Relationship between the tissue level of cyclic AMP and the fat cell size of human adipose tissue.

The relationship between mean fat cell size, maximal tissue cyclic AMP concentration, and glycerol release was investigated in human subcutaneous adipose tissue incubated in vitro with or without isoprenaline or noradrenaline added at maximal effective concentrations. Basal and stimulated glycerol release and cyclic AMP concentration were each related to the fat cell size. Whether or not the phosphodiesterase inhibitor theophylline was present in the incubation system, basal and noradrenaline-induced cyclic AMP levels were significantly correlated with the fat cell size. The noradrenaline-induced cyclic AMP levels resulted in twice as rapid glycerol release as could be expected from the basal ratio between glycerol release and cyclic AMP. Furthermore, both basal and noradrenaline-induced glycerol release in relation to the cyclic AMP levels were more rapid in enlarge fat cells. It is concluded that basal and catecholamine-induced production of cyclic AMP is related to the fat cell size and that a quantitative relationship exists between rate of lipolysis and maximal tissue levels of cyclic AMP in human adipose tissue. Basal and noradrenaline-induced lipolysis are probably regulated by different mechanisms and the lipolytic sensitivity to cyclic AMP seems increased in large fat cells.     

Studies on the mechanisms of epinephrine stimulation of lypolysis

The time course for epinephrine stimulation of lypolysis, cyclic AMP accumulation and activation of protein kinase was studied in adipose tissue from hypophysectomized rats. Triglyceride breakdown, as assessed by glycerol release, increased rapidly in response to epinephrine, maintained a constant rate as long as the hormone was present, and decreased rapidly to basal values when the hormone was removed. Cyclic AMP accumulation was transient peaking within 3 min of exposure to epinephrine and then declining to levels indistinguishable from basal by 9 min. Protein kinase activity in extracts also peaked at 3 min and thereafter declined to a level approximately 25% greater than resting activity. Peak levels of cyclic AMP, steady state levels of protein kinase activity and the rate of glycerol production were all related in a dose dependent manner to the concentration of epinephrine. These observations suggest that the spike in cyclic AMP levels may be necessary to trigger the activation of lipolysis, but was not sufficient to sustain an accelerated rate of tryglyceride breakdown. Continued activation of protein kinase, however, may be essential to sustained lipolysis.     

Studies on the mechanisms of epinephrine stimulation of lypolysis.

The time course for epinephrine stimulation of lypolysis, cyclic AMP accumulation and activation of protein kinase was studied in adipose tissue from hypophysectomized rats. Triglyceride breakdown, as assessed by glycerol release, increased rapidly in response to epinephrine, maintained a constant rate as long as the hormone was present, and decreased rapidly to basal values when the hormone was removed. Cyclic AMP accumulation was transient peaking within 3 min of exposure to epinephrine and then declining to levels indistinguishable from basal by 9 min. Protein kinase activity in extracts also peaked at 3 min and thereafter declined to a level approximately 25% greater than resting activity. Peak levels of cyclic AMP, steady state levels of protein kinase activity and the rate of glycerol production were all related in a dose dependent manner to the concentration of epinephrine. These observations suggest that the spike in cyclic AMP levels may be necessary to trigger the activation of lipolysis, but was not sufficient to sustain an accelerated rate of triglyceride breakdown. Continued activation of protein kinase, however, may be essential to sustained lipolysis.     

The antilipolytic action of insulin on adrenocorticotrophin-stimulated rat adipocytes. The roles of adenosine 3',5'-monophosphate and the protein kinase dependent on adenosine 3',5'-monophosphate

The extent to which a fall in cellular cyclic AMP could account for the antilipolytic action in rat epididymal adipocytes incubated with adrenocorticotrophic hormone was studied. The antilipolytic effect, measured by suppression of glycerol release, was always associated with a decrease in cyclic AMP, but the magnitude of the fall was modified by several factors. For example, it was greater when the cAMP level was high, as when it is at its peak after hormone stimulation, or when cell concentrations are low. Glucose did not modify appreciably the insulin effect on the nucleotide level. The inhibitory effects of insulin on corticotrophin-stimulated lipolysis and cyclic AMP levels were detectable at the concentrations of 1 microU/ml and were biphasic, with maximal effects at 10-100 microU/ml. Protein kinase activity ratio was similarly affected. Activity of cyclic-AMP-dependent protein kinase conformed closely to the level of cyclic AMP. There was no indication that insulin modified the sensitivity of the kinase to cyclic AMP. Insulin did not alter the relationship of cellular cyclic AMP levels to glycerol when adipocytes were incubated with various concentrations of corticotrophin. This was true, irrespective of whether measurements were made when cyclic AMP was on the upward rise after hormone stimulation, or on the decline. The curves obtained with and without insulin were superimposable. It is concluded that the inhibitory action of insulin on lipolysis in fat cells can be fully accounted for by a decrease in cyclic AMP.     

Inhibition of adenosine 3':k'-monophosphate accumulation white fat acids, lactate, and beta-hydroxybutyrate.

The large increase in cyclic AMP accumulation by rat white fat cells seen in the presence of lipolytic agents plus methylxanthines and adenosine deaminase was markedly inhibited by lactate. However, lipolysis was unaffected by lactate. Octanoate, hexanoate, heptanoate, and beta-hydroxybutyrate inhibited both cyclic AMP accumulation and lipolysis by rat fat cells. The mechanism by which these acids inhibit lipolysis differs from that for long chain fatty acids such as oleate. Oleate directly inhibited triglyceride lipase activity of homogenized rat adipose tissue. In contrast, octanoate, beta-hydroxybutyrate, and lacatate had no effect on triglyceride lipase activity. Hormone-stimulated adenylate cyclase activity of rat fat cell ghosts was inhibited by oleate and 4mM octanoate but not by 1.6 mM octanoate, heptanoate, hexanoate, beta-hydroxybutyrate or lactate. None of the acids affected the soluble protein kinase activity of rat adipose tissue. There was no stimulation by lactate, butyrate, beta-hydroxybutyrate, or octanoate of the soluble or particulate cyclic AMP antilipolytic action of a short chain acid such as octanoate or hexanoate was not accompanied by any drop in total fat cell ATP. The mechanism by which lactate lowers cyclic AMP but not lipolysis remains to be established.     

Free fatty acids as feedback regulators of adenylate cyclase and cyclic 3':5'-AMP accumulation in rat fat cells.

Rat fat cells incubated with lipolytic agents released substances to the medium which acted as feedback regulators of cyclic adenosine 3':5'-monophosphate (cyclic AMP) accumulation. The feedback regulators were not removed by adenosine deaminase. Dialyzed medium that had previously been incubated with fat cells in the presence of norepinephrine markedly inhibited cyclic AMP accumulation by fresh cells, whereas dialyzed medium from control cells did not inhibit cyclic AMP accumulation. The effects of lipolytic agents could be mimicked by adding dialyzed medium previously incubated with fat cells in the presence of oleic acid. This suggested that free fatty acids were the nondialyzable and adenosine deaminase-insensitive inhibitors of cyclic AMP accumulation released to the medium by fat cells incubated with lipolytic agents. The regulatory function of free fatty acids was related to the molar ratio of fatty acid to albumin. Profound inhibition of both lipolysis and cyclic AMP accumulation was seen as the free fatty acid/albumin ratio exceeded 3. The inhibition of cyclic AMP accumulation by oleate was seen as soon as there was a detectable increase in cyclic AMP due to lipolytic agents. Protein kinase activity (in the presence of cyclic AMP) of the infranatant obtained after centrifugation of fat cell homogenates at 48,000 x g was inhibited by medium from cells incubated with lipolytic agents or added oleate. Adenylate cyclase activity of rat fat cell ghosts was also inhibited by dialyzed or nondialyzed medium that previously had been incubated with lipolytic agents or added fatty acids. The direct addition of oleate markedly inhibited adenylate cyclase activity as the free fatty acid/albumin ratio exceeded 2. These data suggest that the prolonged drop in cyclic AMP accumulation seen during the incubation of rat fat cells with lipolytic agents is due to the inhibition of adenylate cyclase. This occurs when the free fatty acid/albumin ratio exceeds 3.     

Differential long-term effects of tannic acid on adenyl cyclase activity and lipolysis in rat adipocytes

Plants elaborate a variety of secondary metabolites such as hydrolysable tannins which are relatively abundant in fruits, vegetables and beverages in the human diet. We have studied the in vivo long-term effect consumption of tannic acid-supplemented drinking water (0.05%, w/v) on the rat adipocyte adenyl cyclase system and on lipolysis. We found that 14-day tannic acid supplementation did not significantly affect either body growth or food consumption, while fat pads weight was higher than that of the control, although the difference was not significant. On the other hand, tannic acid supplementation decreased both basal and isoproterenol-stimulated lipolysis significantly whereas cyclic AMP production as well as adenyl cyclase activity increased significantly. These results are at a first glance contradictory as cyclic AMP accumulation and lipolysis are positively correlated in rat adipocytes. They suggest at least that the tannic acid diet led to an inhibition of cyclic AMP-dependent protein kinase activity followed by a decrease in lipolysis in rat adipocytes, and to an increased activity of the type VI adenyl cyclase subunit of rat fat cells. This subunit is known to be negatively regulated under phosphorylation by cyclic AMP-dependent protein kinase. More in-depth studies are required to examine whether tannic acid could at least modify the expression of the catalytic subunit of adenyl cyclase, G-proteins and cyclic AMP-dependent protein kinase and/or alter their activities.     

Role of cyclic AMP and protein kinase on the steroidogenic action of ACTH, prostaglandin E1 and dibutyryl cyclic AMP in normal adrenal cells and adrenal tumor cells from humans.

The role of the cyclic AMP-protein kinase system in mediating the steroidogenic effect of ACTH, prostaglandin E1 and dibutyryl cyclic AMP, induced similar stimulations of protein kinase activity, cyclic AMP was studied using human adrenal cells isolated from normal and adrenocortical secreting tumors. At high concentrations of ACTH, complete activation of protein kinase of normal adrenal cells was observed within 3 min, at the time when cyclic AMP production was slightly increased and there was still no stimulation of steroidogenesis. At supramaximal concentrations, ACTH, PGE1 and dibutyryl cyclic AMP and cortisol productions in adrenal cells isolated from normal and from one adrenocortical tumor. In one tumor in which the adenylate cyclase activity was insensitive to ACTH, the hormone was unable to stimulate protein kinase or steroidogenesis, but the cells responded to both PGE1 and dibutyryl cyclic AMP. In another tumor in which the adenylate cyclase was insensitive to PGE1, this compound also did not increase protein kinase activity or steroidogenesis, but both parameters were stimulated by ACTH and dibutyryl cyclic AMP. After incubation of normal adrenal cells with increasing concentrations of ACTH (0.01-100 nM) marked differences were found between cyclic AMP formation and cortisol production. However at the lowest concentrations of ACTH exerting an effect on steroid production a close linked correlation was found between protein kinase activation and cortisol production, but half-maximal and maximal cortisol production occurs at lower concentration of ACTH than was necessary to induce the same stimulation of protein kinase. Similar findings were found after incubating the adrenal cells with dibutyryl cyclic AMP (0.01-10 mM). The results implicate an important role of the cyclic AMP-protein kinase system during activation of adrenal cell steroidogenesis by low concentrations of steroidogenic compounds.     

Lack of feedback regulation of cyclic 3':5'-AMP accumulation by free fatty acids in chicken fat cells.

Fat cells isolated from the mesenteric adipose tissue of chickens (pullets) responded to glucagon with an increase in lipolysis and a sustained rise in cyclic adenosine 3':5'-monophosphate (cyclic AMP) over a 30-min incubation. The prolonged accumulation of cyclic AMP due to glucagon in chicken fat cells was primarily intracellular. In addition, there was little increase in cyclic AMP accumulation due to theophylline alone or potentiation of the increase due to glucagon. These data indicate that chicken fat cells, unlike rat fat cells, are relatively insensitive to theophylline. Neither lipolysis nor cyclic AMP accumulation by chicken fat cells was inhibited by free fatty acid to albumin ratios (3 to 7) which markedly reduced both events in rat fat cells. However, in the absence of albumin from the medium, lipolysis in chicken fat cells was reduced, but not to the same extent as in rat fat cells. Chicken fat cells did accumulate more intracellular free fatty acids in response to lipolytic agents than did rat fat cells. The uptake of oleate by rat and chicken fat cells was identical. Glucagon-induced accumulation of cyclic AMP by chicken fat cell ghosts was unaffected by added oleate. Under identical conditions glucagon-induced adenylate cyclase activity of rat fat cell ghosts was markedly inhibited by added oleate. Triglyceride lipase activity of the pH 5.2 precipitate from a 40,000 x g infranatant of homogenized fat cells from chickens was less sensitive than that from rat fat cells to the ratio of oleate to albumin. These results suggest that the maintenance of cyclic AMP levels in chicken fat cells incubated with lipolytic agents results from the relative insensitivity of chicken fat cells to free fatty acid inhibition of cyclic AMP accumulation.     

Pertussis toxin effects on adenylate cyclase activity, cyclic AMP accumulation and lipolysis in adipocytes from hypothyroid, euthyroid and hyperthyroid rats

Adipocytes from hypothyroid rats have a decreased responsiveness to agents that activate adenylate cyclase, whereas cells from hyperthyroid rats have an increased responsiveness as compared to the controls. This is reflected in cyclic AMP accumulation as well as lipolysis. Administration of pertussis toxin to rats or its in vitro addition to adipocytes increased basal lipolysis and cyclic AMP accumulation as well as the response to norepinephrine or forskolin. The effects of thyroid status was not abolished by toxin treatment. Pertussis toxin-catalyzed ADP ribosylation of Ni was increased in adipocyte membranes from hypothyroid rats as compared to those from euthyroid rats. However, no change in sensitivity to N6-(phenylisopropyl)adenosine was observed. The data suggest that the amount of Ni might not be rate-limiting for the inhibitory action of adenosine. A consistent decrease in maximal lipolysis was observed in freshly isolated adipocytes from hypothyroid animals as compared to those from the controls. Such defective maximal lipolysis was not corrected by adenosine deaminase or in vivo administration of pertussis toxin. The relationship between cyclic AMP levels and lipolysis suggests that in fat cells from hypothyroid rats either the cyclic AMP-dependent protein kinase or the lipase activity itself may limit maximal lipolysis. There appears to be multiple effects of thyroid status on lipolysis involving factors other than those affecting adenylate cyclase activation.     

Hormonal Regulation of Adipose S-100 Protein Release

The release of S-100 protein from epididymal fat pads was enhanced by epinephrine in vitro, and about 50% of S-100 protein in the tissue was released into the medium after 2-h incubation at 37 degrees C with 10 microM epinephrine. Similar results were obtained with the incubation of isolated adipocytes. The S-100 protein release was also enhanced by isoproterenol, norepinephrine, ACTH, and dibutyryl cyclic AMP, which all increase the lipolysis by increasing cyclic AMP levels in the tissue. Propranolol, a beta-adrenergic blocker, could block the increase of S-100 protein release by catecholamines, indicating that the release was mediated by the beta-adrenergic effect of catecholamines. However propranolol had no suppressive effect on the enhancement of S-100 protein release by ACTH or dibutyryl cyclic AMP. Insulin had an inhibitory effect on the epinephrine-enhanced S-100 protein release. Epinephrine or ACTH could not stimulate the S-100 protein release in the absence of Ca2+, whereas the epinephrine-enhanced glycerol release was not affected under the same conditions. The increase in S-100 protein release was induced by only a pretreatment of the tissue with epinephrine. However, the lipolysis in the tissue was not enhanced by the pretreatment alone. These results indicate that the release of S-100 protein from adipocytes is regulated by the hormones that have been known to control the lipolysis with a manner slightly different from that of lipolysis.     

Inhibitory effect and mode of action of somatostatin on lipolysis in chicken adipocytes

The effect of somatostatin on lipolysis was investigated utilizing isolated chicken adipocytes. Somatostatin-14 and -28 inhibited basal lipolysis. This ability to suppress glycerol release (used as an index of lipolysis) was emphasized in presence of stimulated lipolysis. Concentration of 1 ng/ml somatostatin-14 (0.625 nM) and somatostatin-28 (0.312 nM) was found to inhibit completely the glycerol release induced by concentrations of glucagon up to 2 ng/ml (0.58 nM). The percentage of inhibition was dose-dependent. The antilipolytic effect of somatostatin-14 was also observed during ACTH and aminophylline-stimulated lipolysis. Among the mechanisms which could account for the inhibition, a possible competitive effect of somatostatin-14 with 125I-labelled glucagon binding to adipocyte membranes was excluded. The small inhibiting effect of somatostatin-14 on glycerol release prompted by dibutyryl cyclic AMP, together with the significant inhibiting effect on aminophylline-stimulated lipolysis argued for a reduction of cyclic AMP accumulation. The increase of cyclic AMP levels induced by glucagon was substantially reduced in presence of somatostatin-14. It was concluded that in chicken adipocytes somatostatin inhibited the rate of lipolysis and that reduction on cyclic AMP could be responsible, at least in part, for the antilipolytic effect.     

Inhibition of lipolysis in hamster epididymal adipocytes by selective alpha-adrenergic agents. Evidence for cyclic AMP-dependent and independent mechanisms.

In this study the role of cyclic AMP in the antilpolytic effect of the alpha-adrenergic agents methoxamine and phenylephrine in hamster epididymal adipocytes was studied. Both methozamine and phenylephrine lowered the very high levels of cyclic AMP that were produced by high concentrations of isoproterenol (10 muM) or ACTH (100 MU/ml), and partially inhibited lipolysis. When lower concentrations of isoproterenol were used, the antilipolytic effect of phenylephrine and methoxamine was still evident. Under these conditions methoxamine produced a slight suppression of cyclic AMP levels while phenylephrine increased accumulation of cyclic AMP. It follows, therefore, that the inhibition of lipolysis by the alpha agents is most likely unrelated to changes in cyclic AMP levels; in contrast, phenylephrine promoted lipolysis and increased cyclic AMP levels. When the stimulus for lipolysis was provided by methylxanthines a different picture emerged. Methoxamine antagonized lipolysis and lowered cyclic AMP levels. In the presence of propranolol, phenylephrine lowered cyclic AMP levels and suppressed methylxanthine-accelerated lipolysis. It is suggested that when methy xanthines provide the stimulus for lipolysis the antilipolytic effect of methoxamine and phenylephrine (in the presence of propranolol) may be mediated by the suppression in cyclic AMP levels.     

Regulation of lipolysis and cyclic AMP synthesis through energy supply in isolated human fat cells.

The effects of glucose and of various inhibitors of glycolysis or of oxidative phosphorylation on stimulated lipolysis and on intracellular cyclic AMP and ATP levels were investigated in isolated human fat cells. The glycolysis inhibitors, NaF and monoiodoacetate, inhibited epinephrine or theophylline-stimulated lipolysis and parallely reduced the intracellular cyclic AMP and ATP levels; however, neither NaF nor monoidoacetate significantly affected dibutyryl cyclic AMP-induced lipolysis. Removal of glucose from the medium also reduced the rate of epinephrine-stimulated lipolysis and the intracellular cyclic AMP and ATP levels but failed to modify the lipolytic activity of dibutyryl cyclic AMP. The oxidative phosphorylation inhibitors, antimycin A and, under fixed conditions, 2,4-dinitrophenol also strongly decreased the adipocyte cyclic AMP and ATP levels but inhibited as well the rate of epinephrine- and of dibutyryl cyclic AMP-induced lipolysis. N-Ethylmaleimide, a mixed glycolysis and oxidative phosphorylation inhibitor, not only reduced the intracellular cyclic AMP and ATP levels and epinephrine- or theophylline-induced lipolysis, but also that stimulated by dibutyryl cyclic AMP. When glycolysis was almost fully inhibited, human fat cells were insensitive to epinephrine but remained fully responsive to dibutyryl cyclic AMP. These results, showing a relationship between ATP availability, cyclic AMP synthesis and lipolysis, suggest a different ATP requirement for cyclic AMP synthesis and triacylglycerol lipase activation, a difference which could explain why ATP issued from glucose breakdown appears to be a determinant factor for cyclic AMP synthesis, but not for triacylglycerol lipase activation in human fat cells.     

Divergent effects of adrenocorticotropin and melanotropin on isolated rat and rabbit adipocytes

The stimulation of lipolysis in isolated rat and rabbit fat cells by adrenocorticotropin (ACTH) and α-melanotropin has been studied. The concentration of α-melanotropin required for half maximal stimulation is 0.23 times that of ACTH in rabbit adipocytes but as high as 1140 times that of ACTH in rat fat cells. Chemical modification of the tryptophan residue in ACTH and melanotropin resulted in a loss of lipolytic activity in rat adipocytes and an increase in lipolytic potency in rabbit fat cells. These differences between rat and rabbit fat cells were evident when stimulation of cyclic AMP synthesis was measured in isolated cells or ghosts. The results are discussed in terms of the difference in the hormone receptors of the fat cells of the two species.     

Divergent effects of adrenocorticotropin and melanotropin on isolated rat and rabbit adipocytes.

The stimulation of lipolysis in isolated rat and rabbit fat cells by adrenocorticotropin (ACTH) and alpha-melanotropin has been studied. The concentration of alpha-melanotropin required for half maximal stimulation is 0.23 times that of ACTH in rabbit adipocytes but as high as 1140 times that of ACTH in rat fat cells. Chemical modification of the tryptophan residue in ACTH and melanotropin resulted in a loss of lipolytic activity in rat adipocytes and an increase in lipolytic potency in rabbit fat cells. These differences between rat and rabbit fat cells were evident when stimulation of cyclic AMP synthesis was measured in isolated cells or ghosts. The results are discussed in terms of the difference in the hormone receptors of the fat cells of the two species.     

Effects of adenosine deaminase on cyclic adenosine monophosphate accumulation, lipolysis, and glucose metabolism of fat cells.

In fat cells isolated from the parametrial adipose tissue of rats, the addition of purified adenosine deaminase increased lipolysis and cyclic adenosine 3':5'-monophosphate (cyclic AMP) accumulation. Adenosine deaminase markedly potentiated cyclic AMP accumulation due to norepinephrine. The increase in cyclic AMP due to adenosine deaminase was as rapid as that of theophylline with near maximal effects seen after only a 20-sec incubation. The increases in cyclic AMP due to crystalline adenosine deaminase from intestinal mucosa were seen at concentrations as low as 0.05 mug per ml. Further purification of the crystalline enzyme preparation by Sephadex G-100 chromatography increased both adenosine deaminase activity and cyclic AMP accumulation by fat cells. The effects of adenosine deaminase on fat cell metabolism were reversed by the addition of low concentrations of N6-(phenylisopropyl)adenosine, an analog of adenosine which is not deaminated. The effects of adenosine deaminase on cyclic AMP accumulation were blocked by coformycin which is a potent inhibitor of the enzyme. These findings suggest that deamination of adenosine is responsible for the observed effects of adenosine deaminase preparations. Protein kinase activity of fat cell homogenates was unaffected by adenosine or N6-(phenylisopropyl)adenosine. Norepinephrine-activated adenylate cyclase activity of fat cell ghosts was not inhibited by N6-(phenylisopropyl)adenosine. Adenosine deaminase did not alter basal or norepinephrine-activated adenylate cyclase activity. Cyclic AMP phosphodiesterase activity of fat cell ghosts was also unaffected by adenosine deaminase. Basal and insulin-stimulated glucose oxidation were little affected by adenosine deaminase. However, the addition of adenosine deaminase to fat cells incubated with 1.5 muM norepinephrine abolished the antilipolytic action of insulin and markedly reduced the increase in glucose oxidation due to insulin. These effects were reversed by N6-(phenylisopropyl)adenosine. Phenylisopropyl adenosine did not affect insulin action during a 1-hour incubation. If fat cells were incubated for 2 hours with phenylisopropyl adenosine prior to the addition of insulin for 1 hour there was a marked potentiation of insulin action. The potentiation of insulin action by prior incubation with phenylisopropyl adenosine was not unique as prostaglandin E1, and nicotinic acid had similar effects.     

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

The possible presence of α adrenergic control of lipolysis and cyclic AMP production in brown adipocytes of hamsters was studied in adipocytes isolated from interscapular, subscapular, cervical and axillary regions of normal male hamsters maintained at 25°C. Lipolysis activated by either 3-isobutyl-1-methyl xanthine or isoproterenol was unaffected by the presence of the α adrenergic selective agonists clonidine and methoxamine. Similarly, accumulation of cyclic AMP in response to β-receptor stimulation, alone or in combination with a methyl xanthine, was unaffected by clonidine or methoxamine. In contrast, both lipolysis and cyclic AMP accumulation in brown fat cells were effectively suppressed in the presence of nicotinic acid, prostaglandin E₁ or N⁶-phenylisopropyl adenosine. Accumulation of cyclic AMP in response to the mixed agonist norepinephrine was not influenced when cells were exposed to the alpha adrenergic blocking drugs yohimbine or tolazoline. These observations suggest that alpha-2 adrenergic receptors which are present on hamster white fat cells and control production of cyclic AMP and lipolysis are absent from hamster brown adipocytes. On the other hand, brown fat cells of this species appear to respond to a number of other inhibitory compounds in a manner not markedly different from that of white adipocytes.     

Regulation of cyclic AMP metabolism and lipolysis in isolated rat fat cells by insulin, N6-(phenylisopropyl) adenosine and 2',5'-dideoxyadenosine.

The increases in cyclic AMP accumulation and lipolysis by rat fat cells incubated in the presence of catecholamines were abolished by N6-(phenylisopropyl) adenosine. The same inhibition of cyclic AMP accumulation was seen in the presence of 2',5'-dideoxyadenosine but lipolysis was unaffected. In contrast, insulin inhibited lipolysis without affecting cyclic AMP accumulation by norepinephrine plus adenosine deaminase. These results suggest that there are either multiple pools of cyclic AMP or that ther exists some other mechanism which is involved in the regulation of lipolysis by hormones.     

Effects of adrenalectomy on CRH regulation of ACTH release: adenylate cyclase activity, cyclic AMP-dependent protein kinase activity and ACTH release

Corticotropin releasing hormone (CRH) stimulation of ACTH release and cyclic AMP-mediated events involved in the control of ACTH release were compared in sham-operated and adrenalectomized rats. CRH-stimulated adenylate cyclase activity was decreased in pituitary homogenates from adrenalectomized animals. CRH-stimulated cyclic AMP accumulation was essentially abolished and CRH-stimulated cyclic AMP-dependent protein kinase (A-kinase) activity was decreased in freshly prepared anterior pituitary cells from adrenalectomized animals. Basal and CRH-stimulated ACTH release was elevated in these cells. Since ACTH release is increased in adrenalectomized rats despite the down regulation of CRH-linked pituitary mechanisms, we speculate that the site of action of disinhibition by corticosterone of ACTH release (or synthesis) following adrenalectomy is distal to the generation of cyclic AMP and/or that non-CRH mediated mechanisms assume a greater role in ACTH regulation following adrenalectomy.