Czech Footprints in the Bioenergetics Research

Life manifests as growth, movement or heat production that occurs thanks to the energy accepted from the outside environment. The basis of energy transduction attracted the Czech researchers since the beginning of the 20th century. It further accelerated after World War II, when the new Institute of Physiology was established in 1954. When it was found that energy is stored in the form of adenosine triphosphate (ATP) that can be used by numerous reactions as energy source and is produced in the process called oxidative phosphorylation localized in mitochondria, the investigation focused on this cellular organelle. Although the Czech scientists had to overcome various obstacles including Communist party leadership, driven by curiosity, boldness, and enthusiasm, they characterized broad spectrum of mitochondrial properties in different tissues in (patho)physiological conditions in collaboration with many world-known laboratories. The current review summarizes the contribution of the Czech scientists to the bioenergetic and mitochondrial research in the global context. Keywords: Mitochondria, Bioenergetics, Chemiosmotic coupling.

Increased thermogenic capacity of brown adipose tissue under low temperature and its contribution to arousal from hibernation in Syrian hamsters

Brown adipose tissue (BAT) is thought to play a significant physiological role during arousal when body temperature rises from the extremely low body temperature that occurs during hibernation. The dominant pathway of BAT thermogenesis occurs through the β(3)-adrenergic receptor. In this study, we investigated the role of the β(3)-adrenergic system in BAT thermogenesis during arousal from hibernation both in vitro and in vivo. Syrian hamsters in the hibernation group contained BAT that was significantly greater in overall mass, total protein, and thermogenic uncoupling protein-1 than BAT from the warm-acclimated group. Although the ability of the β(3)-agonist CL316,243 to induce BAT thermogenesis at 36°C was no different between the hibernation and warm-acclimated groups, its maximum ratio over the basal value at 12°C in the hibernation group was significantly larger than that in the warm-acclimated group. Forskolin stimulation at 12°C produced equivalent BAT responses in these two groups. In vivo thermogenesis was assessed with the arousal time determined by the time course of BAT temperature or heart rate. Stimulation of BAT by CL316,243 significantly shortened the time of arousal from hibernation compared with that induced by vehicle alone, and it also induced arousal in deep hibernating animals. The β(3)-antagonist SR59230A inhibited arousal from hibernation either in part or completely. These results suggest that BAT in hibernating animals has potent thermogenic activity with a highly effective β(3)-receptor mechanism at lower temperatures.

Studies of thermogenesis and mitochondrial function in adipose tissues

Brown and white adipose tissues in mammals have a number of similar properties, such as lipid storage and adipokine production, but also distinctive properties. The energy-storing white adipose tissue has few mitochondria and low oxidative capacity. The heat-producing brown adipose tissue has a high density of mitochondria and high oxidative capacity. Mitochondrial function can be investigated in cells and organelles isolated from both brown and white adipose tissues. This chapter describes methods for successful isolation of suitable preparations of adipose tissues and their subsequent use. Questions concerning thermogenic capacity of the tissues, their potential influence on whole body metabolism, and specific properties of the mitochondria and their mode of function may be addressed using these methods.

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.

Effects of age, gender, and senescence on beta-adrenergic responses of isolated F344 rat brown adipocytes in vitro

We previously reported greater age-related attenuation of cold-induced thermoregulation and brown adipose tissue thermogenic capacity in male vs. female F344 rats. With onset of the rapid weight loss that occurs near the end of the lifespan, this age-related attenuation becomes severe. We refer to this "end-of-life" physiological state of older rats as senescence. Here, we measured oxygen consumption of isolated brown adipocytes and found no age (6 vs. 12 vs. 26 mo) or gender effects on maximal norepinephrine (NE)- or CL-316,243 (beta 3-adrenergic agonist)-induced responses. In contrast, brown adipocytes from 22- to 26-mo-old senescent rats (males and females) consumed 51-60% less oxygen during maximal stimulation with NE and CL-316,243 than did cells from 26-mo-old presenescent rats. This attenuation was associated with lower (65-72%) uncoupling protein 1 concentrations but no alterations in NE-induced cAMP levels or lipolysis. Our data indicate that senescence, but not chronological age, significantly impacts NE-/beta 3-mediated thermogenesis of isolated brown adipocytes and that this effect involves altered mitochondrial rather than altered membrane or cytosol events.

Down-regulation of beta3 adrenoreceptor gene expression in brown fat cells is transient and recovery is dependent upon a short-lived protein factor

The regulation of the expression of the beta3 adrenoreceptor gene was examined in the brown adipose tissue of intact mice and in murine brown fat primary cell cultures. Both in vivo and in vitro, high levels of beta3 receptor mRNA were observed. Acute cold exposure of mice resulted in a marked and rapid down-regulation of beta3 gene expression; this down-regulation was, however, transient. Similarly, in brown fat cell cultures, norepinephrine addition led to down-regulation of beta3 gene expression, with a lag phase of 30 min and with an apparent half-life of beta3 mRNA of approximately 30 min. This down-regulation was stimulated via the beta3 receptors themselves and mediated via cAMP; the apparent affinity of norepinephrine was extremely high (<1 nM). The degradation rate after actinomycin was identical to that after norepinephrine and was not affected by the presence of norepinephrine; thus, the down-regulation was due to cessation of transcription but not to an increased rate of degradation. Notably, inhibition of protein synthesis by cycloheximide also led to down-regulation. The norepinephrine-induced down-regulation was transient; spontaneous recovery occurred after approximately 18 h and was not due to depletion of adrenergic agent. Recovery did not occur in the presence of cycloheximide. After recovery, the cells showed a functional desensitization of the down-regulation process itself (EC50 now approximately 10 nM). It is concluded that a down-regulated state cannot explain the functional desensitization of beta3 adrenergic responsiveness observed in brown fat cells isolated from cold-acclimated animals (i.e. physiologically chronically adrenergically stimulated brown fat cells); since the beta3 receptor is not subject to desensitization via phosphorylation processes, no satisfactory explanation for the functional desensitization exists as yet. A model is presented for the down-regulation/recovery process, involving the participation of a phosphorylatable short-lived transcription factor.

Prolonged exposure of hamsters to cold changes the levels of G proteins in brown adipose tissue plasma membranes

The levels of G proteins in plasma membranes prepared from brown adipose tissue of control and cold-exposed hamsters were determined by quantitative immunoblotting and competitive ELISA. Prolonged (four weeks) exposure of hamsters to cold decreased significantly the total content of the alpha subunits of the stimulatory (Gs alpha) as well as inhibitory (Gi alpha (1,2)) G proteins. Interestingly, the reduction in the Gs alpha content was solely due to a large reduction in the content of the short (45 kDa) isoform of Gs alpha, while the level of the long (52 kDa) isoform of Gs alpha remained unchanged. The level of the beta subunit of G protein was decreased comparably to the reduction in the total content of the alpha subunits. Cold-induced alterations in the G protein network associated with plasma membranes of brown adipose tissue were accompanied by changed characteristics of AlF(4-)-sensitive adenylyl cyclase activity.

Fat mobilisation in short days is not associated with altered noradrenergic sensitivity of adipocytes in Djungarian hamsters

One of the primary physiological responses of the Djungarian hamster to short photoperiods is a reduction in body weight with fat mobilisation. The depletion of fat to a minimum level may be regulated either in the periphery, through the sensitivity of adipocytes to hormonal stimulation, or centrally, via adjustments in efferent activity. To investigate this, we examined the lipolytic pathway in fat cells from animals at various stages of entrainment to long or short photoperiod. Short photoperiod exposure of up to a 10-week duration was without effect on basal glycerol release by unstimulated cells or on the ability of norepinephrine or an adenosine analogue to stimulate or inhibit lipolysis, respectively. Prolonged exposure to short photoperiod reduced basal glycerol release, but adipocytes retained their sensitivity to hormonal stimulation. Short photoperiod had no effect on the density or affinity of membrane-bound beta-adrenergic or adenosine receptors, or upon the ability of isoproterenol or forskolin to stimulate adenylate cyclase in adipocyte membranes. This suggests that the regulation of fat depletion in short photoperiod is determined centrally and does not involve alterations in adipocyte sensitivity and, in particular, the desensitisation of the adipocyte beta-adrenergic receptor-linked adenylate cyclase pathway.

Effect of photoperiod on mitochondrial GDP binding and adenylate cyclase activity in brown adipose tissue of Djungarian hamsters

Experiments were designed to investigate the involvement of brown adipose tissue (BAT) thermogenesis in the weight loss exhibited by Djungarian hamsters (Phodopus sungorus campbelli) in response to a short photoperiod. Significant decreases in body weight preceded reductions in food intake, suggesting a photoperiod-induced change in energy expenditure. Sixteen weeks exposure to short photoperiod resulted in large decreases in body weight and interscapular BAT mass that were accompanied by an increase in the thermogenic activity of BAT (estimated by mitochondrial GDP binding). However, exposure to short photoperiod for 8 weeks, that induced smaller but significant reductions in body weight, was without effect on the BAT parameters measured. This suggests that increased BAT thermogenesis is unlikely to initiate, or contribute to, the early stages of photoperiod-induced weight loss. In addition, short photoperiod failed to induce any change in the specific activity or sensitivity of adenylate cyclase in BAT membranes, in contrast to the downregulation of catecholamine-stimulated cAMP production observed in BAT following cold exposure.

Coexisting beta-adrenoceptor subtypes: significance for thermogenic process in brown fat cells

The possible significance of the coexisting beta 1-, beta 2-, and beta 3-adrenoceptors in brown adipose tissue for the thermogenic response was investigated. Oxygen consumption of isolated hamster brown fat cells was analyzed as a measure of thermogenesis. Thermogenesis could be evoked not only by the physiological agent norepinephrine but also by BRL-37344 and CGP-12177. No evidence for biphasic inhibition curves was found with either the selective beta 1-antagonist ICI-89406, the beta 2-antagonist ICI-118551, or the beta 1/beta 2-nonselective beta-antagonist propranolol against 1 microM norepinephrine; pI50 (the negative logarithm of the inhibitory constant for an antagonist, as estimated from the dose-response curve for an antagonist vs. a constant agonist concentration) values for ICI-89406 and ICI-118551 were very low (4-5), implying nonselective inhibition; the pI50 for propranolol was approximately 6 (as expected for the beta 3-receptor). Even with suboptimal norepinephrine, no biphasic inhibition was found. CGP-12177 at concentrations where it is primarily an antagonist to the beta 1-receptor did not influence the dose-response curve for either norepinephrine or BRL-37344. BRL-37344- or CGP-12177-induced thermogenesis was inhibited by the beta-antagonists in a manner similar to norepinephrine-induced thermogenesis. Schild plots for propranolol inhibition of norepinephrine-, isoprenaline-, BRL-37344- and CGP-12177-induced thermogenesis yielded similar pA2 (the negative logarithm of the inhibitory constant for an antagonist, as calculated from a series of agonist dose-response curves at different antagonist concentrations) (approximately 5.5), for interaction with either agonist, implying that the same receptor was stimulated by all agonists. Thus, despite the fact that different beta-receptor subtypes coexist in the tissue, we find no evidence for the participation of beta 1- or beta 2-receptors in the thermogenic response. Within the resolution of the experiments, the results therefore imply that it is predominantly or solely the beta 3-receptor that is coupled to thermogenesis, and it is via this beta-adrenergic receptor that not only norepinephrine but also CGP-12177 and BRL-37344 induce thermogenesis.

Physiological desensitization of beta 3-adrenergic responses in brown fat cells: involvement of a postreceptor process

To investigate a possible physiological desensitization process for beta 3-adrenergic responses, the effect of cold acclimation of hamsters on adrenergically stimulated oxygen consumption of isolated brown fat cells was investigated. Cells were prepared from control and from cold-acclimated hamsters. In agreement with earlier findings, cells isolated from cold-acclimated hamsters responded to norepinephrine addition with a decreased sensitivity (approximately 10 times higher 50% effective concentration) and a decreased maximal rate of oxygen consumption compared with cells from control hamsters. When cells were stimulated with the general beta-adrenergic agonist isoprenaline or with the beta 3-selective agonists BRL-37344 or CGP-12177, a similarly desensitized response was observed, demonstrating that it was indeed a beta 3-adrenergic response that was functionally desensitized. However, when the mitochondria within the cells were directly stimulated with exogenous free fatty acids (palmitate or octanoate), no difference between cells from control and cold-acclimated animals was seen, indicating that a mediatory step must be desensitized. When the cells were stimulated with forskolin (to activate adenylyl cyclase) or with 8-bromoadenosine 3',5'-cyclic monophosphate, the desensitized response was still observed. At post-adenosine 3',5'-cyclic monophosphate levels, a desensitization was not evident. Cyclic nucleotide phosphodiesterase activity was increased in cells from cold-acclimated animals. It is therefore suggested that this increased activity of phosphodiesterase could be (at least partly) responsible for the physiologically induced desensitized responses observed here.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo beta-adrenergic induction of the unmasking of the uncoupling protein in rat brown fat

1. In 28 degrees C adapted rats (WA) both cold stress and norepinephrine (NE) led to a 4-fold increase of uncoupling protein dependent proton conductance which was abolished by propranolol (PRO). 2. In 4-day warm re-exposed rats (after 10 days at 5 degrees C) (WR) the same uncoupling by cold stress was observed but the NE effect was lower. Uncoupling by cold stress was not abolished by PRO. 3. In WR rats, uncoupling was not due to the involvement of an alpha-adrenergic pathway. 4. Both beta-agonist isoproterenol and beta 3-agonists BRL 35135A and ICI D7114 led to high levels of unmasking. 5. Interscapular brown adipose tissue surgical denervation, which abolished cold stress unmasking both in WA and, WR rats, indicates a mediation by direct sympathetic innervation. 6. Depending on the thermal history of the rat, the possibility that unmasking by cold stress could be mediated by different types of beta-receptors is discussed.

Time-course variations of effective proton conductance and GDP binding in brown adipose tissue mitochondria of rats during prolonged cold exposure

1. Time-course variations of the thermogenic pathway in rat brown adipose tissue (BAT) mitochondria were examined. 2. Several parameters of mitochondrial energization, protonmotive force and its components pH gradient and membrane potential were investigated. The specific binding of GDP was compared with the effective proton conductance (CmH+) of the membrane. 3. Ten-days cold exposure led to maximal GDP binding and GDP-dependent CmH+. 4. The subsequent relative decrease in GDP binding observed during prolonged cold exposure (40 days) was functional and led to a lower GDP-dependent CmH+. CmH+ showed greater variation than GDP binding. 5. The CmH+ decrease was not due to a masking of active sites of the uncoupling protein. 6. Basal GDP-independent CmH+ was not modified. 7. Results are discussed with reference to the significance of biochemical measures and to the physiological regulation of BAT thermogenesis.

Cold acclimation induces desensitization to adenosine in brown fat cells without changing receptor binding

The ability of brown fat cells isolated from control and cold-acclimated hamsters to respond to adenosine was investigated. In measurements of the rate of oxygen consumption, it was observed that cells from control hamsters responded as expected to addition of adenosine deaminase, 3-isobutyl-1-methylxanthine (IBMX), or 2-chloroadenosine (i.e., norepinephrine dose-response curves were shifted to left in presence of adenosine deaminase or IBMX and to right with 2-chloroadenosine). However, brown fat cells isolated from cold-acclimated hamsters, under identical conditions, showed almost complete absence of adenosine control. Thus acclimation to cold induced a desensitization to adenosine by physiological means. To evaluate the molecular mechanism underlying desensitization to adenosine, [3H]phenylisopropyladenosine ([3H]PIA) binding to brown fat membranes from control and cold-acclimated hamsters was investigated. [3H]PIA bound with similar high affinity (KD approximately 5 nM) and saturability (Bmax approximately 15 fmol/mg protein) in both membrane preparations, demonstrating that desensitization to adenosine was not due to changes in adenosine receptor number or receptor affinity for adenosine. Furthermore, GTP induced a reduction in [3H]PIA affinity in brown fat membranes from both control and cold-acclimated hamsters, indicating that desensitization was probably not due to an uncoupling between the receptor and Gi protein. It was therefore concluded that the adenosine desensitization process may be located at the Gi protein-adenylate cyclase interaction.

Lipid peroxidation inhibits norepinephrine-stimulated lipolysis in rat adipocytes. Reduction of beta-adreno-ceptor number

The effect of lipid peroxidation on lipolysis depends on the intactness of the adipocyte plasma membrane. In the intact cells, the norepinephrine-stimulated lipolysis was inhibited, while the basal one was elevated. In the lysed cells, lipid peroxidation had no effect upon hormone-stimulated lipolysis, but the basal one was strongly inhibited. The effects of free radical damage (iron plus ascorbate ions) were compared to those of malondialdehyde, a non-radical product of lipid peroxidation. Although qualitatively similar, deterioration of plasma membrane induced by malondialdehyde was much lower than by free radicals. The changes in lipolytic response to norepinephrine were accompanied by a drastic reduction in the number of beta-adrenergic receptors.

In vitro study of adrenergic stimulation of 32P incorporation into phospholipids of brown adipose tissue of control and cold-acclimated rats

1. 32P-labelled inorganic phosphate incorporation into total and mitochondrial phospholipids was studied, in vitro, on brown adipose tissue (BAT) of control and cold-acclimated rats. 2. It was found that norepinephrine acts as in vivo, on BAT phospholipid metabolism via alpha 1 adrenergic receptors specifically increasing phosphatidic acid and phosphatidylinositol turnover with the same magnitude in both groups. 3. Cold-induced alpha 1 adrenergic desensitization is not as important as cold-induced beta adrenergic desensitization. 4. No specific effect of norepinephrine was seen in mitochondrial phospholipid turnover.

Modulation of receptors and adenylate cyclase activity during sucrose feeding, food deprivation, and cold exposure

Thermogenesis in brown adipose tissue (BAT) serves as a regulator of body temperature and weight maintenance. Thermogenesis can be stimulated by catecholamine activation of adenylate cyclase through the beta-adrenergic receptor. To investigate the effects of sucrose feeding, food deprivation, and cold exposure on the beta-adrenergic pathway, adenylate cyclase activity and beta-adrenergic receptors were assessed in rat BAT after 2 wk of sucrose feeding, 2 days of food deprivation, or 2 days of cold exposure. beta-Adrenergic receptors were identified in BAT using [125I]iodocyanopindolol. Binding sites had the characteristics of mixed beta 1- and beta 2-type adrenergic receptors at a ratio of 60/40. After sucrose feeding or cold exposure, there was the expected increase in BAT mitochondrial mass as measured by total cytochrome-c oxidase activity but a decrease in beta-adrenergic receptor density due to a loss of the beta 1-adrenergic subtype. This BAT beta-adrenergic receptor downregulation was tissue specific, since myocardial beta-adrenergic receptors were unchanged with either sucrose feeding or cold exposure. In contrast, food deprivation did not alter BAT beta-adrenergic receptor density. Forskolin-stimulated adenylate cyclase activity increased in BAT after sucrose feeding or cold exposure but not after food deprivation. The ratio of isoproterenol-stimulated to forskolin-stimulated adenylate cyclase activity decreased in the sucrose-fed and cold-exposed rats but not in the food-deprived rats. These data suggest that in BAT, sucrose feeding or cold exposure result in downregulation of beta-adrenergic receptors and that isoproterenol-stimulated adenylate cyclase activity was limited by receptor availability.

Involvement of the sympathetic nervous system in lipolytic activity in brown adipose tissue of cold acclimated rats

In cold acclimated rats, in vitro, NE led to a significant increase in release of FFA and glycerol in denervated IBAT. In vivo, study of arteriovenous differences showed that the denervated BAT loses its full capacity to utilize FFA and glycerol released by NE. After denervation an increase of blood flow in Sulzer's vein was observed. This effect appeared immediately after intervention whereas the effect on fat metabolism appeared later. In cold acclimated rats, the sympathetic nervous system appears to be an important regulator of fatty acid metabolism in BAT.

Beta-adrenergic receptors and cAMP levels of rat parotid and submandibular glands during chronic isoproterenol treatment

The number of cell surface beta-adrenergic receptors and the level of cyclic AMP of the parotid and the submandibular gland were examined in rats treated for up to 10 days with twice daily injections of the beta-adrenergic agonist, isoproterenol. Receptor densities of 125 +/- 8.7 fmol/mg membrane protein for the parotid and 60.1 +/- 5.6 fmol/mg for the submandibular glands were found with [3H]dihydroalprenolol (beta-adrenergic receptor antagonist) binding of glands from control rats. No change from levels of controls was found in the number of beta-receptors of the submandibular gland with chronic isoproterenol stimulation; the parotid glands, on the other hand, showed a 22% decrease in dihydroalprenolol binding from the 4th until the 8th day of treatment. By day 10 of isoproterenol treatment the parotid gland demonstrated a shift from a population consisting of primarily beta-adrenergic receptors to one consisting of equal numbers of beta 1- and beta 2-adrenoceptors. The basal level of cAMP present in cell lysates remained unchanged in the isoproterenol-treated submandibular gland while the parotid gland showed a 30-40% decrease. Control and isoproterenol-treated animals demonstrated the same time course of cAMP accumulation after a single challenge with isoproterenol.

Thermogenic and metabolic consequences of thyroid hormone treatment in brown and white adipose tissue

Male rats were treated with triiodothyronine in the drinking water for 12 days. In vitro rates of isoprenaline stimulated lipolysis were significantly greater in brown but not white adipose tissue. Rates of [14C]glucose incorporation into triacylglycerols were significantly reduced in BAT (brown adipose tissue) and WAT (white adipose tissue) under basal and isoprenaline stimulated conditions. In a second experiment, hyperthyroid animals showed impaired weight gain, despite increased food intake during 19 days' treatment. Energy expenditure on days 5 and 12, and BAT core temperature differences (TBAT - TCORE) on day 19, were significantly greater than in control animals. Epididymal white fat pad weight was reduced and interscapular brown fat pad weight increased by triiodothyronine treatment.

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.

Characteristics of beta-adrenergic receptors in isolated cells and in crude membranes of brown adipose tissue

In relation to decreased metabolic sensitivity to catecholamines observed, in vitro, in brown fat of cold-acclimated rats, beta-adrenergic receptors were studied in isolated cells and in a crude membrane preparation from rat interscapular brown adipose tissue. [3H] dihydroalprenolol binding had the same characteristics in both types of preparation; competition studies of [3H] dihydroalprenolol binding led to the characterization of beta 1 subtype adrenergic receptors with a lower affinity of beta-adrenergic agonists for [3H] dihydroalprenolol binding sites in membranes than that found in isolated cells. Cold acclimation produced, in isolated cells only, a decrease of 41% in the [3H] dihydroalprenolol binding sites and a beta-adrenergic agonist affinity increase. It is concluded that beta-adrenergic receptor decrease could be a factor, at the hormone receptor interaction level, in the regulation of the transmission of biological action responsible for the cold-induced decrease in catecholamine responsiveness in brown adipose tissue. For a study of the desensitization process in brown fat, isolated cells seem to offer certain advantages over a crude membrane preparation.

Brown adipose tissue and thermogenesis in hypophysectomized rats in relation to temperature acclimation

The aim of this work was to test the role of pituitary dependent hormones in cold-induced non-shivering thermogenesis. In the 28 degrees C-acclimated rat, hypophysectomy inhibited body growth and led to an atrophy of thyroid and adrenals. In brown adipose tissue (BAT) some alterations were induced which are usually observed after cold acclimation of the animal: increase in relative weight, decreases in the relative amount of lipids, increases in the amounts of protein and DNA and modification of the proportions of several phospholipid fatty acids; moreover, basal lipolysis, in vitro, was enhanced to the same extent as that following cold acclimation of the normal rat. The in vivo stimulation by norepinephrine (NE) of O2 consumption (test for nonshivering thermogenesis) and of fatty acid release into blood were suppressed. Progressive cold acclimation of the hypophysectomized rats at 15 degrees C led to a hypertrophy of BAT to the same extent as in the sham-operated animals. The in vivo sensitivity to NE was partially restored. The results suggest that hypophysectomy does not suppress the ability to acclimate to moderate cold by means of BAT dependent non-shivering thermogenesis. However, the low ability to produce heat seems to indicate that pituitary or pituitary-dependent hormones are necessary to optimize the cold stimulation of brown fat thermogenesis.

The molecular basis for adrenergic desensitization in hamster brown adipose tissue: uncoupling of adenylate cyclase activation

A crude membrane fraction was prepared from hamster brown adipose tissue. Extensive washing of the crude membranes was crucial for the appearance of specific beta-adrenergic receptor binding as assessed by (-)-[3H]dihydroalprenolol. Adrenergic agents competed for the specific binding sites with beta 1-specificity. Binding characteristics were very similar to those earlier found in intact cells, supporting our previous finding that a single (non-tumour) mammalian cell may contain as many as 60,000 beta-adrenergic receptors. Desensitization in situ (i.e. chronic norepinephrine stimulation due to cold acclimation) only marginally affected the number of beta 1-receptors and their affinity (Ki) for norepinephrine. Total (fluoride-stimulated) adenylate cyclase increased somewhat, but the Kact for norepinephrine slightly decreased. Thus the ratio Ki/Kact was rather unaffected by cold acclimation. However, the fraction of adenylate cyclase which could be stimulated by norepinephrine decreased drastically. GTP introduced a low-affinity form (for agonist) of the receptor. The form observed in isolated cells must primarily be the high-affinity form. The basis for desensitization must reside in a diminished ability to transfer the signal from the receptor to the cyclase. This change may be molecularly located in the N-protein or in its interaction with the receptor.