Alpha-adrenergic inhibition of cyclic AMP accumulation in hamster adipocytes. Similarity of receptor with alpha-2 adrenergic receptors

The present communication shows the effects of several alpha-adrenergic agonists and antagonists on cyclic AMP levels in hamster epididymal adipocytes. In response to ACTH (30 mU/ml) in combination with 1-methyl-3-isobutylxanthine (0.10 mM) or adenosine deaminase (1.0 micrograms/ml), cyclic AMP levels increased to a maximum by 10 min and this level was maintained for another 20 min. Elevated cyclic AMP levels were partially suppressed by the alpha-adrenergic agents clonidine, methoxamine, methyl norepinephrine and phenylephrine. The lowest effective concentration of each of these agonists required to suppress cyclic AMP levels was 10 nM clonidine; 3 microM methoxamine; 10 microM methyl norepinephrine; 10 microM phenylephrine. Clonidine and methoxamine suppressed cyclic AMP levels by nearly 65% while phenylephrine and methyl norepinephrine caused only a 30% decline. Studies of the relative potencies of alpha-adrenergic blocking drugs on prevention of the inhibitor effect of clonidine on cyclic AMP levels disclosed that phentolamine and yohimbine were more potent blockers of clonidine action than phenoxybenzamine and prazosin. The rank order of potencies of agonists at causing suppression of cyclic AMP levels and the rank order of potencies of antagonists of clonidine action suggest similarity of the alpha-adrenergic receptors present on hamster adipocytes, which affect cyclic AMP accumulation to alpha-2 adrenergic receptors.     

Effect of adrenergic agents on alpha-amylase release and adenosine 3',5'-monophosphate accumulation in rat parotid tissue slices.

The role of cyclic AMP in stimulus-secretion coupling with investigated in rat parotid tissue slices in vitro. Isoproterenol and norepinephrine stimulated a rapid intracellular accumulation of cyclic AMP, which reached a maximum level of 20-30 times the control value by 5 to 10 min after addition of the drug. Isoproterenol was approximately ten times more potent in stimulating both alpha-amylase release and cyclic AMP accumulation than were norepinephrine and epinephrine, which had nearly equal effects on these two parameters. Salbutamol and phenylephrine were less effectivema parallel order of potency and sensitivity was observed for the stimulation of adenylate cyclase activity in a washed particulate fractionmthe results suggest that these drugs are acting on a parotid acinar cell through a beta1-adrenergic mechanismmat the lowest concentrations tested, each of the adrenergic agonists stimulated significant alpha-anylase release with no detectable stimulation of cyclic AMP accumulationmeven in the presence of theophylline, phenylephrine at several concentrations increased alpha-amylase release without a detectable increase in cyclic AMP levels. However, phenylephrine did stimulate adenylate cyclase. These data suggest that, under certain conditions, large increases in the intra-cellular concentration of cyclic AMP may not be necessary for stimulation of alpha-amylase release by adrenergic agonists. Also consistent with this idea was the observation that stimulation of cyclic AMP accumulation by isoproterenol was much more sensitive to inhibition by propranolol than was the stimulation of alpha-amylase release by isoproterenol. Stimulation of alpha-amylase release by phenylephrine was only partially blocked by either alpha- or beta-adrenergic blocking agents, whereas stimulation of adenylate cyclase by phenylephrine was blocked by propranolol and not by phentolaminemphenoxybenzamine and phentolamine potentiated the effects of norepinephrine and isoproterenol on both cyclic AMP accumulation and alpha-amylase release by N-6,O-2'-dibutyryl adenosine 3',5'-monophosphate; These observations may indicate a non-specific action of phenoxybenzamine, and demonstrate the need for caution in interpreting evidence obtained using alpha-adrenergic blocking agents as tools for investigation of alpha- and beta-adrenergic antagonism.     

Multiple Adrenergic Receptor Subtypes Controlling Cyclic AMP Formation: Comparison of Brain Slices and Primary Neuronal and Glial Cultures

The adrenergic receptor subtypes involved in cyclic AMP responses to norepinephrine (NE) were compared between slices of rat cerebral cortex and primary neuronal and glial cultures from rat brain. In neuronal cultures, NE and the beta-adrenergic receptor agonist isoproterenol (ISO) caused similar increases in cyclic AMP, which were not altered by the alpha-adrenergic receptor antagonist phentolamine. In glial cultures, NE caused a much smaller cyclic AMP response than did ISO, and this difference was reversed by alpha-adrenergic receptor antagonists (phentolamine greater than yohimbine greater than prazosin). alpha 2-Adrenergic receptor agonists partially inhibited the ISO response in glial cultures to a level similar to that observed with NE alone (clonidine = UK 14,304 greater than NE greater than 6-fluoro-NE greater than epinephrine). In slices from cerebral cortex, NE caused a much larger increase in cyclic AMP than did ISO, and this difference was reversed by alpha-adrenergic receptor antagonists with a different order of potency (prazosin greater than phentolamine greater than yohimbine). alpha 1-Adrenergic receptor agonists potentiated the response to ISO to a level similar to that observed with NE alone (epinephrine = NE greater than phenylephrine greater than 6-fluoro-NE greater than methoxamine). In all three tissue preparations, large responses to both alpha 1-receptor activation (increases in inositol phosphate accumulation) and alpha 2-receptor activation (decreases in forskolin-stimulated cyclic AMP accumulation) were observed. These data indicate that all of the major adrenergic receptor subtypes (beta, alpha 1, alpha 2) are present in each tissue preparation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cyclic AMP and the positive inotropic effect of norepinephrine and phenylephrine.

Time-response studies of the effects of norepinephrine and phenylephrine revealed that both agonists caused an increase in cyclic AMP levels before increases in contractile force in either the electrically stimulated left atria or spontaneously beating right atria of the rat. Norepinephrine caused a nearly sixfold increase in cyclic AMP, whereas phenylephrine produced only a 50% increase in the nucleotide. Pretreatment with reserpine did not affect the norepinephrine cyclic AMP response; however, the phenylephrine cyclic AMP response was abolished. Reserpine pretreatment did not significantly affect the contractile responses of either amine. In the presence of propranolol, norepinephrine was found to have the ability to produce an increace in contractile force in which cyclic AMP was apparently not involved. The time course of the contractile response induced by adrenergic amines was found to be remarkably influenced by the chronotropic response in spontaneously beating preparations while the cyclic AMP response was not greatly affected. This difference in the contractile response may be due to the ability of the chronotropic response to influence the flux of calcium through the cell membrane.     

Mediation of Norepinephrine-Stimulated Cyclic AMP Accumulation by Adrenergic Receptors in Hypothalamic and Preoptic Area Slices: Effects of Estradiol

Adrenergic receptor agonists and antagonists were employed to establish (a) which receptor subtypes mediate the cyclic AMP response to norepinephrine in hypothalamic and preoptic area slices from gonadectomized female rats and (b) which receptor subtypes might be modulated by the steroid hormone estradiol. Slice cyclic AMP levels were elevated by the beta receptor agonist isoproterenol, but not by alpha 1 (phenylephrine, methoxamine) or alpha 2 (clonidine) agonists. However, the alpha agonist phenylephrine potentiated the effect of the beta agonist isoproterenol on slice cyclic AMP accumulation. In slices from rats given no hormone treatment, the beta antagonist propranolol inhibited norepinephrine-stimulated cyclic AMP production, while the alpha 1 antagonist prazosin was without effect. In contrast, the cyclic AMP response to norepinephrine in slices from estradiol-treated rats was blocked more effectively by prazosin than by propranolol. Estradiol treatment also attenuated the production of cyclic AMP by the beta agonist isoproterenol. The data suggest (a) that norepinephrine induction of cyclic AMP accumulation in hypothalamic and preoptic area slices is mediated by beta receptors and potentiated by alpha receptor activation and (b) that estradiol depresses beta and increases alpha 1 receptor function in slices from brain regions associated with reproductive physiology.     

Effect of adrenergic agents on α-amylase release and adenosine 3′,5′-monophosphate accumulation in rat parotid tissue slices

The role of cyclic AMP in stimulus-secretion coupling was investigated in rat parotid tissue slices in vitro. Isoproterenol and norepinephrine stimulated a rapid intracellular accumulation of cyclic AMP, which reached a maximum level of 20–30 times the control value by 5 to 10 min after addition of the drug. Isoproterenol was approximately ten times more potent in stimulating both α-amylase release and cyclic AMP accumulation than were norepinephrine and epinephrine, which had nearly equal effects on these two parameters. Salbutamol and phenylephrine were less effective. A parallel order of potency and sensitivity was observed for the stimulation of adenylate cyclase activity in a washed particulate fraction. The results suggest that these drugs are acting on the parotid acinar cell through a β₁-adrenergic mechanism.At the lowest concentrations tested, each of the adrenergic agonists stimulated significant α-amylase release with no detectable stimulation of cyclic AMP accumulation. Even in the presence of theophylline, phenylephrine at several concentrations increased α-amylase release without a detectable increase in cyclic AMP levels. However, phenylephrine did stimulate adenylate cyclase. These data suggest that, under certain conditions, large increases in the intracellular concentration of cyclic AMP may not be necessary for stimulation of α-amylase release by adrenergic agonists. Also consistent with this idea was the observation that stimulation of cyclic AMP accumulation by isoproterenol was much more sensitive to inhibition by propranolol than was the stimulation of α-amylase release by isoproterenol.Stimulation of α-amylase release by phenylephrine was only partially blocked by either α- or β-adrenerg blocking agents, whereas stimulation of adenylate cyclase by phenylephrine was blocked by propranolol and not by phentolamine. Phenoxybenzamine and phentolamine potentiated the effects of norepinephrine and isoproterenol on both cyclic AMP accumulation and α-amylase release. However, phenoxybenzamine also potentiated the stimulation of α-amylase release by N⁶,O^2′-dibutyryl adenosine 3′,5′-monophosphate. These observations may indicate a non-specific action of phenoxybenzamine, and demonstrate the need for caution in interpreting evidence obtained using α-adrenergic blocking agents as tools for investigation of α- and β-adrenergic antagonism.     

Negative control of norepinephrine on the thyroid cyclic AMP system

Negative control on the thyroid cyclic AMP system has been studied. The increase of cyclic AMP levels induced by TSH in dog thyroid slices and its consequent secretion were inhibited by norepinephrine. This effect was different from the previously described activation of cyclic AMP disposal by acetylcholine: it was not calcium-dependent, was observed in the presence of isobutyl methylxanthine and was not inhibited by atropine. The inhibitory action of norepinephrine was abolished by phentolamine but not by L-propranolol. Clonidine and epinephrine also markedly inhibited the elevation of cyclic AMP levels, but phenylephrine did not. The inhibitory effect of norepinephrine and clonidine was abolished by yohimbine but not by prazosin. These results suggest that the effect of adrenergic agents on dog thyroid follicular cells is mediated by alpha 2-receptors. Similar results were obtained on dog thyroid adenylate cyclase activity: norepinephrine diminished the activation of adenylate cyclase induced by TSH, in a sodium-dependent process. This inhibition was abolished by phentolamine and yohimbine, but not by L-propranolol and and prazosin. This shows that the negative alpha 2-adrenergic effect bears directly on adenylate cyclase.     

Effect of monoamine receptor agonists and antagonists on cyclic AMP accumulation in human cerebral cortex slices.

In human cerebral cortex slices noradrenaline, isoproterenol (a beta-adrenergic agonist), dopamine, apomorphine (a dopaminergic agonist), and serotonin stimulated cyclic AMP formation: noradrenaline greater than or equal to isoproterenol greater than dopamine = apomorphine = serotonin. Clonidine (and alpha-adrenergic agonist) was ineffective in stimulating cyclic AMP formation in temporal cortex slices. The stimulatory effect of noradrenaline and isoproterenol was blocked by propranolol (a beta-adrenergic blocker) but not by phentolamine (an alpha-adrenergic blocker). Pimozide (a selective dopaminergic antagonist) inhibited the increase of cyclic AMP formation induced by dopamine or apomorphine but not that induced by noradrenaline, isoproterenol, or serotonin. Neither propranolol or phentolamine had any effect on dopamine- or serotonin-stimulated cyclic AMP formation. Chlorpromazine blocked the increase of cyclic AMP formation induced by noradrenaline, dopamine or serotonin, while cyproheptadine, a putative central serotonergic antagonist, was ineffective. These observations suggest that there may be at least two monoamine-sensitive adenylate cyclases in human cerebral cortex which have the characteristics of a beta-adrenergic and a dopaminergic receptor, respectively, and also possibly a serotonergic receptor.     

α-adrenergic inhibition of cyclic AMP accumulation in hamster adipocytes similarity of receptor with α-2 adrenergic receptors

The present communication shows the effects of several α-adrenergic agonists and antagonists on cyclic AMP levels in hamster epididymal adipocytes. In response to ACTH (30 mU/ml) in combination with 1-methyl-3-isobutylxanthine (0.10 mM) or adenosine deaminase (1.0 μg/ml), cyclic AMP levels increased to a maximum by 10 min and this level was maintained for another 20 min. Elevated cyclic AMP levels were partially suppressed by the α-adrenergic agents clonidine, methoxamine, methyl norepinephrine and phenylephrine. The lowest effective concentration of each of these agonists required to suppress cyclic AMP levels was 10 nM clonidine; 3 μM methoxamine; 10 μM methyl norepinephrine; 10 μM phenylephrine. Clonidine and methoxamine suppressed cyclic AMP levels by nearly 65% while phenylephrine and methyl norepinephrine caused only a 30% decline. Studies of the relative potencies of α-adrenergic blocking drugs on prevention of the inhibitory effect of clonidine on cyclic AMP levels disclosed that phentolamine and yohimbine were more potent blockers of clonidine action than phenoxybenzamine and prazosin. The rank order of potencies of agonists at causing suppression of cyclic AMP levels and the rank order of potencies of antagonists of clonidine action suggest similarity of the α-adrenergic receptors present on hamster adipocytes, which affect cyclic AMP accumulation to α-2 adrenergic receptors.     

Long-term regulation by theophylline of fatty acid synthetase, acetyl-CoA carboxylase and lipid synthesis in cultured glial cells.

The long-term regulation of fatty acid synthetase and acetyl-CoA carboxylase and of fatty acid and sterol synthesis was studied in C-6 glial cells in culture. When theophylline (10(-3) M) was added to the culture medium of these cells, rates of lipid synthesis from acetate and activities of synthetase and carboxylase became distinctly lower than in cells that were untreated. This effect appeared after approximately 12 h, and after 48 h enzymatic activities were reduced approx. 2-fold and rates of lipid synthesis from acetate 3- to 4-fold. The likelihood that the decrease in fatty acid synthesis from acetate was caused by the decrease in activities of fatty acid synthetase and acetyl-CoA carboxylase was established by several observations. These indicated that the locus of the effect probably did not reside at the level of acetate uptake into the cell, alterations in acetate pool sizes or conversion of acetate to acetyl-CoA. Moreover, de novo fatty acid synthesis was found to be the predominant pathway in these glial cells, whether treated with theophylline or not. The mechanism of the effect of theophylline on fatty acid synthetase was shown by immunochemical techniques to involve an alteration in content of enzyme rather than in catalytic efficiency. The change in content of fatty acid synthetase was shown by isotopic-immunochemical experiments to involve a decrease in synthesis of the enzyme. The mechanism whereby theophylline leads to a decrease in lipogenesis and in the synthesis of fatty acid synthetase may not be mediated entirely by inhibition of phosphodiesterase and an increase in cyclic AMP levels, because dibutyryl cyclic AMP (10(-3) M) only partially reproduced the effect.     

Roles of the AMP-activated and cyclic-AMP-dependent protein kinases in the adrenaline-induced inactivation of acetyl-CoA carboxylase in rat adipocytes.

1. In isolated rat adipocytes, acetyl-CoA carboxylase is inactivated by treatment of the cells with adrenaline or the beta-agonist isoproterenol, but not by the alpha-agonist phenylephrine. The inactivation is stable during purification in the presence of protein phosphatase inhibitors, and is associated with a 30-40% increase in the labelling of enzyme isolated from 32P-labelled cells. 2. Increased phosphorylation occurs within peptide T1, which was identified by sequencing to be the peptide Ser-Ser77-Met-Ser79-Gly-Leu-His-Leu-Val-Lys, containing Ser-77 (phosphorylated by cyclic-AMP-dependent protein kinase) and Ser-79 (phosphorylated by the AMP-activated protein kinase). Analysis of the release of radioactivity as free phosphate during Edman degradation of peptide T1 revealed that all of the phosphate was in Ser-79 in both basal and hormone- or agonist-stimulated cells. Treatment of adipocytes with various agents which activate cyclic-AMP-dependent protein kinase by receptor-independent mechanisms (forskolin, cyclic AMP analogues, isobutylmethylxanthine) also produced inactivation of acetyl-CoA carboxylase and increased phosphorylation at Ser-79. 3. The (Rp)-[thio]phosphate analogue of cyclic AMP, which is an antagonist of binding of cyclic AMP to the regulatory subunit of cyclic-AMP-dependent protein kinase, opposes the effect of adrenaline on phosphorylation and inactivation of acetyl-CoA carboxylase. Together with the effects of isobutylmethylxanthine and the stimulatory cyclic AMP analogues, this strongly indicates that cyclic-AMP-dependent protein kinase is an essential component of the signal transduction pathway, although clearly it does not directly phosphorylate acetyl-CoA carboxylase. 4. As shown by okadaic acid inhibition, greater than 95% of the acetyl-CoA carboxylase phosphatase activity in extracts of rat adipocytes or liver is accounted for by protein phosphatase-2A, with less than 5% attributable to protein phosphatase-1. Inhibition of protein phosphatase-1 via phosphorylation of inhibitor-1 is therefore unlikely to be the mechanism by which cyclic-AMP-dependent protein kinase indirectly increases phosphorylation of acetyl-CoA carboxylase. Various other potential mechanisms are discussed.     

Inhibition of lipolysis in hamster adipocytes with selective alpha-adrenergic stimuli. Functional characterization of the alpha-receptor

This communication shows the relative potencies of the alpha-agonists clonidine, methoxamine, methyl norepinephrine and phenylephrine in producing inhibition of lipolysis. At cell densities greater than 15 mg cell/ml lipolysis activated by either 1-methyl-3-isobutyl xanthine or adenosine deaminase was inhibited by alpha-adrenergic stimuli with a rank order of potency of clonidine greater than methoxamine greater than methyl norepinephrine; phenylephrine produced a further stimulation of lipolysis. At the same cell density isoproterenol-accelerated lipolysis was inhibited by alpha-adrenergic stimuli with a rank order of potency of phenylephrine greater than methoxamine greater than clonidine greater than methyl norepinephrine. When the density of fat cells was reduced to less than 5 mg/ml, clonidine was a more effective inhibitor of isoproterenol-activated lipolysis thatn phenylephrine. Lipolysis that was activated by dibutyryl cyclic AMP, ACTH or cholera enterotoxin was not reduced by any alpha-adrenergic agent. Under conditions when clonidine failed to inhibit catecholamine-activated lipolysis (i.e., at cell densities greater than 15 mg/ml), it failed to antagonize the antilipolytic activity of phenylephrine. The antilipolytic activities of clonidine and phenylephrine were most effectively antagonized by the blocking drugs phentolamine and yohimbine; in contrast, phenoxybenzamine and prazosin were less effective blockers. These data indicate that the alpha-adrenergic receptor on hamster fat cells is similar to presynaptic alpha-adrenergic receptors. The data further suggest the possibility that phenylephrine may exert its action through a separate alpha-adrenergic receptor mechanism.     

Cyclic GMP and cyclic GMP-dependent protein kinase in brown adipose tissue of developing rats

In order to ascertain the possible involvement of cyclic GMP in the physiological regulation of the function and development of brown fat of the rat, we have determined its tissue concentration in vivo under a variety of conditions. The steady-state concentration of cyclic GMP in interscapular brown adipose tissue of late foetus was about 80 pmol per g fresh weight. The concentration gradually declined during the first 2 weeks after birth to reach 40 pmol/g fresh weight and then remained constant into adulthood. The cyclic GMP content of brown fat was decreased by chemical sympathectomy and was increased after complete acclimatization of the animals to the cold. The activity of cyclic GMP-dependent protein kinase was also highest in tissue from newborn and cold-acclimatized rats.Both acute cold stress and injection of norepinephrine resulted in a significant but temporary increase in the concentration of cyclic GMP in brown fat, which was followed by a depression of the concentration below values in untreated animals. The concentration of cyclic AMP showed similar pattern of changes. Injection of phenylephrine was followed by a pronounced increase in the cyclic GMP content of brown fat, with little effect upon cyclic AMP. Injection of isoproterenol raised the content of cyclic AMP but not that of cyclic GMP. The ability of norepinephrine and phenylephrine to increase the concentration of cyclic GMP was abolished by pre-treatment of the animals with phenoxybenzamine, but not by pre-treatment with propranolol. Conversely, propranolol but not phenoxybenzamine abolished the effects of norepinephrine on the cyclic AMP content of the tissue.Thus we have established the responsiveness of the cyclic GMP content of brown fat to physiological and pharmacological stimuli and have evidence of the possible participation by cyclic GMP in the α-adrenergic stimulation and in the regulation of proliferative processes in the tissue.     

Action of phenylephrine on protein synthesis in liver cells.

The alpha-adrenergic agonist phenylephrine was found to inhibit protein labelling from [3H]valine in isolated liver cells. This effect is only observable under conditions of partial Ca2+ depletion and in cells displaying maximal rates of protein labelling, i.e. cells isolated from fed animals or from starved animals when incubated in the presence of alanine. The ability of phenylephrine to inhibit protein labelling at near-saturating concentrations of the amino acid precursor indicates that this alpha-agonist actually decreases the rate of protein synthesis. The possibility that phenylephrine acts by making cellular Ca2+ availability further limiting can be ruled out, since alanine stimulates protein labelling under conditions of severe Ca2+ depletion obtained by pretreatment of the cells with EGTA. The following observations indicate that the phenylephrine action may be mediated by an increase in cellular cyclic AMP content: (1) a close relationship was found between the abilities of phenylephrine to inhibit protein labelling and to increase cyclic AMP content; (2) cyclic AMP mimics the phenylephrine action only in cells partially depleted of Ca2+; (3) the alpha 1-antagonist prazosin, which inhibited the phenylephrine-mediated increase in cyclic AMP, also abolished the effect on protein synthesis.     

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 rat liver acetyl-CoA carboxylase. Stimulation of phosphorylation and subsequent inactivation of liver acetyl-CoA carboxylase by cyclic 3':5'-monophosphate and effect on the structure of the enzyme.

The effects of citrate and cyclic AMP on the rate and degree of phosphorylation and inactivation of rat liver acetyl-CoA carboxylase were examined. High citrate concentrations (10 to 20 mM), which are generally used to stabilize and activate the enzyme, inhibit phosphorylation and inactivation of carboxylase. At lower concentrations of citrate, the rate and degree of phosphorylation are increased. Furthermore, phosphorylation and enzyme inactivation are affected by cyclic AMP under these conditions. At high citrate concentrations, cyclic AMP has little or no effect on inactivation and phosphorylation of acetyl-CoA carboxylase. Phosphorlation and inactivation of carboxylase is accompanied by depolymerization of the polymeric form of the enzyme into intermediate and protomeric forms. Depolymerization of carboxylase requires the transfer of the gamma-phosphate group from ATP to carboxylase. Inactivation occurs in the absence of CO2, which indicates that phosphorylation of the enzyme is the cause of inactivation and depolymerization, i.e. carboxylation of the enzyme is not responsible for inactivation of the enzyme.     

Free‐running rhythmicity in the life spans of hydranths of the marine cnidarian,Campanularia flexuosa

Abstract

The adrenergic agonists and antagonists, norepinephrine, phenylephrine, iso‐proterenol, phentolamine, and propranolol, were administered to rats in different phases of the diurnal cycle, and their effects on tyrosine aminotransferase activity were studied. All substances tested, regardless whether being typically α‐ or ß ‐adrenergic or whether being agonists or antagonists, elevated tyrosine aminotransferase during the minimum of enzyme activity. The effects at the enzyme maximum, however, were generally more or less depressive. The extent of the depressions by norepinephrine and by propranolol highly depended on the duration of treatment. The hepatic concentration of cyclic AMP did not exhibit a significant rhythmicity. The results do not favour the idea of an adrenergic control of the diurnal rhythm in tyrosine aminotransferase activity.     

Alpha-adrenergic interaction with stimulators of cyclic AMP concentrations in canine thyroid slices.

Thyroid stimulating hormone (TSH) increased cyclic AMP levels approximately 10–20 fold in canine thyroid slices after 30 min incubation. Thereafter the cyclic AMP level declined reaching about 50% of the maximal by 90 min even in the presence of 10 mM theophylline. When phentolamine, an α-adrenergic blocker, was added with TSH to the incubation medium, the decline of cyclic AMP levels that followed the peak was markedly diminished. The maximal effect of phentolamine was observed at a concentration of 10^?6M. A similar decline of the cyclic AMP levels after the peak was observed when the tissues was stimulated by prostaglandin E₁ or cholera toxin and the decline was again prevented by phentolamine. Phentolamine alone had no significant effect on the basal cyclic AMP levels. Phenylephrine, an α-adrenergic agonist, diminished the rise of cyclic AMP levels induced by TSH.Norephinephrine, a physiologic adrenergic stimulator, caused a marked inhibition of the elevation of cyclic AMP levels induced by prostaglandin E₁ or cholera toxin as was the case by TSH (Life Sciences 21, 607, 1977). The norepinephrine effect was abolished by phentolamine, but not by propranolol, a β-adrenergic blocker.These results indicate that α-adrenergic actions may be involved in the counter-regulation of cyclic AMP levels in canine thyroid glands.     

The actions of cyclic AMP on biosynthetic processes are mediated indirectly by cyclic AMP-dependent protein kinase.

Adrenalin and glucagon inhibit glycogen, fatty acid and cholesterol synthesis by elevation of cyclic AMP, activation of cyclic AMP-dependent protein kinase and increased phosphorylation of the rate-limiting enzymes of these pathways. Here, we review recent evidence which indicates that inhibition of these biosynthetic pathways in muscle, adipose tissue and liver is much more indirect than has previously been supposed. In particular, cyclic AMP-dependent protein kinase does not appear to inhibit glycogen synthase, acetyl-CoA carboxylase and HMG-CoA reductase by phosphorylating them directly. It appears to achieve the same end result by inactivation of the protein phosphatases which dephosphorylate these regulatory enzymes in vivo, although this has only been established definitively in the case of glycogen synthesis.     

Roles of alpha and beta adrenergic receptors in control of glucose oxidation in hamster epididymal adipocytes.

Oxidation of [14C] glucose in isolated epididymal adipocytes from Golden hamsters was stimulated by isoproterenol, epinephrine and norepinephrine, which all interact with beta-adrenergic receptors and by adrenocorticotrophic hormone. In contrast alpha-receptor agonists, such as phenylephrine, methoxamine or clonidine did not increase basal glucose oxidation. The beta-adrenergic blocking drug propranolol inhibited both lipolysis and glucose oxidation when these had been stimulated by isoproterenol, epinephrine or norepinephrine. Conversely, the alpha-adrenergic blocking drugs phentolamine and phenoxybenzamine did not influence lipolysis or glucose oxidation when isoproterenol provided the stimulus and increased both lipolysis and glucose metabolism in the present of either epinephrine or norepinephrine. All alpha-adrenergic agonists tested (phenylephrine, methoxamine and clonidine) lowered lipolysis and glucose oxidation isolated adipocytes exposed to isoproterenol. However, when adrenocorticotropin provided the stimulus for glucose oxidation and lipolysis, only clonidine produced a significant reduction in lipolysis and glucose oxidation. None of the alpha-agonists influenced glucose metabolism which had been increased by insulin. These data confirm the presence of both alpha and beta adrenergic receptors on hamster epididymal adipocytes and suggest that they exert antagonistic influences on lipolysis and glucose oxidation. These data are also consistent with the view that adrenergic stimulation of glucose oxidation and lipolysis in adipocytes are both mediated through beta receptors.