Divergent biological effects of adenosine and dibutyryl adenosine 3',5'-monophosphate on the isolated fat cell

The insulin-like effects of phorbol myristate acetate (PMA) in the isolated fat cell

Recent data from many laboratories suggest that insulin stimulates diacylglycerol formation. Data presented in this manuscript demonstrate an insulin-like effect of PMA, a tumor promoting agent that mimics the action of diacylglycerol, in isolated adipocytes on; (a) glucose oxidation using uniformly labelled, C-1-labelled and C-6-labelled glucose, (b) epinephrine-induced lipolysis and (c) low Km cAMP phosphodiesterase activity. Additionally, a lipolytic effect of PMA is identified when unopposed by epinephrine. These data not only demonstrate an insulin-like effect of phorbol esters in adipose tissue but they lend support to the concept of diacylglycerol involvement in the mechanism of insulin action.

Adenosine 3',5'-cyclic-monophosphate and outflow facility in monkey eyes with intact and retrodisplaced ciliary muscle

Bolus intracameral infusion of 100 micrograms cAMP, corresponding to an initial intracameral concentration of 3 mM, significantly increased total outflow facility in cynomolgus monkey eyes with intact or disinserted-retrodisplaced ciliary muscles by 41% and 37% respectively. Lower cAMP doses had little or no effect in both types of eyes, as did dibutyryl cAMP doses of 1-100 micrograms (20 microM-2mM). In eyes with intact ciliary muscles, intracameral infusion (1 and 10 micrograms) of the cAMP analog 8-methylthio cAMP (0.25 mM and 2.5 mM) significantly increased facility by 55% and 69% respectively, while 100 micrograms of the inactive cAMP metabolite 5'AMP (3 mM) did not alter facility. The facility increases produced by cAMP and 8-methylthio cAMP represented a constant percentage of starting facility independent of the absolute value of starting facility. These findings indicate that the facility increase produced by cAMP and appropriate analogs is not mediated by ciliary-muscle contraction and that the disinsertion procedure itself does not compromise the ability of the outflow pathways to respond to cAMP. The facility increase probably represents a cAMP effect directly on the endothelial cells of the trabecular meshwork-Schlemm's canal inner wall.

Total iridectomy does not alter outflow facility responses to cyclic AMP in cynomolgus monkeys

Total outflow facility was measured by two-level constant pressure anterior chamber perfusion in surgically untouched and totally iridectomized cynomolgus monkey eyes receiving bolus intracameral infusions of cyclic AMP, dibutyryl cyclic AMP, or 5'AMP. Cyclic AMP doses of 50 and 100 micrograms (corresponding to initial intracameral concentrations of 1.5- and 3 mM) increased facility by approximately 20 and 40% respectively in both surgically untouched and aniridic eyes. Higher doses produced no greater effect, and lower doses were ineffective. Dibutyryl cyclic AMP and 5'AMP were ineffective in doses up to 100 micrograms (2- and 3 mM respectively). These findings indicate that the iris plays no mechanical role in mediating the facility response to cyclic AMP, and that the iridectomy procedure per se does not compromise the ability of the outflow pathways to respond to cyclic AMP. Since untouched and aniridic eyes exhibit similar facility responses to epinephrine and norepinephrine, the present findings also suggest that the iris is not the source of cyclic AMP mediating facility responses to catecholamines.

Properties of cAMP-induced transmembrane current in mollusc neurons

The effects of different substances affecting some or other chains of cAMP metabolism on the properties of transmembrane current induced by cAMP injection have been studied in Helix pomatia neurons. It was found that preinjection of papaverine or adenosine triphosphate from a microelectrode into the neuron increases the cAMP-current amplitude. Injection of dibutyryl-cAMP does not induce transmembrane current by itself but results in a noticeable reversible inhibition of cAMP-current. Extracellular administration of trifluoperazine produces either an increase or a decrease of the cAMP-current amplitude in different cells but in both cases its removal restores the initial amplitude of cAMP-current. Furosemide has no appreciable effect on cAMP-current. An increase in intracellular Ca2+ concentration (by iontophoretic injection through a microelectrode, generation of a burst of action potentials, application of dinitrophenol, tetraphenylphosphonium or caffeine) considerably enhances the amplitude of cAMP-current. The amplitude of cAMP-current remains increased for many minutes after return of the intracellular Ca2+ level to its initial value. A possible physiological role of the observed effect is discussed.

Anti-lipolytic action of prazosin in hamster fat cells

Using isolated white fat cells of the hamster, we studied the possible inhibitory effects on lipolysis of the preferentially selective alpha-1-adrenoceptor blocking and anti-hypertensive agent prazosin. Parallel studies were undertaken with the preferentially alpha-2-adrenoceptor blocking agent yohimbine. Neither of the compounds had any significant effects on basal (non-stimulated) lipolysis. When lipolysis was stimulated by noradrenaline or isoprenaline, however, prazosin showed significant anti-lipolytic effects from a concentration of 10(-6) M. Lipolysis stimulated by methylisobutylxanthine was antagonized by prazosin only at a concentration of 10(-4) M, and dibutyrylcyclic adenosine 3'5' monophosphate-stimulated lipolysis was not inhibited at all. It is suggested that prazosin inhibits lipolysis by preventing adenyl cyclase activation by an indirect mechanism not identifiable with an alpha-2-adrenergic function but associated possibly with the formation of a local inhibitory mediator. These observations are compatible with a reduction of the outflow of free fatty acids from adipose stores in patients undergoing prazosin therapy and thus a breaking of the "vicious circle" leading to elevated plasma lipoproteins and may explain, in part at least, the reduction in plasma lipid levels observed during prazosin treatment in man.

Cyclic AMP and lipogenesis in fat cells from thyroidectomized rats

Thyroid hormones regulate lipid metabolism by affecting lipogenesis as well as lipolysis. The present paper discusses the way thyroidectomy induced an enhancement in lipogenesis in rat fat cells. The doubling in the conversion of glucose to CO2 and fatty acids seen after thyroidectomy was found to be due to a modification in the actual pathway of glucose metabolism: there was a preferential stimulation of the conversion of glucose to CO2 by the pentose cycle (utilisation of [1-14C]glucose) while the production of fatty acids and glyceride-glycerol proceeded, respectively, much more, or only slightly more, via the pathway of [6-14C]glucose metabolism. Studies employing the phosphodiesterase inhibitor MIX, or the cyclic AMP analogue, DBcAMP showed that the lipogenic process depends on cyclic AMP. As the stimulatory effect of thyroidectomy was not abolished, however, lipogenesis must be under the independent control of both cyclic AMP and absence of thyroid hormones. Insulin, a further mediator of lipogenesis was found to further enhance the already preexisting high conversion of glucose to CO2 in fat cells from thyroidectomized rats. It is concluded that at least three factors modify lipogenesis: thyroidectomy, cyclic AMP and insulin; each achieving its effect in an independent manner.

On the secretagogue effect of dibutyryl cyclic AMP in the rat exocrine pancreas

DbcAMP greater than or equal to 0.1 mM induces the discharge of exportable enzymes from rat pancreas fragments incubated in vitro. This effect is qualitatively similar to the action of physiological secretagogues acting via hormone receptors: 1) it is accompanied by the appearance of exocytotic images at the acinar cell apex; 2) it is energy dependent but energy supply is low while that required for the carbamylcholine or caerulein response is high and can only be afforded by oxidative phosphorylation; 3) it is calcium dependent, but no alteration of inward or outward calcium movement can be observed; 4) it is altered by agents known to disrupt the microfilamentous microtubular system [41]. However, the secretory response to DbcAMP is quantitatively less than that obtained with hormonal stimuli. A damaging effect of DbcAMP on pancreatic acinar cells is ruled out on histological and biochemical grounds: there is no significant leakage of LDH; protein synthesis, 2-deoxy-D-glucose and L-leucine uptake are unaltered. The secretagogue effect of DbcAMP is reversible, dose-related and specific. It is not mediated by neurotransmitter release or by interaction with their receptors. The evidence presented points to a direct interaction of DbcAMP on the pancreatic acinar cell and suggests the last step of the secretory cycle as the most probable site of action of the nucleotide derivative.

Stimulation by N6,O2'-dibutyrul andenosine 3',5'-cyclic monophosphate of RNA and DNA synthesis and of cell proliferation of rat hepatocytes in primary tissue culture

The effects of DBcAMP in doses from 1.5 x 10(-8) to 1.5 x 10(-3) M on the compartmental apparent surface area (ASA) and (5(-3H)uridine radioactivity concentration (URC), (methyl-3H)thymidine labelling index per 1 hour ([Me-3H]Tdr LI/h) and per cent mitotic index (MI%) and colchicine metaphase index (CMI%) of young rat differentiated hepatocytes in primary tissue culture were investigated by morphometric and radioautographie methods. In these cells DBcAMP was found to elicit: (1) progressive increments in the ASA of nucleoli, karyoplasm and cytoplasm; (2) peak increases in nucleolar URC at 1.5 x 10(-8) and 10(-5) M, but a slight decrease at 1.5 x 10(-3) M; (3) singificant increments in karyoplasmic and total nuclear URC at all doses, except at 1.5 x 10(-6) and 10(-4) M, when such parameters remained at control levels; (4) steady and progressive increases in cytoplasmic and total cell URC values; (5) marked increments in (Me-3H)Tdr LI/h, MI% and CMI% up to the dose of 1.5 x 10(-4) M, but at 1.5 x 10(-3) M these parameters were found to be either much less enhanced or to approach closely to control values. cAMP in doses from 1.5 x 10(-8) to 10(-4) M also markedly incremented the in vitro hepatocyte CMI%, while having a lesser stimulatory effect at 1.5 x 10(-3)M. Finally of the various possible metabolites of DBcAMP administered at 1.5 x 10(-8) M to liver cultures, N6- and O2'-MBcAMP and, again, cAMP significantly increased the CMI%, of cultured hepatocytes, whereas 5'-AMP, adenosine and allantoin had no significant effect and Na-butyrate slightly decreased it. The present observations strengthen the hypothesis that cAMP and its butyrated derivatives, by possibly amplifying the template activity of the liver chromatin, accelerate the flow of differentiated primary young rat hepatocytes into the various stages of the mitotic cell cycle.

Stimulation of cartilage macromolecule synthesis by adenosine 3',5'-monophosphate

The role of cyclic AMP in the regulation of cartilage macromolecule synthesis in vitro was studied in pelvic cartilage from 10-12 day chick embryos. Incubation of cartilages in medium containing 0.5 mM cyclic AMP resulted in a 30% inhibition of 35SO4-2, [3H]leucine and [3H]uridine incorporation into proteoglycan, total protein and RNA, respectively. Higher concentrations of cyclic AMP had no greater effects. In contrast, butyrylated cyclic AMP derivatives (0.5-5.0 mM) added to the incubation medium stimulated (50-100%) the incorporation of these radiolabeled precursors into cartilage macromolecules. Theophylline, in concentrations (0.1-0.5 mM) which raise intracellular cyclic AMP, also increases the incorporation of radiolabeled precursors into macromolecules. The data indicate that exogenous cyclic AMP and butyrylated cyclic AMP derivatives have paradoxical effects on cartilage macromolecule synthesis. Butyrylated cyclic AMP derivatives, not exogenous cyclic AMP, mimic the effects of intracellular cyclic AMP. Incubation of embryonic chicken cartilage with exogenous cyclic AMP results in the extracellular degradation of the cyclic AMP to adenosine. Adenosine (0.125 mM) inhibits precursor incorporation into cartilage macromolecules. The metabolism of exogenous cyclic AMP generates sufficient adenosine to account for the observed inhibitory effects of exogenous cyclic AMP on cartilage macromolecule synthesis. Butyrylated cyclic AMP derivatives are not degraded during incubation with cartilage. The data indicate that cartilage is a tissue in which the effect of cyclic AMP is to stimulate anabolic processes.

The action of dibutyryl adenosine-3':5'-cyclic monophosphoric acid and theophylline on the isolated cat parotid gland

In the isolated cat parotid gland intraarterially applied dibutyryl cyclic AMP (db-cAMP) (1.0 mM) produced a slow, but often maintained salivary flow. db-cAMP was also able to potentiate secretion evoked by supramaximal doses of intraarterially applied acetylcholine. Similar secretory effects were obtained also after intraarterial application of theophylline (1.0 mM). The secretory responses due to db-cAMP or theophylline were unaccompanied by measurable acinar membrane potential changes and stayed unchanged after cutting the parasympathetic innervation of the gland and after blocking both cholinergic and beta-adrenergic receptors with atropine (10-minus 7 M), and with D-(minus)-N-isopropyl-p-nitrophenol-ethanolamine (INPEA) (10-minus 5 M), respectively. The possibility of the existence of an acinar adenylate cyclase system functionally linked to the beta-adrenergic membrane receptor is discussed.

Modulation of adenylate cyclase activity in liver and fat cell membranes by insulin

Insulin depresses both the activity of adenylate cyclase stimulated by glucagon, epinephrine, and sodium fluoride in liver cell membranes and the activity of adenylate cyclase stimulated by epinephrine and adrenocorticotropin in particulate preparations from homogenates of isolated fat cells. Significant inhibition is detected with very low concentrations (10(-11) molar) of insulin but not with unphysiologically high (10(-9)molar) concentrations of the hormone. These direct effects of insulin on an enzymatic system in broken-cell -preparations suggest a fundamental role of adenylate cyclase activity and of cyclic adenosine monophosphate in the mechanism of action of insulin.

Metabolic control mechanisms in mammalian systems. Involvement of adenosine 3':5'-cyclic monophosphate in androgen action

1. The ability of exogenously administered cyclic AMP (adenosine 3':5'-monophosphate) to exert andromimetic action on certain carbohydrate-metabolizing enzymes was investigated in the rat prostate gland and seminal vesicles. 2. Cyclic AMP, when injected concurrently with theophylline, produced marked increases in hexokinase, phosphofructokinase, glyceraldehyde phosphate dehydrogenase, pyruvate kinase, and two hexose monophosphate-shunt enzymes, as well as alpha-glycerophosphate dehydrogenase activity in accessory sexual tissues of castrated rats. The 6-N,2'-O-dibutyryl analogue of cyclic AMP caused increases of enzyme activity that were greater than those induced by the parent compound. 3. Time-course studies demonstrated that, whereas significant increases in the activities of most enzymes occurred within 4h after the injection of cyclic AMP, maximal increases were attained at 16-24h. 4. Increase in the activity of the various prostatic and vesicular enzymes was dependent on the dose of cyclic AMP; in most instances, 2.5mg of the cyclic nucleotide/rat was sufficient to elicit a statistically significant response. 5. Administration of cyclic AMP and theophylline also produced stimulation of enzyme activities in secondary sexual tissues of immature rats. 6. Cyclic AMP and theophylline did not affect significantly any of the enzymes studied in hepatic tissue. 7. Stimulation of various carbohydrate-metabolizing enzymes in the prostate gland and seminal vesicles by cyclic AMP was independent of adrenal function. 8. Concurrent treatment with actinomycin or cycloheximide prevented the cyclic AMP- and theophylline-induced increases in enzyme activities in both castrated and adrenalectomized-castrated animals. 9. Administration of a single dose of testosterone propionate (5.0mg/100g) to castrated rats caused a significant increase in cyclic AMP concentration in both accessory sexual tissues. 10. In addition, treatment with theophylline potentiated the effects of a submaximal dose of testosterone (1.0mg/100g) on all those prostatic and seminal-vesicular enzymes that are increased by exogenous cyclic AMP. 11. The evidence indicates that cyclic AMP may be involved in triggering the known metabolic actions of androgens on secondary sexual tissues of the rat.

Human lymphocytic metabolism. Effects of cyclic and noncyclic nucleotides on stimulation by phytohemagglutinin

The effects of extracellular nucleotides and agents which elevate intracellular cyclic adenosine 3',5'-monophosphate (cyclic AMP) concentrations on human lymphocyte metabolism have been studied. Aminophylline, isoproterenol, and prostaglandins, all of which elevate lymphocyte cyclic AMP levels, inhibited incorporation of (3)H-labeled thymidine, uridine, and leucine into the DNA, RNA, and protein of phytohemagglutinin (PHA)-stimulated lymphocytes. Aminophylline inhibition was maximal only when the inhibitor was added within 1 hr after exposure of cells to PHA, suggesting that a relatively early step in the lymphocyte transformation process may be affected. The addition of various nucleotides to the culture medium also inhibited incorporation of labeled precursors. The best inhibitor, dibutyryl cyclic AMP (DU cyclic AMP), produced maximal inhibition only if present during the 1st hr after initial exposure to PHA. Among the various cyclic nucleotides derivatives of guanosine and adenine were the most effective inhibitors (substantial inhibition at 0.1 mM concentrations). However, the inhibition was not specific for nucleotides containing the cyclic phosphodiester moiety since the tri-, di-, and monophosphates of adenosine and guanosine were equally effective in diminishing thymidine uptake. The above inhibitions were not due to secondary effects of the inhibitors on the interaction of PHA with lymphocytes as judged by (125)I-labeled PHA binding studies.Low concentrations (1-10 mumoles/liter) of cyclic AMP produced slight stimulation of thymidine-(3)H uptake in resting lymphocytes (lymphocytes not stimulated with PHA). However, the effects were quite small as compared with those produced by PHA itself. Attempts to demonstrate increased thymidine uptake 48 hr after pulsing lymphocytes with aminophylline or isoproterenol were unsuccessful. The relationship of these observations to a possible regulatory role for cyclic AMP in PHA-stimulated lymphocytes is discussed.