Biochemical aspects of acclimation to cold

Metabolism, temperature, and ventilation

In mammals and birds, all oxygen used (VO2) must pass through the lungs; hence, some degree of coupling between VO2 and pulmonary ventilation (VE) is highly predictable. Nevertheless, VE is also involved with CO2 elimination, a task that is often in conflict with the convection of O2. In hot or cold conditions, the relationship between VE and VO2 includes the participation of the respiratory apparatus to the control of body temperature and water balance. Some compromise among these tasks is achieved through changes in breathing pattern, uncoupling changes in alveolar ventilation from VE. This article examines primarily the relationship between VE and VO2 under thermal stimuli. In the process, it considers how the relationship is influenced by hypoxia, hypercapnia or changes in metabolic level. The shuffling of tasks in emergency situations illustrates that the constraints on VE-VO2 for the protection of blood gases have ample room for flexibility. However, when other priorities do not interfere with the primary goal of gas exchange, VE follows metabolic rate quite closely. The fact that arterial CO2 remains stable when metabolism is changed by the most diverse circumstances (moderate exercise, cold, cold and exercise combined, variations in body size, caloric intake, age, time of the day, hormones, drugs, etc.) makes it unlikely that VE and metabolism are controlled in parallel by the condition responsible for the metabolic change. Rather, some observations support the view that the gaseous component of metabolic rate, probably CO2, may provide the link between the metabolic level and VE.

Toward an animal model of childhood-onset obesity: follow-up of OLETF rats during pregnancy and lactation

The Otsuka Long-Evans Tokushima Fatty (OLETF) rat model of obesity (a spontaneous CCK1 receptor knockout) has been extensively studied as model of hyperphagia-induced obesity. In previous studies, young OLETF rats presented abnormal eating patterns [compared with Long-Evans Tokushima Otsuka (LETO) controls] in a variety of independent ingestion and nursing tests during the suckling period. The aim of the present study was to characterize the early emergence of abnormal adiposity in the pups. Moreover, because both the dams and the pups present the genetic mutation, a close follow-up of the dams' body weight and intake during pregnancy and lactation was performed to examine the circumstances that contribute to build up the pups' early adiposity. Compared with controls, OLETF pups presented higher fat levels, larger adipocytes, and increased waist circumference as early as postnatal day 7 and this profile persisted to the age of weaning. While LETO dams gained weight throughout pregnancy and lactation, OLETF dams were obese and hyperphagic during pregnancy but lost weight during lactation, probably as a result of rearing hyperphagic pups. Current and previous results suggest a possible influence of the dams' obesity during gestation and a high investment in nursing time during lactation on the pups' obesity levels during childhood. This, combined with the innate hyperphagia repeatedly observed in the pups at these early ages, makes the OLETF strain a useful tool in the research of childhood-onset obesity.

Metabolic response to cooling temperatures in chicken embryos and hatchlings after cold incubation

We asked to what extent cold exposure during embryonic growth, and the accompanying hypometabolism, may interfere with the normal development of thermogenesis. White Leghorn chicken eggs were incubated in control conditions (38 degrees C) or at 36 or 35 degrees C. Embryos incubated at a lower temperature (34 degrees C) failed to hatch. The cold-incubated embryos had lower oxygen consumption (VO2) and body weight (W) throughout incubation, and hatching was delayed by about, respectively, 1 and 2 days. The W-VO2 relationship of the cold-incubated embryos was as in controls, indicating that cold-induced hypometabolism was at the expense of the growth, not the maintenance, component of VO2. At embryonic day E11, the metabolic response to changes in ambient temperature (T) over the 30-39 degrees C range was typically poikilothermic, with Q10 = 1.8-1.9, and similar among all sets of embryos. Toward the end of incubation (E20), the thermogenic responses of the cold-incubated embryos were significantly lower than in controls. This difference occurred also in the few-hour old hatchlings (H1), even though, at this time, W was similar among groups. Exposure to cold during only the last 3 days of incubation (from E18 to H1), i.e. during the developmental onset of the endothermic mechanisms, did not lower the thermogenic capacity of the hatchlings. In conclusion, sustained cold-induced hypometabolism throughout incubation blunted the rate of embryonic growth and the development of thermogenesis. This latter phenomenon could be an example of epigenetic regulation, i.e. of environmental factors exerting a long-term effect on gene expression.

Deiodinating activity in the brown adipose tissue of rats following short cold exposure after strenuous exercise

Interscapular brown adipose tissue (IBAT) activity is controlled by the sympathetic nervous system, and factors that influence thermogenesis appear to act centrally to modify the sympathetic outflow to IBAT. Cold exposure produces a rise in IBAT temperature as a result of the increase in sympathetic outflow to IBAT. This is associated with an increased thyroid activity. 3,5,3'-triiodothyronine (T3) and T4 levels increase during strenuous exercise, and, at the end of the exercise bout, a decrease of T3 and T4 levels, with an increase in TSH during the following 4-5 days, is seen. We evaluated the effect of strenuous exercise on 5'-deiodinase (5'-D) activity in IBAT in normal environmental conditions and after short (30 min) cold exposure. 5'-D activity is lower in rats at basal condition. Short cold exposure (SCE) increases 5'-D in IBAT both in exercising rats and in sedentary rats. However, this increase is lower in exercising animals. Strenuous exercise can reduce 5'-D activity in normal environmental conditions and after SCE. Probably, other compensatory mechanisms of heat production are active in exercising rodents.

Serotonin effect on deiodinating activity in the rat

Serotonin (5-HT) and thyroid hormones are part of a complex system modulating eating behaviour and energy expenditure. 5-Deiodinase (5-D) converts the relatively inactive thyroxine (T4) to triiodothyronine (T3), and its activity is an indirect measure of T3 production in peripheral tissues, particularly in the brain, intrascapular brown adipose tissue (IBAT), heart, liver, and kidney. We evaluated the effect of 5-HT on 5'-D activity during basal conditions and after short (30 min) cold exposure (thyroid stimulating hormone stimulation test, TST). 5'-D activity was assessed in the liver, heart, brain, kidney, and IBAT. TST increases 5'-D activity in the brain, heart, and IBAT and decreases it in kidney, leaving it unchanged in the liver. 5-HT alone did not modify 5'-D activity in the organs under study but decreased it in the IBAT, heart, and brain when injected before the TST was administered. Our results confirm the important role of 5-HT in thermoregulation, given its peripheral site of action, in modulating heat production controlling intracellular T3 production. These effects are more evident when heat production is upregulated during cold exposure in organs containing type II 5'-D, such as the brain, heart, and IBAT, which are able to modify their function during conditions that alter energy balance. In conclusion, 5-HT may also act peripherally directly on the thyroid and organs containing type II 5'-D, thus controlling energy expenditure through heat production.

Thermal and respiratory control in young rats exposed to cold during postnatal development

We questioned to what extent sustained increases in metabolic rate during the neonatal period may influence the development of thermal and respiratory control. Male rats were exposed to cold (14 degrees C) for the first 3 weeks, which increased metabolic rate with small effects on body growth. Measurements were performed at 1 month of age, when the body weight of the Cold group averaged approximately 88% of Controls. In Cold rats, the concentration of the uncoupling protein of the brown adipose tissue was increased. Acute exposures to different ambient temperatures (5, 15, 25 and 35 degrees C) provoked changes in body temperature similar in Cold and in Control rats. At these temperatures, small differences in the absolute values of oxygen consumption (Vdot;(O(2))) between the two groups could be explained by the differences in body weight. Hematocrit and lung weight of Cold rats were as in Controls, but the lung protein-DNA ratio was increased because of a drop in lung cellularity. The resting ventilation-oxygen consumption ratio (Vdot;(E)/Vdot;(O(2))) was similar between Cold and Controls. Also the changes in Vdot;(O(2)) and Vdot;(E) during acute hypoxia (10% O(2)) or hypercapnia (5% CO(2)), and the corresponding hyperventilatory responses (increases in Vdot;(E)/Vdot;(O(2))) did not significantly differ between the two groups. In conclusion, in the rat, the increased metabolic requirements caused by cold exposure during the early postnatal phases improved the thermogenic capacity, while having negligible impact on the development of respiratory control.

Thermogenesis in newborn rats after prenatal or postnatal hypoxia

Oxygen consumption (VO2) was measured in normoxia as ambient temperature (Ta) was lowered from 40 to 15 degrees C, at the rate of 0.5 degrees C/min (thermoneutrality approximately 33 degrees C). In 2-day-old rats born in hypoxia after hypoxic gestation, the Ta-VO2 relationship was as in controls; their interscapular brown adipose tissue (IBAT) was hypoplastic (less proteins and DNA), with lower concentration of the mitochondrial uncoupling protein thermogenin. In 8-day-old rats exposed to hypoxia postnatally (day 2 to day 8), at any Ta below thermoneutrality VO2 was higher than in controls; also, in this group IBAT was hypoplastic with decreased thermogenin. Additional measurements under various experimental conditions indicated that the increased thermogenic capacity was not explained by the smaller body mass and increased blood oxygen content or by the eventuality of intermittent cold stimuli during the chronic hypoxia. On the other hand, chronic hypercapnia (3% CO2 in normoxia, from day 2 to day 8) also resulted in increased normoxic thermogenesis. We conclude that chronic hypoxia in the perinatal period 1) reduces IBAT mass and thermogenin concentration and 2) can increase the newborn's thermogenic capacity because of stress-related mechanisms not specific to hypoxia.

Long photophase is not a sufficient stimulus to reduce thermogenic capacity in winter-acclimatized short-tailed field voles (Microtus agrestis) during long-term cold acclimation

The thermogenic capacity of brown adipose tissue in winter- and summer-acclimatized short-tailed field voles (Microtus agrestis) was investigated by examining changes in mass of brown adipose tissue, the ratio of white adipose tissue to brown adipose tissue, the concentration of the uncoupling protein (thermogenin) in whole depots (micrograms) and in mitochondrial mass (micrograms.mg-1) and the activity of cytochrome c oxidase in the depots (mmol.min-1). The concentration of thermogenin in winter-acclimatized voles (n = 8), per brown adipose tissue depot and per mitochondrial mass, was significantly higher than in summer-acclimatized voles (n = 6). There was no significant difference in the level of cytochrome c oxidase activity between these two groups. Four groups of winter-acclimatized voles (n = 6 in each group) were exposed to 5 degrees C for 10, 20, 50 and 100 days in a 14L:10D photoperiod. Body mass, brown adipose tissue mass, white adipose tissue mass and basal metabolic rate were significantly positively related to the length of time cold exposed up to 100 days. There was a significant inverse relationship between the ratio of white to brown adipose tissue mass and the duration of cold exposure. There was no significant relationship between thermogenin concentration, either per depot or in mitochondrial mass of brown adipose tissue, with the length of time cold exposed. The level of cytochrome c oxidase activity increased significantly from control levels to a maximum after 10 days in the cold but decreased from 10 days onwards. In winter-acclimatized M. agrestis, a 14L:10D photoperiod is not a sufficient stimulus to reduce thermogenic capacity during cold acclimation.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostaglandins, brown fat and weight loss

In obesity, a situation is created in which energy intake exceeds energy expenditure. The three components of energy expenditure are resting metabolism, physical activity, and thermogenesis. Increasing attention is being paid to the role of impaired energy expenditure in obesity. Evidence indicates that impairment in activity of the sympathetic nervous system, which stimulates thermogenic processes, contributes to the etiology of obesity. In addition, insulin resistance, a well-recognized metabolic consequence of obesity, appears to interfere with feeding-related, insulin-mediated increases in thermogenesis in brown adipose tissue. This thermogenic defect results in reduced energy buffering by brown adipose tissue leading to deficient energy expenditure and an increased efficiency in weight gain. A unique weight loss program, The Princeton Metabolic Diet Program, is presented. The Program stimulates metabolism by stimulating the sympathetic nervous system and correcting insulin resistance, thereby enhancing thermogenesis in brown adipose tissue. Methods include: 1) alternating diet composition and caloric intake and, 2) the use of nutritional metabolic stimulants. This type of non-toxic therapy, directed at correcting biochemical defects, will enhance metabolic mechanisms and induce weight loss.

Thermogenic capabilities of the opossum Monodelphis domestica when warm and cold acclimated: similarities between American and Australian marsupials

1. Monodelphis domestica is a small marsupial mammal from South America. Its thermogenic abilities in the cold were determined when the opossums were both warm (WA) and cold (CA) acclimated. Maximum heat production of M. domestica was obtained at low temperatures in helium-oxygen. 2. Basal metabolic rate (BMR) in the WA animals was 3.2 W/kg and mean body temperature was 32.6 degrees C at 30 degrees C. These values were lower than those generally reported for marsupials. Nevertheless, these M. domestica showed considerable metabolic expansibility in response to cold. Sustained (summit) metabolism was 8-9 times BMR, while peak metabolism was 11-13 times BMR. These maximum values were equal to, or above, those expected in small placentals. 3. Cold acclimation altered the thermal responses of M. domestica, particularly in warm TaS. However, summit metabolism was not significantly increased; nor did M. domestica show a significant thermogenic response to noradrenaline, which in many small placentals elicits non-shivering thermogenesis. The thermoregulatory responses of this American marsupial were, in most aspects, similar to those of Australian marsupials. This suggests that the considerable thermoregulatory abilities of marsupials are of some antiquity.

Great adaptability of brown adipose tissue mitochondria to extreme ambient temperatures in control and cold-acclimated gerbils as compared with mice

1. The gerbil (Gerbillus campestris) is a desert rodent able to tolerate high (38 degrees C) and low (-20 degrees C) ambient temperatures, probably due to both its low resting metabolic rate in hot environment and its high peak metabolic rate in cold. 2. Measurement of mitochondrial state IV respiration and cytochrome-oxidase activity (COX) were made in interscapular brown adipose tissue (IBAT), liver and hind limb muscles of gerbils and mice of nearly equal body mass, acclimated for 4 weeks at cold ambient temperature (CA) or reared at thermoneutrality (TN). 3. The most striking difference between these two animal species appears to be in IBAT mitochondria: in TN animals, the level of state IV respiration and COX activity was lower in gerbils than in mice, but the cold acclimation-induced increase in these parameters was greater in gerbils than in mice. 4. Alternatively, in gerbils as in mice, cold acclimation induced a reduction in muscle mitochondrial COX activity. No important change due to cold acclimation was observed in liver mitochondria, either in gerbils or in mice. 5. As compared with mice, the lower state IV respiration in IBAT mitochondria from TN gerbils may explain their low RMR, whereas the higher COX activity of IBAT mitochondria from CA gerbils may explain their higher PMR. 6. As a result of this great adaptability of BAT mitochondria, the gerbil seemed to be able to live in a wide range of ambient temperatures in its natural habitat.

The effect of intermittent cold treatment on the adipose tissue of the cat. Apparent transformation from white to brown adipose tissue

Young cats (Felis domestica), aged 10-13 weeks, were intermittently exposed to a temperature of -30 degrees C for two periods of 1 hr per day. Animals were sacrificed on the 7th day and adipose tissue from the perirenal, pericardial, axillary, interscapular, and subcutaneous-inguinal depots was examined by electron microscopy and analysed stereologically. All examined depots were morphologically changed after cold treatment. Adipose tissue of perirenal, pericardial, and axillary depots showed a greater decrease in lipid content than the interscapular and subcutaneous-inguinal depots, but other changes were similar. Compared to the control group, which consisted of typical white adipose tissue, the diameter of adipose cells examined after cold treatment was diminished, in extreme cases to 18 micron (from 75 micron in the control group). The number of capillaries per cell was doubled (as evaluated on semithin sections). The most dramatic changes were observed in the mitochondria. Their volume increased to 0.48 micron 3 (from 0.13 micron 3 in the control), and the surface density of mitochondrial cristae per mitochondrial volume increased to 50 micron 2/micron 3 (from 32 in the control). Pleomorphism in mitochondrial size and inner structure and the presence of intramitochondrial electron-dense bodies and crystalline structures led us to conclude that the cold stress induced an increase in the absolute number of mitochondria in the adipose cells. The adipose tissue after cold treatment thus morphologically resembled the brown adipose tissue of cold-acclimated rodents. This implies that the adipose tissue of young cats can change its morphology and function, depending on the requirements of the organism.

Parallel measurements of heat production and thermogenin content in brown fat cells during cold acclimation of rats

The maximum thermogenic capacity of brown fat cells from control and cold acclimated rats was measured using a continuous-flow microcalorimetric system. The content of the 32.000 D, brown fat specific protein, thermogenin, was measured in the cells used for heat production measurements by competitive ELISA. The ratio between the maximal thermogenic capacity and the amount of thermogenin for control and cold acclimated rats was compared. It was found that the ratio between the two parameters decreased during cold acclimation due to a decrease in maximal thermogenic capacity and an increase in the amount of thermogenin, indicating regulation of heat production either at thermogenin or receptor level.

Inhibition of acetyl-carnitine oxidation in rat brown-adipose-tissue mitochondria by erucoyl-carnitine is due to sequestration of CoA

The cause underlying the inhibitory effect of erucoyl-carnitine on acetyl-carnitine oxidation in rat brown-adipose-tissue mitochondria was investigated. The inhibition was shown to be of a noncompetitive nature, with an I50 of about 10 microM erucoyl-carnitine. Erucoyl-carnitine did not inhibit the respiratory chain or the citric acid cycle to a significant degree. Incubation of mitochondria with erucoyl-carnitine led to about 2/3 of all CoA being sequestered in the form of acid-insoluble esters (probably erucoyl-CoA). This sequestration had an apparent Km of about 10 microM. Erucoyl-carnitine also inhibited pyruvate oxidation with an I50 of about 10 microM. When added at this concentration, it also inhibited the oxidation of a wide variety of acyl-carnitines by about 50%, despite very different oxidation rates of these different acyl-carnitines. It was concluded that the inhibitory effect of erucoyl-carnitine on all CoA-dependent substrates could be adequately explained by the suggestion that erucoyl sequesters a significant fraction of mitochondrial matrix CoA as slowly metabolizable erucoyl-CoA esters. Possible physiological effects of this sequestration for brown adipose tissue thermogenesis are discussed.

Limitations in the use of the enzyme-linked immunosorbent assay (ELISA) for identification and quantification of thermogenin

The use of immunological assays, ELISA and RIA, for the identification and quantification of thermogenin (the brown adipose tissue-specific, GDP-binding, 32 kDa uncoupling protein) raises doubts regarding the exclusive occurrence of thermogenin in brown adipose tissue. Weak reactions between mitochondria from rat liver, rat skeletal and heart muscle and hamster white adipose and thermogenin antibodies have been observed (Cannon et al., 1982; Lean et al., 1983; Hansen et al., 1984). In order to study whether these reactions were due to thermogenin in tissues other than brown adipose tissue (BAT) or due to non-specific binding of thermogenin antibodies, a protein from rat liver mitochondria and a protein from tubifex mitochondria were isolated by the same procedure as thermogenin. The 2 proteins had almost the same molecular weight as thermogenin and reacted with thermogenin antibodies in ELISA and dot-blotting, but did not bind GDP and had an amino acid composition different from that of thermogenin. It is concluded that the weak reactions seen between thermogenin antibodies and mitochondria from different tissues other than BAT are due to non-specific binding, and that antibody cross-reactivity alone is unsuitable for the identification of thermogenin.

[3H]GDP binding and thermogenin amount in brown adipose tissue mitochondria from cold-exposed rats

Brown fat mitochondria were isolated from cold-exposed and control rats, and their content of the brown-fat-specific 32-kDa "uncoupling" protein thermogenin determined both by the traditional [3H]GDP-binding method and by the recently developed enzyme-linked immunosorbent assay (ELISA). In mitochondria isolated from both cold-acclimated (3 wk at 4 degrees C) and cold-exposed rats (24 h), an increase in thermogenin content was observable, both when estimated by the [3H]GDP-binding method and by the ELISA assay, and there was no statistically significant difference in the magnitude of these increases in the two methods. In 1 h cold-exposed rats there was no increase in [3H]GDP binding or in the ELISA reaction. When the amount of thermogenin was plotted against [3H]GDP binding in the different states, a relationship of 75,000 g thermogenin per mole GDP bound was obtained. Based on the resolution of these two methods, and under the three conditions investigated, it was concluded that there was no reason to postulate the existence of a "masked" form of thermogenin or of an "unmasking" process and that thermogenin in the mitochondria, as in the isolated state, has apparently one GDP binding site per dimer.

Brown adipose tissue in cafeteria-fed hamsters

Hamsters consuming a "cafeteria diet" had more brown adipose tissue than did chow-fed hamsters. The growth of the brown fat depots in cafeteria-fed hamsters was accompanied by increases in tissue protein and cytochrome oxidase. To assess the thermogenic capacity of brown fat mitochondria, the binding of GDP to isolated mitochondria was measured. Mitochondrial GDP binding was not affected by feeding the cafeteria diet for 4 wk, but more prolonged cafeteria feeding for 8 wk did, however, increase the binding of GDP to isolated mitochondria. The morphology of brown adipose tissue was altered during cafeteria feeding. The brown adipose tissue of cafeteria-fed hamsters had more large unilocular cells than did the brown adipose tissue of chow-fed hamsters. In addition, the average adipocyte diameter was greater in brown adipose tissue of cafeteria-fed hamsters. These data support the presence of a dietary regulation of brown adipose tissue growth in hamsters. The growth of brown adipose tissue in hamsters eating the cafeteria diet appears to result largely from proliferation of adipocytes, as evidenced by the increases in tissue protein and cytochrome oxidase during cafeteria feeding, but some hypertrophy of the adipocytes also occurs. A dietary regulation of brown fat thermogenic capacity is also apparent but this regulation is evident only after more prolonged periods of cafeteria feeding. Hamsters eating a cafeteria diet increase their caloric intake but have the same or greater body weight gain efficiency as do chow-fed animals. The absence of dietary stimulation of thermogenesis may underlie the similar efficiencies of weight gain in chow- and cafeteria-fed hamsters.

Effects of fasting and aminophylline on norepinephrine-stimulated non-shivering thermogenesis

In an attempt to further elucidate the mechanisms of fasting-depressed maximum thermogenesis and cold tolerance, norepinephrine (NE)-stimulated non-shivering thermogenesis (NST) in cold-acclimated rats was used as a functional index of possible alterations in adrenergic efficacy after fasting. Fasting decreased the magnitude of maximum NE-Stimulated NST by 18.2% [6.87 +/- 0.47 Kcal (Kg X 75 X min)-1 well-fed vs. 5.81 +/- 0.39 Kcal (Kg X 75 X min)-1 fasted], but the apparent adrenergic binding affinity was not affected [Ke = 0.43 micrograms NE min-1 well-fed vs. 0.55 micrograms NE min-1 fasted]. Pretreatment with aminophylline [15 mg Kg-1, i.p.], a phosphodiesterase inhibitor, restored the fasting-depressed NE-stimulated NST to the fed level. The results suggest that the depression of maximum thermogenesis after fasting is not due to changes in adrenergic binding characteristics but to alteration in cAMP production/degradation, resulting in decreased substrate mobilization for thermogenesis.

Enzyme-linked immunosorbent assay (ELISA) studies of the interaction between mammalian and avian anti-thermogenin antibodies and brown-adipose-tissue mitochondria from different species

Three different antibody preparations, rabbit anti-hamster and rabbit anti-rat thermogenin sera and chicken anti-rat thermogenin IgG, were tested for cross-reactivity towards isolated thermogenin and BAT mitochondria from different mammalian species using an ELISA-technique. It was found that the antibody preparations readily cross-reacted with different species, but that the affinity of the antibody preparations was greater towards the homologous species than the other species. The reactivity of an antibody preparation towards mitochondria from different tissues from the homologous species was also tested, and the exclusive occurrence of thermogenin in BAT could be confirmed.

Seasonal changes in white and brown adipose tissues in Clethrionomys gapperi (red-backed vole) and in Microtus pennsylvanicus (meadow vole)

Mass and gross composition of white and brown adipose tissues and of skeletal muscle were determined for C. gapperi and for M. pennsylvanicus from monthly samples of a 1-year period. Amounts of brown fat increased throughout autumn to a maximum in late winter and then declined to a minimum in the spring. Gross composition remained relatively constant throughout the year. Changes in white fat showed a trend similar to changes in brown fat. Relatively low mean values for muscle mass during summer were due to a change in the age structure of the vole population.

The relationship between extramitochondrial Ca2+ concentration, respiratory rate, and membrane potential in mitochondria from brown adipose tissue of the rat

The ability of isolated mitochondria from rat brown-adipose tissue to regulate extramitochondrial Ca2+ (measured by arsenazo) was studied in relation to their ability to produce heat (measured polarographically). The energetic state of the mitochondria was expressed as a membrane potential, delta psi (estimated with safranine), and was varied semi-physiologically by the use of different GDP concentrations. In these mitochondria GDP binds to the 32-kDa polypeptide, thermogenin, which regulates coupling. Ca2+ uptake (at 5 microM extramitochondrial Ca2+) was maximal at delta psi greater than 150 mV. Basal Ca2+ release increased from 1 to 2 nmol x min-1 x mg-1 below 150 mV. Na+ -stimulated rate of Ca2+ release was stable within the investigated delta psi span (100-160 mV). Initial Ca2+ levels were maintained below 0.2 microM for 100 mV less than delta psi less than 160 mV. Ca2+ levels maintained after Ca2+ challenge (20 nmol Ca2+ x mg-1) were below 0.4 microM for delta psi greater than 135 mM. Respiration was unstimulated for delta psi greater than 150 mV and was maximal at delta psi less than or equal to 135 mV. In the presence of well-oxidised substrates, the respiration at maximally activated thermogenin was markedly below fully uncoupled respiration and was probably limited by thermogenin activity--i.e. by a limited H+ reentry (OH- exit) and therefore by a membrane potential maintained at about 135 mV. It is concluded that at membrane potentials of 135 mV and above the mitochondria exhibit full Ca2+ control and are able to regulate thermogenic output up to maximum without interfering with this Ca2+ control. Membrane potential probably does not decrease below 135 mV in vivo. Therefore, Ca2+ homeostasis and thermogenesis are non-interfering and can be hormonally independently regulated, e.g. by alpha-adrenergic and beta-adrenergic stimuli, respectively.