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

Bidirectional Ca2+ coupling of mitochondria with the endoplasmic reticulum and regulation of multimodal Ca2+ entries in rat brown adipocytes

How the endoplasmic reticulum (ER) and mitochondria communicate with each other and how they regulate plasmalemmal Ca(2+) entry were studied in cultured rat brown adipocytes. Cytoplasmic Ca(2+) or Mg(2+) and mitochondrial membrane potential were measured by fluorometry. The sustained component of rises in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) produced by thapsigargin was abolished by removing extracellular Ca(2+), depressed by depleting extracellular Na(+), and enhanced by raising extracellular pH. FCCP, dinitrophenol, and rotenone caused bi- or triphasic rises in [Ca(2+)](i), in which the first phase was accompanied by mitochondrial depolarization. The FCCP-induced first phase was partially inhibited by oligomycin but not by ruthenium red, cyclosporine A, U-73122, a Ca(2+)-free EGTA solution, and an Na(+)-free solution. The FCCP-induced second phase paralleling mitochondrial repolarization was partially blocked by removing extracellular Ca(2+) and fully blocked by oligomycin but not by thapsigargin or an Na(+)-deficient solution, was accompanied by a rise in cytoplasmic Mg(2+) concentration, and was summated with a high pH-induced rise in [Ca(2+)](i), whereas the extracellular Ca(2+)-independent component was blocked by U-73122 and cyclopiazonic acid. The FCCP-induced third phase was blocked by removing Ca(2+) but not by thapsigargin, depressed by decreasing Na(+), and enhanced by raising pH. Cyclopiazonic acid-evoked rises in [Ca(2+)](i) in a Ca(2+)-free solution were depressed after FCCP actions. Thus mitochondrial uncoupling causes Ca(2+) release, activating Ca(2+) release from the ER and store-operated Ca(2+) entry, and directly elicits a novel plasmalemmal Ca(2+) entry, whereas Ca(2+) release from the ER activates Ca(2+) accumulation in, or release from, mitochondria, indicating bidirectional mitochondria-ER couplings in rat brown adipocytes.

Actin filaments play a permissive role in the inhibition of store-operated Ca2+ entry by extracellular ATP in rat brown adipocytes

Stimulation of P2 receptors with micromolar concentration of ATP evokes a transient increase in [Ca2+]i (intracellular free Ca2+ concentration), primarily due to release of Ca2+ from intracellular stores; such stimulation also triggers almost complete suppression of thapsigargin-evoked sustained [Ca2+]i increase mediated through a store-operated Ca2+ entry pathway in rat brown adipocytes. We investigated the role of cytoskeletal actin in the inhibitory effect of the extracellular ATP on store-operated Ca2+ entry, using fura 2 fluorescence for continuous measurement of [Ca2+]i, and using Alexa fluor 488-phalloidin staining of actin. Disassembly of actin networks by cytochalasin D (1 microM) or latrunculin A (3 microM) prevented the inhibitory effect of ATP (10 microM) on the thapsigargin (100 nM)-evoked store-operated Ca2+ entry, without changing the effect of ATP in increasing [Ca2+]i. In normal cells, bath application of ATP induced a transient [Ca2+]i increase, consisting of a rapid increase (the rising phase) and the subsequent decrease (the declining phase) to a lower steady level despite the continued presence of the agonist. Disruption of actin assemblies did not significantly affect the rising phase, but prevented the declining phase. Cells incubated with 10 microM ATP for 4 min demonstrated marked accumulations of actin filaments at the cell periphery, showing protrusions at the cell surface; this actin-assembly process is mediated through P2 receptors. In cells treated with cytochalasin D or latrunculin A, extracellular ATP did not induce actin redistribution. These results suggest that the actin reorganization plays a role in ATP-induced inhibition of store-operated Ca2+ entry in rat brown adipocytes.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Inhibition of store-operated Ca2+ entry by extracellular ATP in rat brown adipocytes

1. Modulation of intracellular free Ca2+ concentration ([Ca2+]i) by extracellular ATP was investigated in cultured adult rat brown adipocytes using the fluorescent Ca2+ indicator fura-2. 2. Bath application of ATP in micromolar concentrations caused a large increase in [Ca2+]i in cells previously stimulated with noradrenaline. This ATP-induced [Ca2+]i increase exhibited a monotonic decline to near the resting levels within approximately 2 min, even in the continued presence of the agonist. 3. The magnitude and time course of the [Ca2+]i increase in response to ATP were not significantly affected by removal of extracellular Ca2+, suggesting that a mobilization of intracellular Ca2+ primarily contributes to the increase. 4. The [Ca2+]i increase in response to ATP was sensitive to inhibition by suramin, suggesting the involvement of P2 purinoceptors in the response. 5. Thapsigargin (100 nM) evoked a gradual and irreversible increase in [Ca2+]i which was entirely dependent upon extracellular Ca2+, providing functional evidence for the expression of store-operated Ca2+ entry in these brown adipocytes. 6. Extracellular ATP at a concentration of 10 microM depressed this thapsigargin (100 nM)-induced [Ca2+]i increase by 92 +/- 3 % (n = 8 cells), strongly suggesting that ATP inhibits an influx of Ca2+ across the plasma membrane through the store-operated pathway. Bath application of phorbol 12-myristate 13-acetate (PMA, 5 microM) did not affect the thapsigargin-induced [Ca2+]i increase, indicating that the inhibitory action of ATP is not mediated by activation of protein kinase C (PKC). 7. These results indicate that extracellular ATP not only mobilizes Ca2+ from the intracellular stores but also exerts a potent inhibitory effect on the store-operated Ca2+ entry process in adult rat brown adipocytes.

ATP can stimulate exocytosis in rat brown adipocytes without apparent increases in cytosolic Ca2+ or G protein activation

Extracellular ATP activates large increases in cell surface area and membrane turnover in rat brown adipocytes (Pappone, P. A., and Lee, S. C. 1996. J. Gen. Physiol. 108:393-404). We used whole-cell patch clamp membrane capacitance measurements of membrane surface area concurrently with fura-2 ratio imaging of intracellular calcium to test whether these purinergic membrane responses are triggered by cytosolic calcium increases or G protein activation. Increasing cytosolic calcium with adrenergic stimulation, calcium ionophore, or calcium-containing pipette solutions did not cause exocytosis. Extracellular ATP increased membrane capacitance in the absence of extracellular calcium with internal calcium strongly buffered to near resting levels. Purinergic stimulation still activated exocytosis and endocytosis in the complete absence of intracellular and extracellular free calcium, but endocytosis predominated. Modulators of G protein function neither triggered nor inhibited the initial ATP-elicited capacitance changes, but GTPgammaS or cytosolic nucleotide depletion did reduce the cells' capacity to mount multiple purinergic responses. These results suggest that calcium modulates purinergically-stimulated membrane trafficking in brown adipocytes, but that ATP responses are initiated by some other signal that remains to be identified.

Effects of P2 purinergic receptor stimulation in brown adipocytes

Sympathetic stimulation of brown adipocytes plays a major role in body energy homeostasis by activating energy-wasting pathways. Sympathetic neuronal input initiates a variety of metabolic, developmental, and membrane responses in brown fat cells. Many of these actions are mediated by adrenergic pathways mobilized by released norepinephrine. However, since sympathetic stimulation may also release vesicular ATP, we tested brown fat cells for ATP responses. Micromolar concentrations of extracellular ATP had a number of effects on brown adipocytes. We have shown previously that ATP elicits substantial (average of approximately 30%) increases in cell membrane capacitance (P. A. Pappone and S. C. Lee, J. Gen. Physiol. 108: 393-404, 1996). Here, we show that cytosolic calcium levels were increased by ATP, both through release from intracellular stores and through influx, as assessed by fura 2 imaging. In addition, ATP indirectly activated a nonselective cation conductance that was independent of cytosolic calcium levels in patch voltage-clamped brown fat cells. Similar calcium, conductance, and capacitance responses could be activated by 2-methylthio-ATP and ADP, consistent with mediation by a P2 type purinergic receptor. Calorimetric measurements from cell suspensions showed that ATP increased basal heat production of isolated brown fat cells by approximately 40% but had no effect on the greater than fivefold increase in heat production seen with maximal adrenergic stimulation. These myriad responses to extracellular ATP suggest that P2 receptor-mediated signaling is important in brown adipocyte physiology and that sympathetic stimulation may normally activate purinergic as well as adrenergic pathways in brown fat.

Adrenergically activated Ca2+ increases in brown fat cells: effects of Ca2+, K+, and K channel block

We measured intracellular calcium concentration ([Ca2+]i) during adrenergic stimulation using fura-2 ratio imaging of individual cultured neonatal rat brown fat cells. One micromolar norepinephrine (NE) increased [Ca2+]i from an average resting value of 105 nM to 555 nM in approximately 30 s. [Ca2+]i remained elevated as long as NE was present but returned to resting levels within 2-3 min after NE removal. The response was half maximal at approximately 50 nM NE and was primarily alpha-adrenergic. The sustained, but not the initial, increase in [Ca2+]i required extracellular calcium. Cells stimulated in high-K media had [Ca2+]i responses like those in 0 Ca2+, suggesting that depolarization abrogates calcium influx. Parallel perforated-patch recordings showed that the increase in [Ca2+]i activates a calcium-activated K conductance. Blocking K channels with moderate concentrations of tetraethylammonium (TEA) had only small effects on NE-induced changes in [Ca2+]i, but high concentrations of TEA significantly reduced the response. We conclude that cytoplasmic calcium is modulated by fluxes from both intracellular and extracellular sources and that K channels may not be required for normal short-term [Ca2+]i responses to hormone.

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.

Stimulation of generation of inositol phosphates by carbamoylcholine and its inhibition by phorbol esters and iodide in dog thyroid cells

The action of carbamoylcholine (Cchol), NaF and other agonists on the generation of inositol phosphates (IPs) was studied in dog thyroid slices prelabelled with myo-[2-3H]inositol. The stimulation by Cchol (0.1 microM-0.1 mM) of IPs accumulation through activation of a muscarinic receptor [Graff, Mockel, Laurent, Erneux & Dumont (1987) FEBS Lett. 210, 204-210] was pertussis- and cholera-toxin insensitive. Ins(1,4,5)P3, Ins(1,3,4)P3 and InsP4 were generated. NaF (5-20 mM) also increased IPs generation (Graff et al., 1987); this effect was potentiated by AlCl3 (10 microM) and unaffected by pertussis toxin. Although phorbol dibutyrate (5 microM) abolished the cholinergic stimulation of IPs generation (Graff et al., 1987), it did not affect the fluoride-induced response. Cchol and NaF did not require extracellular Ca2+ to exert their effect, and neither KCl-induced membrane depolarization nor ionophore A23187 (10 microM) had any influence on basal IPs levels, or on cholinergic stimulation. However, more stringent Ca2+ depletion with EGTA (0.1 or 1 mM) decreased basal IPs levels as well as the amplitude of the stimulation by Cchol without abolishing it. Dibutyryl cyclic AMP, forskolin, cholera toxin and prostaglandin E1 had no effect on basal IPs levels and did not decrease the response to Cchol. Iodide (4 or 40 microM) also strongly decreased the cholinergic action on IPs, this inhibition being relieved by methimazole (1 mM). Our data suggest that Cchol activates a phospholipase C hydrolysing PtdIns(4,5)P2 in the dog thyroid cell in a cyclic AMP-independent manner. This activation requires no extracellular Ca2+ and depends on a GTP-binding protein insensitive to both cholera toxin and requires no extracellular Ca2+ and depends on a GTP-binding protein insensitive to both cholera toxin and pertussis toxin. The data are consistent with a rapid metabolism of Ins(1,4,5)P3 to Ins(1,3,4)P3 via the Ins(1,4,5)P3 3-kinase pathway, followed by dephosphorylation by a 5-phosphomonoesterase. Indeed, a Ca2+-sensitive InsP3 3-kinase activity was demonstrated in tissue homogenate. Stimulation of protein kinase C and an organified form of iodine inhibit the Cchol-induced IPs generation. The negative feedback of activated protein kinase C could be exerted at the level of the receptor or of the receptor-G-protein interaction.

Alpha 1- and beta-adrenergic regulation of intracellular Ca2+ levels in brown adipocytes

In order to monitor changes in cytosolic Ca2+ levels, brown-fat cells were incubated with the fluorescent Ca2+-indicator fura-2 and the fluorescence intensity ratio followed. The addition of norepinephrine led to a rapid and persistent increase in the cytosolic Ca2+ level, which was dose-dependent with a maximal effect at about 1 microM. The response was diminished in the absence of extracellular Ca2+ and was inhibited more efficiently by phentolamine and prazosin than by propranolol or yohimbine, indicating alpha 1-adrenergic mediation. Accordingly, selective alpha 1-adrenergic stimulation also increased the cytosolic Ca2+ level. However, selective beta-adrenergic stimulation, as well as the adenylate cyclase activator forskolin, were also able to increase the cytosolic Ca2+ level in these cells to a certain extent. It was concluded that the major part of the increase in cytosolic Ca2+ was mediated, as in other cell types, via alpha 1-adrenergic receptors, but that Ca2+ levels were also positively modulated by a cAMP-mediated process. These observations are discussed in relation to known alpha 1/beta synergisms in brown adipose tissue.

Effect of thyroid status on catecholamine stimulation of thyroxine 5'-deiodinase in brown adipocytes

We examined type II 5'-iodothyronine deiodinase activation by adrenergic agonists in dispersed brown adipocytes from euthyroid and hypothyroid rats. In euthyroid cells, basal deiodinase activity was 30-100 fmol I-.h-1.10(6) cells-1 and increased four- to fivefold during exposure to norepinephrine, an effect that was enhanced by alprenolol. In cells from hypothyroid rats, norepinephrine caused a three- to fourfold greater deiodinase stimulation than occurred in euthyroid cells but alprenolol inhibited the response. In euthyroid cells, phenylephrine caused greater stimulation than did norepinephrine, but this was inhibited by alprenolol. Isoproterenol and 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) inhibited the phenylephrine response but were modestly stimulatory alone. Although both alpha 1- and beta-adrenergic agonists increased deiodinase activity modestly in hypothyroid cells, in combination they caused a marked synergistic stimulation. This synergism was induced by 8-BrcAMP and forskolin, as well as by isoproterenol. The stimulation of deiodinase in both cell types was due to an increase in Vmax without an alteration in the Km and required mRNA synthesis. The markedly greater deiodinase response of the hypothyroid brown adipocyte to catecholamines may serve to enhance the impaired thermogenic response of this tissue to cold exposure.

Role of cell calcium in alpha-1 adrenergic receptor control of arachidonic acid release from brown adipocytes

Exposure of brown fat cells to phenylephrine, an agonist of alpha-1 adrenergic receptors, activates a phospholipase A2 which releases arachidonic acid. Since receptor activation of phospholipase A2 requires calcium, experiments were undertaken to define more precisely the role played by calcium in the regulation of enzyme activity. In this study, adipocytes were loaded with the fluorescent calcium chelator quin2 in order to buffer intracellular calcium and block receptor stimulated changes in its concentration. When quin2 loaded adipocytes were incubated in buffer containing 0.10 mM calcium, the ability of phenylephrine to stimulate release of arachidonic acid was severely reduced. At an intracellular quin2 concentration of 6.6 mM stimulated arachidonic acid release was inhibited by more than 50% and at 13 mM it was completely blocked. In contrast, phenylephrine stimulation of inositol phosphate accumulation was unaffected by quin2. Quin2 also did not affect the liberation of arachidonic acid in response to exogenous phospholipase C, A23187 or forskolin. The intracellular calcium antagonist TMB-8 also inhibited phenylephrine-stimulation of arachidonic acid release and this effect was reversed by ionomycin. Basal phospholipase A2 activity was increased by introduction of high calcium concentrations into cells rendered permeable with digitonin, but phenylephrine still caused a further increase in enzyme activity. These findings show a selective inhibition of phenylephrine activation of phospholipase A2 by either the chelation of intracellular calcium with quin2 or by the calcium antagonist TMB-8 and suggest an essential role for intracellular calcium in alpha adrenergic stimulation of enzyme activity. However, because phenylephrine still stimulates enzyme activity in cells rendered permeable with digitonin, we suggest that the action of phenylephrine cannot be attributed solely to changes in intracellular calcium.

Rapid action of 1,25-dihydroxyvitamin D3 on hepatocyte phospholipids

Recent studies have reported cellular effects of 1,25-dihydroxyvitamin D3 within 15 minutes, a time period too rapid to be mediated by nuclear activation. The vitamin increases hepatocyte cytosolic calcium levels in the absence of extracellular calcium within 5 minutes. Since metabolites of phosphatidylinositol have been implicated as second messengers in the regulation of cytosolic calcium, we examined the effect of 1,25-dihydroxyvitamin D3 on hepatocyte phosphatidylinositol turnover and compared these effects to those produced by vasopressin. In isolated hepatocytes labeled with [3H]inositol, 1,25-dihydroxyvitamin D3 (4 nM) increased [3H]glycerophosphorylinositol by 16% (p less than 0.01) within 2.5 minutes, by 18% (p less than 0.01) after 5 minutes, and by 11% (p less than 0.05) after 10 minutes. At a concentration of 20 nM, 1,25-dihydroxyvitamin D3 increased [3H]glycerophosphorylinositol by 27% (p less than 0.01) after 5 minutes. Vitamin D did not affect [3H]inositol polyphosphates. Conversely, vasopressin had no effect on [3H]glycerophosphorylinositol but significantly increased [3H]inositol phosphate, [3H]inositol bisphosphate, and [3H]inositol triphosphate. 1,25-Dihydroxyvitamin D3 (4 nM) decreased [3H]phosphatidylinositol by 10% (p less than 0.05) after 5 minutes and by 16% (p less than 0.01) after 10 minutes. At a concentration of 20 nM, 1,25-dihydroxyvitamin D decreased [3H]phosphatidylinositol by 18% (p less than 0.01) after 5 minutes. The vitamin did not affect [3H]phosphatidylinositol bisphosphate or [3H]phosphatidylinositol trisphosphate. 24,25-Dihydroxyvitamin D had no effect on inositol phospholipids. The effects of 1,25-dihydroxyvitamin D3 on inositol phospholipids were blocked by quinacrine. Bromophenacylbromide inhibited the effects of 1,25-dihydroxyvitamin D3 on inositol phospholipids and also blocked the vitamin-induced increments in cytosolic calcium.(ABSTRACT TRUNCATED AT 250 WORDS)

The alpha 1-adrenergic transduction system in hamster brown adipocytes. Release of arachidonic acid accompanies activation of phospholipase C

Previous studies of brown adipocytes identified an increased breakdown of phosphoinositides after selective alpha 1-adrenergic-receptor activation. The present paper reports that this response, elicited with phenylephrine in the presence of propranolol and measured as the accumulation of [3H]inositol phosphates, is accompanied by increased release of [3H]arachidonic acid from cells prelabelled with [3H]arachidonic acid. Differences between stimulated arachidonic acid release and formation of inositol phosphates included a requirement for extracellular Ca2+ for stimulated release of arachidonic acid but not for the formation of inositol phosphates and the preferential inhibition of inositol phosphate formation by phorbol 12-myristate 13-acetate. The release of arachidonic acid in response to phenylephrine was associated with an accumulation of [3H]arachidonic acid-labelled diacylglycerol, and this response was not dependent on extracellular Ca2+ but was partially prevented by treatment with the phorbol ester. The release of arachidonic acid was also stimulated by melittin, which increases the activity of phospholipase A2, by ionophore A23187, by lipolytic stimulation with forskolin and by exogenous phospholipase C. The arachidonic acid response to phospholipase C was completely blocked by RHC 80267, an inhibitor of diacylglycerol lipase, but this inhibitor had no effect on release stimulated with melittin or A23187 and inhibited phenylephrine-stimulated release by only 40%. The arachidonate response to forskolin was additive with the responses to either phenylephrine or exogenous phospholipase C. These data indicate that brown adipocytes are capable of releasing arachidonic acid from neutral lipids via triacylglycerol lipolysis, and from phospholipids via phospholipase A2 or by the sequential activities of phospholipase C and diacylglycerol lipase. Our findings also suggest that the action of phenylephrine to promote the liberation of arachidonic acid utilizes both of these reactions.

Adenosine inhibits phenylephrine activation of phospholipase A in hamster brown adipocytes

Exposure of brown adipocytes to phenylephrine activates a phospholipase A2 producing arachidonic acid and lysophospholipids. When adipocytes were incubated with adenosine deaminase, a greater release of arachidonic acid and accumulation of lysophosphatidyl-choline in response to phenylephrine was noted. The potentiating effect of adenosine deaminase was also observed in the presence of A23187 and for both stimuli, the effect of adenosine deaminase was reversed by phenylisopropyladenosine. These results suggest the presence of an heretofore unrecognized action of adenosine, namely inhibition of phospholipase A2 activity in brown fat cells.

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.

Effect of phorbol esters on the hormonal responsiveness of isolated white fat cells

Phorbol 12-myristate 13-acetate (PMA) did not alter the basal cyclic AMP accumulation or lipolysis in isolated rat fat cells. Similarly, PMA was unable to modify the cyclic AMP accumulation induced by isoprenaline and forskolin or the inhibition of cyclic AMP accumulation and lipolysis induced by prostaglandin E2 and phenylisopropiladenosine. PMA inhibited the alpha 1-adrenergic increase in the labeling of phosphatidylinositol in a dose-dependent fashion. By itself, PMA was without significant effect on the labeling of phosphatidylinositol and phosphatidylethanolamine but increased the labeling of phosphatidylcholine and its precursor phosphatidic acid.

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.

Carbachol and sodium fluoride, but not TSH, stimulate the generation of inositol phosphates in the dog thyroid

In dog thyroid slices prelabeled with myo-[2-3H]inositol, carbachol (10(-7)-10(-4) M) and NaF (10-20 mM) stimulated IP1, IP2 and IP3 generation. These effects did not require the presence of extracellular calcium. Atropine and PDBu inhibited the action of the cholinergic agonist. No effect of TSH (1-100 mU/ml) could be detected on PIP2 hydrolysis and IP production. These results suggest that IP3 could play a role in the metabolic actions of carbachol in the thyroid; a G-protein coupling the hormone-receptor binding to phospholipase C activation exists in the thyroid membrane; the well known TSH-induced increased PI turnover does not result in IP3 accumulation.

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.