Cold acclimation (4 degrees C) and "cafeteria diets" increased the thermic response of rats to catecholamines. This phenomenon was accompanied by six- to eightfold increases of interscapular brown adipose tissue (IBAT) weight, total tissue cytochrome oxidase activity, and total number of brown adipocytes. Quantitative radioautographic experiments using [3H]thymidine disclosed that cold exposure markedly enhanced the mitotic activity in blood capillaries and small-venule endothelial cells, adipose tissue interstitial cells, and preadipocytes rather than in fully differentiated brown adipocytes. IBAT mitotic index increased 70 times over control values after only 2 days of cold exposure. Thereafter, the proliferative activity progressively decreased. IBAT cell composition was modified during cold acclimation as the percentage of interstitial cells and preadipocytes increased over the other cellular types. Because brown adipose tissue is the principal site of norepinephrine-induced thermogenesis in homeothermal animals, it is suggested that brown adipocyte proliferation from precursor cells represents the fundamental phenomenon explaining the increased capacity of cold-acclimated animals to respond calorigenically to catecholamines.

The effects of exercise training and food restriction on the regulation of lipolysis were studied comparatively in adipocytes isolated from male and female rats. Exercise training inhibited cell proliferation in parametrial, but not in epididymal adipose tissue, whereas it significantly reduced adipocyte size in both fat depots. Adipocyte capacity for responding lipolytically to epinephrine (10 microns) or to ACTH (1 micron) was markedly increased by exercise training. Enhanced lipolysis was also observed when cells isolated from exercise-trained animals were stimulated by bypassing with dibutyryl cyclic AMP (5 mM) or theophylline (5 mM) the early metabolic steps associated with hormonal activation of the adenylate cyclase complex. Significantly, binding of (-)-[3H]dihydroalprenolol to cellular receptor sites was not affected by exercise training. It is therefore concluded that exercise training increases adipocyte responsiveness to lipolytic hormones at a metabolic step distal to stimulus recognition by adrenoreceptors, possibly at the level of protein kinases or lipases. Food restriction markedly reduced adipocyte size and partially mimicked the effects of exercise training on adipocyte proliferation and lipolysis.

Regulation of respiration by catecholamines was studied in adipocytes isolated from interscapular brown adipose tissue of warm-acclimated rats by rapid digestion of collagenase. (-)-Norepinephrine stimulated adipocyte respiration 10–12 times above basal values in less than 3 min. (Vmax = 410 +/- 29.5 nmol O2 . min-1 . 10(-6) cells-1). Stimulated respiration remained stable for at least 20 min, provided that cells were incubated in balanced salt media containing bicarbonate. The maximal capacity of total brown adipose tissue for norepinephrine-stimulated respitarion was estimated at 1.5 ml O2/min per rat. beta-Adrenergic agonists increased calorigenesis stereospecifically with an order of potency expected for respiratory stimulation via adrenoceptors of the beta 1-subtype: (-)-isoproterenol (1/2 Vmax = 2 nM) greater than (-)-norepinephrine (1/2 Vmax = 20 nM) approximately equal to (-)-epinephrine (1/2 Vmax = 40 nM) greater than corresponding (+)-stereoisomers. The alpha-adrenergic agonist phenylephrine (1/2 Vmax = 5 microM) stimulated adipocyte respiration as rapidly and as effectively as beta-agonists. Although alpha-adrenoreceptors are present in brown adipose tissue, studies with alpha- and beta-adrenergic antagonists revealed that norepinephrine elicits thermogenesis at physiological concentrations (less than or equal to 1 microM) predominantely via beta 1-adrenergic pathways.

No abstract

Rats consuming Coca-Cola and Purina chow ad libitum increased their total energy intake by 50% without excess weight gain. Their resistance to cold was markedly improved. These phenomena were characterized by significant increases in interscapular brown adipose tissue weight (IBAT) (91%), cellularity (59%), triglyceride content (52%), protein content (94%), and cytochrome oxidase activity (167%). In contrast, Coca-Cola consumption did not significantly affect the cellularity or triglyceride content of parametrial white adipose tissue (PWAT), although it slightly augmented PWAT weight. The effects of Coca-Cola on cold resistance, IBAT cellularity, and composition were entirely reproduced by sucrose, but not caffeine, consumption. Although caffeine also increased IBAT cellularity and composition, it significantly decreased the rate of body weight gain, PWAT weight, and adipocyte size. Moreover, it markedly inhibited adipocyte proliferation in PWAT thereby mimicking the effects of exercise training and food restriction (Bukowiecki et al., Am. J. Physiol. 239 (Endocrinol. Metab. 2): E422-E429, 1980). It is concluded a) that sucrose and Coca-Cola consumption improve the resistance of rats to cold, most probably by increasing brown adipose tissue cellularity, and b) that moderate caffeine intake might be useful for inhibiting proliferative activity in white adipose tissue, thereby preventing obesity.

The capacity of brown adipose tissue to respond calorigenically to catecholamines increases markedly during cold‐acclimation of adult rats. To investigate this phenomenon, we have used the potent radioactive ligand (‐)‐[3H]dihydroalprenolol to directly estimate the number, the density and the affinity of β‐adrenergic receptors in brown adipose tissue membranes from control, cold‐exposed and cold‐acclimated rats. Binding of (−)‐[3H]dihydroalprenolol to unfractionated membranes was rapid, stable, saturable and reversible. It displayed the affinity, specificity and stereoselectivity expected of binding to adenylate cyclase‐coupled β‐adrenergic receptors. β‐Adrenergic agonists competed for binding sites with an order of potency typical of the β1 subtype of adrenergic receptors: (−)‐Isoproterenol > (−)‐norepinephrine ≥ (−)‐epinephrine. Binding exhibited a remarkable stereoselectivity, the (−)‐isomers of β‐adrenergic agonists and antagonists being 34 to 280 times more potent than the (+)‐isomers in competing for (−)‐[3H]dihydroalprenolol binding sites. Total interscapular brown adipose tissue of the adult, warm‐acclimated rat contained 1.12 ± 0.08 pmol of (−)‐[3H]dihydroalprenolol binding sites. During cold‐acclimation, growth of the tissue was accompanied by a 4–5‐fold increase in the total number of recptor sites. However, this increase did not keep pace with the increse in brown adipose tissue cellularity (as estimated by total tissue DNA content), resulting in a 40–50% reduction in receptor density. The decrease in receptor density was associated with cold‐exposure rather than with cold‐acclimation. The affinity of (−)‐[3H]dihydroalprenolol receptor sites was not significantly altered by cold‐acclimation. The results of this study are interpreted as indicating that catecholamines released via activation of the sympathetic nervous system regulate both the density and the number of their own receptors in brown adipose tissue of cold‐exposed animals. The development of the hyperadrenergic response of this thermogenic tissue during cold‐acclimation may result from a marked organ hyperplasia associated with an increased number of β‐adrenergic receptor sites and cannot be explained by alterations in receptor density or affinity.

To investigate a possible role of catecholamines in mammary gland growth and differentiation, we have studied the characteristics of a specific beta-adrenergic receptor population during the different reproductive phases of the rat mammary gland, namely pregnancy and lactation. The functional response to mammary beta-adrenergic receptor stimulation was assessed by measurement of adenylate cyclase activity during the same physiological states of the gland [125I]Cyanopindolol (CYP) binds specifically to membranes prepared from lactating mammary glands. Scatchard analysis of the binding data shows the presence of a single class of high affinity sites, with an apparent Kd value of 25.0 +/- 0.4 pM and a binding capacity of 32.5 +/- 1.2 fmol/mg protein in lactating mammary glands at random stages of lactation. The order of potency of a series of agonists to compete for [125I]CYP binding is consistent with the interactions with a beta 2-subtype receptor. The binding of [125I]CYP to mammary glands also shows a marked stereoselectivity; the (-)isomers of isoproterenol and propranolol are more potent than their respective enantiomers. The radioautographic localization of [125I]CYP reveals the presence of specific beta-adrenergic receptors in the epithelial cells, alveoles, ducts, as well as adipocytes. [125I]CYP binding shows a 2- to 3-fold increase during pregnancy. Such a result correlates with parallel increases in stimulation of adenylate cyclase activity, the cytosolic progesterone receptor concentration, as well as plasma 17 beta-estradiol and progesterone levels. At parturition, a sharp decline in beta-adrenergic receptor concentration is observed, a finding concomitant with a drop in progesterone receptor levels as well as plasma estradiol and progesterone concentrations. During midlactation, beta-adrenergic receptors reach their maximal levels. The presence of specific beta-adrenergic receptors functionally coupled to the adenylate cyclase system and the marked changes in receptor capacity and distribution measured during the different physiological states of the mammary gland suggest that the mammary beta-adrenergic receptors are highly sensitive to changes in the hormonal milieu and provide a mechanism for a direct catecholaminergic influence on mammary gland growth and differentiation.

In order to gain further knowledge about the potential role of catecholamines in mammary carcinoma, we have used the potent beta-adrenergic antagonist cyanopindolol (CYP) as iodinated ligand to characterize beta-adrenergic receptors in membranes prepared from mammary tumors induced by dimethylbenz(a)anthracene (DMBA) administration in the rat. The binding of [125I]CYP to membrane preparations of DMBA-induced rat mammary tumors is rapid at room temperature, reaching half maximal specific binding at 30 min of incubation. Scatchard analysis of the data indicates that [125I]CYP binds to a single class of high affinity sites (114 +/- 2.1 fmoles/mg protein) at an apparent KD value of 38.0 +/- 0.3 pM. The order of potency of a series of agonists to compete for [125I]CYP binding is consistent with interaction with a beta 2-subtype receptor: zinterol greater than (-)isoproterenol greater than (-)epinephrine much greater than (-)norepinephrine. In addition, the potency of a series of specific beta 1 and beta 2 synthetic compounds to displace [125I]CYP in mammary tumors is similar to their potency in typical beta 2-adrenergic tissues. The binding of [125I]CYP to DMBA-induced rat mammary tumors shows a marked stereoselectivity, the (-)isomers of isoproterenol and propranolol being 150 and 80 times more potent, respectively, than their respective enantiomers. The autoradiographic localization of [125I]CYP performed on frozen sections revealed the presence of specific beta-adrenergic receptors in all the malignant cells. Spontaneous mammary tumors of aging (18-22 months) female rats have high levels of beta-adrenergic receptors. Castration decreased the concentration of [125I]CYP binding sites in DMBA-induced mammary tumors. A close correlation was observed between progressing, static, and regressing tumors after ovariectomy and beta-adrenergic receptor concentration. The presence of beta-adrenergic receptors in mammary tumors as well as the modulation of their level by ovarian hormones provides a mechanism for catecholaminergic influence in mammary cancer tissue.