

We have previously demonstrated mobilization of Ca2+ in and efflux of Rb+ (K+) from isolated hamster brown adipocytes as a consequence of norepinephrine stimulation. We have now investigated the adrenoceptor subtype specificity of these responses and found them both to be of the alpha 1-subtype. Further, we have found that the Rb+ (K+) efflux was dependent upon a primary Ca2+ mobilization event in response to the alpha 1-adrenergic stimulation, since the Rb+ efflux could also be demonstrated by the addition of the Ca2+ ionophore A23187 to the cells. The norepinephrine- and A23187-stimulated Rb+ effluxes were both inhibited by the Ca2+-dependent K+-channel blocker apamin. Apamin also significantly attenuated Ca2+ mobilization in cells in response to a submaximal concentration of norepinephrine. We conclude that alpha 1-adrenergic stimulation of brown fat cells leads to a mobilization of intracellular Ca2+ which, in itself or via other mechanisms, leads to an increase in cytosolic Ca2+ concentration which, in turn, activates a Ca2+-dependent K+ channel, leading to a K+ release from these cells. A possible role for this channel to sustain and augment the response to alpha 1-adrenergic stimulation is discussed.

Norepinephrine stimulation of phosphoinositide hydrolysis in rat cerebral cortex is associated with the alpha1-adrenoceptor

Norepinephrine (NE) and the selective alpha1-adrenoceptor agonist phenylephrine (PE) both markedly stimulate the formation of [3H]inositol phosphates in a concentration-dependent manner upon incubation with [3H]myo-inositol. The selective alpha2 agonist, clonidine, did not significantly alter [3H]inositol phosphate formation, even at concentrations as high as 10(-3) M. The alpha1 antagonist prazosin (IC50, 0.036 microM) was 300 times more potent than the alpha2 antagonist yohimbine (IC50, 10.7 microM) as an inhibitor of NE (10(-4) M)-stimulated phosphatidylinositol (PI) hydrolysis. These results indicate that the alpha1-, but not the alpha2-adrenoceptor subtype in rat brain is coupled to phosphoinositide hydrolysis.

Increased alpha 1-adrenergic receptor density in brown adipose tissue of cold-acclimated rats and hamsters

Binding sites for [3H]prazosin were characterized in crude membrane fractions from rat brown adipose tissue. Based on agonist (norepinephrine approximately equal to phenylephrine much greater than isoprenaline) and antagonist (prazosin much greater than yohimbine greater than propranolol) potencies to compete with [3H]prazosin, the binding sites were identified as alpha 1-receptors, not previously described in rat brown adipose tissue. As the [3H]prazosin binding sites could be observed in isolated brown fat cell preparations, they were probably postsynaptic. The effect of cold acclimation was studied in crude membrane fractions from control and cold-acclimated (4 degrees C) rats and hamsters. Cold acclimation did not change the affinity of the receptor for agonists and antagonists, but there was a significant increase in the number of alpha 1-receptors (per mg protein), both in rat (100% increase) and hamster (40% increase) brown fat. Based on these results and on earlier results on beta-receptors from this and other laboratories, it is suggested that activation of brown adipose tissue is associated with an increase in the relative density of alpha 1-receptors (i.e. in the alpha 1/beta ratio) and an increased significance of alpha 1-adrenergic pathways for the function of the tissue.

Increased alpha 1-adrenergic receptor density in brown adipose tissue of cafeteria-fed rats

A possible general corollary between alpha 1-receptor density in brown adipose tissue and the degree of activation of the tissue was investigated. For this purpose, the effect of cafeteria feeding on alpha 1-adrenergic receptors in brown adipose tissue of seven-week-old female rats was studied by the use of the alpha 1-antagonist (3H)-prazosin. In cafeteria-fed rats, the KD of the alpha 1-receptor for (3H)prazosin was unchanged (about 0.35 nM), but the receptor density was doubled (up to 40 fmol per mg of membrane protein). This was also observed when the results were expressed per unit of a plasma-membrane marker (5'-nucleotidase). It was concluded that an increased alpha 1-receptor density is seen not only in cold-acclimated rats, but also in other conditions where brown fat is activated, and a possible general physiological significance of alpha 1-adrenergic pathways in brown adipose tissue is discussed.

Na+-dependent, alpha-adrenergic mobilization of intracellular (mitochondrial) Ca2+ in brown adipocytes

The existence and significance of a hormone-sensitive, rapidly mobilizable intracellular pool of Ca2+ in hamster brown-fat cells was investigated with 45Ca2+-labelling techniques. It was shown that such a pool existed and was probably located within the abundant mitochondria. It was rapidly mobilized by norepinephrine (median effective concentration 50 nM) through alpha-adrenergic mechanisms. The mobilization of Ca2+ from the intracellular stores (mitochondria) required the presence of extracellular Na+, but not of Ca2+, K+ or Mg2+. It is concluded that the experiments are in agreement with a hypothesis linking the mobilization of intracellular Ca2+ pools with an alpha-adrenergically-induced increase in plasma membrane Na+ permeability (observed as a membrane depolarization), and a subsequent activation of the mitochondrial Na+/Ca2+ exchange, leading to mobilization of mitochondrial Ca2+ and the mediation of alpha-adrenergic effects as a result of an elevated cytosolic Ca2+ level.

Metabolic effects of beta, alpha 1, and alpha 2 adrenoceptor activation on brown adipocytes isolated from the perirenal adipose tissue of fetal lambs

Brown adipocytes can be readily isolated by collagenase digestion of perirenal adipose tissue from fetal lambs. In isolated cells the addition of phenylephrine in the presence of alprenolol (to specifically stimulate alpha adrenoceptors) resulted in an increase in de novo synthesis of phosphatidylinositol and phosphatidic acid. The stimulatory effects were preferentially inhibited by prazosin while yohimbine had little effect, indicating that the adrenoceptors were alpha 1 in character. Isoproterenol stimulated cyclic adenosine monophosphate (AMP) accumulation and lipolysis as well as respiration. Forskolin also mimicked the effects of beta adrenergic stimulation. Clonidine, a specific alpha 2 adrenergic agonist, inhibited lipolysis and cyclic AMP accumulation. Insulin inhibited cyclic AMP accumulation and stimulated glucose metabolism in the adipocytes. The present studies indicate that beta, alpha 1, and alpha 2 adrenergic as well as insulin responses can be detected in ovine perirenal adipocytes.

Quantitative differentiation of alpha- and beta-adrenergic respiratory responses in isolated hamster brown fat cells: evidence for the presence of an alpha 1-adrenergic component

The respiratory (thermogenic response of brown fat cells has been investigated for differentiation between alpha- and beta-adrenergic components. The relative sensitivity of the cells generally followed the pattern of the EC50 for isoprenaline less than norepinephrine = epinephrine much less than phenylephrine and the response to all these agonists was much more sensitive to propranolol than to phentolamine. Based on these criteria the response was primarily beta 1. However, the biphasic nature of the dose-response curves and the antagonist inhibition curves indicated additionally the presence of an alpha-component. Inhibition studies demonstrated the IC50 series: prazosin less than phentolamine less than yohimbine, indicating that the alpha-component is of the alpha 1-subtype. The effects of selective alpha- and beta-stimulation were additive. The maximal oxygen consumption of isolated hamster brown fat cells was composed of an 80% beta 1 adrenergic component and a 20% alpha 1 adrenergic component. Different mechanisms (beta 1 through cyclic AMP and alpha 1 possibly through Ca2+) and perhaps different purposes (e.g. short-term and long-term regulation, respectively) may explain the coexistence of two stimulatory adrenergic responses in one cell type.

Identification of [3H]prazosin binding sites in crude membranes and isolated cells of brown adipose tissue as alpha 1-adrenergic receptors

The alpha 1-receptor selective adrenergic antagonist [3H]prazosin was used to study adrenergic binding sites in crude membranes and isolated cells from hamster brown adipose tissue. The antagonist labelled a site which fulfilled the criteria for being the alpha 1-receptor which participates in mediation of a part of the norepinephrine-induced respiration (thermogenesis) in intact cells. The similarity between the characteristics of the binding site in crude membrane fractions and in isolated brown fat cells suggested that the site is of postsynaptic origin. In equilibrium binding studies [3H]prazosin bound with very high affinity (Kd = 0.4 nM), and the maximal binding capacity was 72 fmol/mg protein and 207 fmol/10(6) cells, equal to 120 000 receptors per cell. The kinetically calculated Kd had a value of 0.17 nM, in good agreement with that determined in the equilibrium binding experiments. The relative potencies of adrenergic agents to displace [3H]prazosin revealed a typical alpha 1-specificity: WB-4101 = prazosin greater than phentolamine greater than dihydroergocryptine much greater than yohimbine greater than propranolol for antagonists, and L-phenylephrine = L-norepinephrine = L-epinephrine greater than L-isoprenaline greater than D-norepinephrine for agonists. Thus stereoselectivity was also shown. The actual Ki values for antagonists were closely similar in crude membranes and isolated cells whereas the alpha 1-receptor showed a 10-20 times higher affinity for agonists in the cellular preparation than in the crude membranes. The Ki values for the different antagonists and agonists derived from binding studies in isolated cells were compared with the IC50 and EC50 values for these agents obtained from studies on alpha 1-mediated cellular effects. It is suggested that tight coupling exists between alpha 1-receptor occupancy and alpha 1-mediated effects.

Alpha 1-adrenergic stimulation of hamster brown adipocyte respiration

Respiration was increased approximately 5-fold with 0.05 microM norepinephrine and to a maximum of 10-fold by 0.30 microM norepinephrine. Prazosin, an alpha-adrenergic blocking agent highly selective for alpha 1-type receptors, partially inhibited the response to norepinephrine (0.05 microM) by 20-25% at a concentration of 0.10-1 microM. In contrast, when the stimulus for respiration was provided by isoproterenol or 3-isobutyl-1-methylxanthine, prazosin was without effect up to a concentration of 10 microM. Yohimbine, an alpha-adrenergic blocking drug preferential for alpha 2-receptors, did not influence norepinephrine-stimulated oxygen uptake. Respiration was increased two- to fourfold by phenylephrine or methoxamine, agents preferential for alpha 1-adrenergic receptors but not at all by clonidine, an agent preferential for alpha 2-adrenergic receptors. The stimulatory effect of phenylephrine on oxygen uptake was fully blocked by prazosin but not propranolol. Removal of extracellular calcium with ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid prevented phenylephrine stimulation of respiration but was without effect when isoproterenol was the stimulus. These results support the participation of alpha 1-adrenergic receptors in control of respiration and are consistent with the possibility that changes in cell calcium are intimately involved in this response.

Apparent absence of alpha-2 adrenergic receptors from hamster brown adipocytes

The possible presence of alpha adrenergic control of lipolysis and cyclic AMP production in brown adipocytes of hamsters was studied in adipocytes isolated from interscapular, subscapular, cervical and axillary regions of normal male hamsters maintained at 25 degrees C. Lipolysis activated by either 3-isobutyl-l-methyl xanthine or isoproterenol was unaffected by the presence of the alpha adrenergic selective agonists clonidine and methoxamine. Similarly, accumulation of cyclic AMP in response to beta-receptor stimulation, alone or in combination with a methyl xanthine, was unaffected by clonidine or methoxamine. In contrast, both lipolysis and cyclic AMP accumulation in brown fat cells were effectively suppressed in the presence of nicotinic acid, prostaglandin E1 or N6-phenylisopropyl adenosine. Accumulation of cyclic AMP in response to the mixed agonist norepinephrine was not influenced when cells were exposed to the alpha adrenergic blocking drugs yohimbine or tolazoline. These observations suggest that alpha-2 adrenergic receptors which are present on hamster white fat cells and control production of cyclic AMP and lipolysis are absent from hamster brown adipocytes. On the other hand, brown fat cells of this species appear to respond to a number of other inhibitory compounds in a manner not markedly different from that of white adipocytes.