Opioid peptide effects on insulin release and c-AMP in islets of Langerhans

The time course and specificity of the effect of opioid peptides on c-AMP production in the islets of Langerhans was examined. An enkephalin analogue, d-Ala2Me Phe4 Met(O)-ol enkephalin (DAMME, Sandoz) produced a significant stimulation of basal c-AMP levels, with a peak of stimulation at 5 minutes and a decline thereafter. These changes in intracellular c-AMP levels were of the same order of magnitude as those induced by other secretagogues, but did not coincide in time with the more rapid peak of enkephalin-induced insulin release. The rise in islet c-AMP and insulin secretion induced by DAMME and alpha-endorphin but not leu enkephalin was antagonised by naloxone. The effects of high and low concentrations of a variety of opioid peptides and naloxone on insulin release and islet c-AMP levels were determined, alpha-endorphin, dynorphin, leu enkephalin and met enkephalin all stimulated insulin secretion significantly, though not to the same extent. Higher concentrations of alpha-endorphin, dynorphin and met enkephalin inhibited insulin release relative to effects at low opiate concentrations. However, higher concentrations of leu enkephalin stimulated insulin release further. We conclude from these results that the mode of action of opioid peptides in stimulating insulin release is not via increased islet c-AMP exclusively. Furthermore, the results obtained with different classes of opioid suggest the presence of distinctive types of opiate receptor in islets of Langerhans.     

Augmentation of cholinergic-mediated amylase release by forskolin in mouse parotid gland

Cholinergic-mediated amylase release in mouse parotid acini was augmented by forskolin; the potency but not the maximal response to carbachol was altered. Amylase released by carbachol plus forskolin was dependent on extracellular calcium and was mimicked by the calcium ionophore, A23187 plus forskolin. Forskolin was also shown to enhance carbachol-stimulated 45Ca2+ uptake into isolated acini. Hydroxylamine, nitroprusside, and 8-bromo-c-GMP each in combination with forskolin mimicked the effects of carbachol plus forskolin on amylase release. In the presence of carbachol (10(-8)M) forskolin did not augment c-AMP levels. However, in the presence of carbachol (5 X 10(-7) M) or hydroxylamine (50 microM) forskolin did significantly augment c-AMP accumulation. These results suggest that calcium and c-GMP may mediate the augmentation of cholinergic-mediated amylase release by effects on c-AMP metabolism.     

Interconversion of myocardial adrenoceptors: its relationship to adenylate cyclase activation.

Hypothyroidism in rats induces a shift from β toward α in the properties of myocardial adrenoceptors mediating the rise in cyclic AMP (c-AMP). The change occurs in contracting but not in quiescent myocardial preparations, it can be reversed by treating hypothyroid rats with thyroxin, and it is similar to changes in the properties of adrenoceptors mediating inotropic responses in the same preparations. The results indicate that adrenoceptors mediating the rise in c-AMP and contractility are similarly interconvertible and suggest that the α-adrenoceptor mediated rise in c-AMP is a consequence of events leading to an inotropic response rather than their cause.     

Effect of sodium ions on cyclic AMP and cyclic GMP levels in mouse parotid acini

The ability of the β-adrenergic agonist, isoproterenol, to elevate intracellular levels of cyclic-AMP (c-AMP) and cyclic GMP (c-GMP) in mouse parotid acini was dependent upon the extracellular sodium concentration. In the absence of extracellular sodium isoproterenol-stimulated c-GMP and c-AMP levels were significantly reduced; carbachol-stimulated c-GMP levels were not affected. Monensin, a sodium ionophore, mimicked the effects of isoproterenol in elevating c-GMP levels; this effect was abolished in the absence of extracellular sodium. Monensin did not mimic the effects of isoproterenol in elevating c-AMP levels. The data presented suggests that sodium ions may play a role in β-adrenergic regulation of cyclic nucleotide levels in mouse parotid gland and that the mechanisms involved in regulation of c-AMP and c-GMP levels appear to be different.     

Effects of c-AMP, caffeine, theophylline, and vinblastine on spontaneous transmitter release at locust nerve-muscle junctions

The effects of c-AMP, phosphodiesterase inhibitors (caffeine and theophylline) and vinblastine on spontaneous transmitter release was investigated at locust neuromuscular junctions. c-AMP, theophylline, caffeine, and vinblastine caused facilitation of transmitter release. None of these drugs had any effect on the amplitude of miniature excitatory postsynaptic potentials (min. E.P.S.P.'s), or on the resting membrane potential, but vinblastine increased the proportion of 'large' min. E.P.S.P.'s. The effect of theophylline (but not c-AMP and caffeine) on min. E.P.S.P. frequency was found to be calcium dependent. The effects of these drugs on the locust glutamatergic synapse are compared with their actions at other synapses.     

Effect of NH3 on c-AMP associated activities and extracellular c-AMP production in Dictyostelium discoideum.

A model of morphogenetic regulation in $\text{Dictyostelium discoideum}$ (1) is based on the assumption that NH₃ inhibits the synthesis and/or release of extracellular 3′,5′-cyclic AMP and that by topographical restriction of c-AMP production to specified zones within the cell aggregate, NH₃ is presumed to set up the conditions for apical dominance and directed morphogenetic movements. This study indicates that: exposure of preaggregative cells to exogenous NH₃ + NH₄⁺ inhibits the acquisition of c-AMP-induced properties associated with aggregation competence (accumulation of specific contact sites required to form EDTA resistant aggregates and the synthesis of extracellular and membrane-bound c-AMP phosphodiesterase); exposure of aggregation competent cells which are actively producing extracellular c-AMP to exogenous NH₃ + NH₄⁺ is followed by the immediate cessation of extracellular c-AMP release. The pH dependence of these effects suggests that the active species is NH₃.     

Induction of alkaline phosphatase by cyclic AMP or its dibutyryl derivative in a hybrid line between mouse and Chinese hamster in culture

Cyclic AMP (c-AMP) and its derivative, dibutyryl-cyclic AMP (DB c-AMP) induced the activity of alkaline phosphatase (ALP) in a hybrid line. Theophylline was also effective and greatly potentiated the action of c-AMP when given along with the nucleotide. This effect was attributable neither to a direct activation of the enzyme nor to the formation of activators in the cells grown in DB c-AMP. In the presence of actinomycin S₃ or cycloheximide, the induction by DB c-AMP did not occur or was reduced very much. It is therefore suggested that c-AMP and DB c-AMP induce alkaline phosphatase activity through a new synthesis of both RNA and protein in the cells.     

Does cyclic GMP mediate amylase release from mouse parotid acini?

In mouse parotid acini both cholinergic and beta-adrenergic agonists increased intracellular levels of cyclic-GMP (c-GMP) as well as amylase release. The derivative of c-GMP, 8-bromo-c-GMP, mimicked the effects of cholinergic and beta-adrenergic stimulation on amylase release. Nitroprusside (NP), hydroxylamine (HA) and sodium azide (NaA) increased c-GMP levels and also enhanced amylase release in a dose-dependent manner; cyclic-AMP (c-AMP) levels were not affected. The phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (MIX) enhanced the effects of carbachol on both c-GMP accumulation and amylase release. These results suggest that c-GMP may mediate the actions of cholinergic agonists and at least partially mediate the actions of beta-adrenergic agonists on mouse parotid enzyme release.     

Reduction of beta-adrenoceptor function by oxidative stress in the heart

The effect of oxidative stress on beta-adrenoceptor function in the heart was determined. To this end ventricle membranes, field-stimulated rat left atria and field-stimulated rat right ventricle strips were exposed to 0.1 mM cumene hydroperoxide for 20 min. It was found that oxidative stress increased beta-adrenoceptor number and reduced c-AMP formation in the ventricle membranes. In the rat left atria and rat right ventricle strips the efficacy of beta-adrenoceptor agonists was reduced to approximately 30% of the control value, whereas maximal beta-adrenoceptor-mediated response was reduced to 50%. Using membranes from control atria and from atria exposed to oxidative stress, it was found that oxidative stress had no effect on beta-adrenoceptor density, nor on the affinity of (-)isoproterenol for the receptor. c-AMP production in membranes prepared from atria exposed to oxidative stress was reduced to approximately 30% of the c-AMP production in membranes prepared of control atria. In addition, it was found that the shape of the function that transduces the stimulus which is generated by receptor activation into an effect, is not altered by oxidative stress. It was concluded that the reduction of the efficacy of beta-adrenoceptor agonists by oxidative stress is probably caused by the reduction of c-AMP formation. Because the efficacy of forskolin and of dibutyryl c-AMP was not affected by oxidative stress, the reduced c-AMP formation is probably caused by an impaired coupling between the receptor and adenylate cyclase. The reduction of maximal beta-adrenoceptor-mediated response might be the result of cytotoxic aldehydes that are produced during oxidative stress. In ischemia, catecholamine release and subsequent beta-adrenoceptor hyperstimulation lead to cardiotoxicity. As shown in the present study, oxidative stress reduces beta-adrenoceptor function. This might represent a protective physiological feedback mechanism that protects the heart against excessive beta-adrenoceptor stimulation.     

Lack of alteration in regional brain adenosine-3',5'-cyclic monophosphate levels after acute and chronic treatment with ethanol.

Previously reported studies have suggested that acute and chronic treatment with ethanol induces alterations in adenosine-3′, 5′-cyclic monophosphate (c-AMP) levels in the brain. Because the methods used in those studies to minimize postmortem accumulation of c-AMP are now considered to be inadequate, the effects of ethanol were reinvestigated using focused microwave irradiation to prevent postmortem c-AMP accumulation. These studies were extended to include measurements in seven areas of the rat brain after acute administration of ethanol and in animals rendered ethanol-dependent. Three treatment groups were examined: acutely treated while intoxicated (6 g/kg, p.o.), ethanol-dependent while intoxicated, and ethanol-dependent while undergoing a withdrawal syndrome. No changes in c-AMP levels were observed in any of the brain areas studied after any of the ethanol treatments. The data suggest that changes in c-AMP levels in the brain do not play any role in the acute and chronic effects of ethanol.     

c-AMP and the cell cycle: inhibition of growth stimulation

This paper attempts to elucidate the role of fluctuating intracellular cyclic AMP levels in the cell cycle. Insulin- and serum-induced growth stimulation is inhibited by low doses of c-AMP administered together with the stimulator. The degree of inhibition is dependent upon the concentration of this nucleotide and can be detected first as inhibition of DNA synthesis. Controls with other cyclic nucleotides, butyric acid, and 5′-AMP confirm the unique role of c-AMP. Although dbc-AMP blocks protease-induced stimulation of growth, it has no effect on the surface alterations induced by proteases, indicating that alterations in intracellular levels of this cyclic nucleotide may be a consequence of the surface alteration and may be instrumental in governing the cell cycle. A tentative working model is presented, which considers how c-AMP levels may fluctuate and influence the cell cycle.     

Cyclic Adenosine 3′,5′-Monophosphate in Escherichia coli

The concentration of cyclic adenosine 3',5'-monophosphate (c-AMP) in Escherichia coli growing on different sources of carbon was studied. Cultures utilizing a source of carbon that supported growth relatively poorly had consistently higher concentrations of c-AMP than did cultures utilizing sugars that supported rapid growth. This relationship was also observed in strains defective in c-AMP phosphodiesterase and simultaneously resistant to catabolite repression; in such strains the c-AMP concentration was slightly higher for several sources of carbon tested. Cultures continued to synthesize c-AMP and secreted it into the medium, under conditions that brought about an inhibition of the intracellular accumulation of the cyclic nucleotide. Transient repression of the synthesis of beta-galactosidase was not associated with an abrupt decrease in the cellular concentration of c-AMP.     

Isoproterenol-stimulated renin secretion in the rat: second messenger roles of Ca and cyclic AMP

These experiments were designed to elucidate which of two second messengers (cyclic 3',5' adenosine monophosphate [c-AMP]; intracellular calcium [Cai]) was more closely related to the renin secretory process. The rat renal cortical slice preparation was used. Agents which previously were shown to inhibit basal renin secretion by increasing Cai (ouabain, vanadate, angiotensin II, antidiuretic hormone, and 60 mM K) antagonized and/or blocked isoproterenol-stimulated secretion, which is thought to be mediated by adenylate cyclase activation and increased levels of c-AMP. The stimulatory effect of dibutyryl c-AMP was antagonized and/or blocked by the same agents which antagonized and/or blocked isoproterenol-stimulated secretion. Thus, the inhibitory effects of these agents on isoproterenol-stimulated secretion cannot be explained by a Ca-induced decrease in c-AMP production. Secretory rate was stimulated by a potent phosphodiesterase inhibitor (3-isobutyl-1-methylxanthine). A combination of this and dibutyryl c-AMP produced even greater stimulation. Ouabain blocked the stimulatory effect of this combination. These results are not consistent with an invariant direct relationship between c-AMP and renin secretory rate, but are consistent with an inverse relationship between Ca; and renin secretion. Further, they are consistent with the hypothesis that in isoproterenol-stimulated renin secretion. c-AMP is the second and Cai the third or the final messenger.     

Effect of L-asparaginase on insulin secretion from isolated rat islets of Langerhans.

The effects of L-asparaginase were evaluated on glucose-induced insulin release from isolated rat islets of Langerhans. Islets were obtained by enzymatic digestion of pancreas from Sprague-Dawley rats. The study of L-asparaginase effects on insulin secretion was performed in a static incubation of islets. Insulin secretion was measured at 60 min of incubation with different secretagogues with and without L-asparaginase. L-Asparaginase at concentrations from 310 to 5,000 U/ml could inhibit the glucose-induced insulin secretion in a dose-dependent manner. This effect was not recovered after incubation in the absence of the drug for another 2 h. The half-maximal inhibitory effect of the enzyme on insulin secretion was observed at L-asparaginase concentrations of 1,000 U/ml. Tolbutamide (200 microM) and ketoisocaproic acid (20 mM) did not induce insulin secretion in the presence of moderately high L-asparaginase concentrations. L-Asparaginase did not inhibit glucose-induced insulin secretion in the presence of isobutyl-methyl-xanthine (IBMX) (20 microM) or forskolin (20 microM). L-Asparaginase promoted a decrease in total c-AMP in isolated rat islets at concentrations from 500 to 1,500 U/ml when they were stimulated by glucose. If islets were treated with IBMX or forskolin, L-asparaginase did not inhibit the glucose-induced total c-AMP levels in islets.     

Culic 3', 5', -adenosine monophosphate and catabolic repression in Escherichia coli.

Several strains of $\text{E. coli}$ were grown on different sources of carbon and β-galactosidase activity as well as intracellular and extracellular concentrations of c-AMP were determined. There was a good (inverse) correlation between extracellular concentrations of c-AMP and the intensity of catabolite repression, whereas the relationship between intracellular c-AMP levels and catabolite repression was not clear-cut.     

c-AMP plasmatic levels in Scrapie infected sheep and goats from Sicily

Scrapie is a fatal degenerative disease of the central nervous system of sheep and goats. Adenosine 3′,5′-cyclic monophosphate (c-AMP) plays a key role in many biological processes, membrane permeability, lipogenesis, metabolism, neuronal activity, muscular contraction, cellular differentiation, hormonal and enzymatic activities, proteins synthesis, etc. and in inflammation, immune processes, cellular growth regulation and carcinogenesis. In this work the c-AMP plasmatic levels in Scrapie infected sheep and goats were studied. The study was carried out on 31 sheep of Comisana breed and 35 autochthonous goats belonging to two farms from Sicily. The evaluation of c-AMP levels in blood samples, taken from the jugular vein, was carried out by radioimmunoassay (RIANEN” c-AMP 125 - Dupont NEN Diagnostic). The results obtained show significantly higher c-AMP levels in animals with Scrapie (40.88 ± 2.08 pmol/L in sheeps; 43.54 ± 1.62 pmol/L in goats) than healthy animals used as controls (26.45 ± 0.76 pmol/L in sheeps; 28.17 ± 1.58 pmol/L in goats). The increase in c-AMP plasmatic levels could be correlated to alterations of central nervous system functioning and variations of neurotransmitters (NA, Ach, GABA, etc.) responsible for behavioural and neurological changes in Scrapie infected sheep and goats.     

Effects of ATP synthesis inhibitors, temperature, 8-bromo c-AMP, and calcium omission on release of corticotropin releasing factor from superfused rat hypothalami

The release of immunoreactive corticotropin-releasing factor (I-CRF) was studied in vitro using a hypothalamic superfusion system. Omission of calcium, or inclusion of ATP synthesis inhibitors in the medium significantly reduced the rates of basal and K+-induced I-CRF release. Reducing the incubation temperature of the hypothalami to 4 degrees C caused a reversible inhibition of basal I-CRF release. I-CRF release was stimulated in a dose-dependent fashion by 8-bromo c-AMP. Those results suggest that the in vitro release of CRF is energy-dependent and may involve a calcium and c-AMP-mediated mechanism.     

An examination of baseline and drug-induced levels of cyclic nucleotides in the cerebrospinal fluid of control and psychiatric patients

Baseline c-AMP levels, and probenecid-induced accumulations of c-AMP and c-GMP in the lumbar CSF of depressed, manic, and schizophrenic patients failed to show differences when compared to controls screened to exclude affective, schizophrenic, and CNS neurological disorders. Cyclic-GMP baseline levels in all three psychiatric groups did approach a level of significance when compared to controls. The administration of psychotropic drugs to these patients failed to show significant differences in paired comparisons. Levels of c-GMP from lumbar CSF were found to be positively correlated to concentrations of c-AMP. Categorization of data according to sex, and then age did not result in a discernible trend. It seems unlikely on the basis of this investigation that measurement of cyclic nucleotides in lumbar CSF will be of significance in differentiating psychiatric disorders.     

Observations on the level of cyclic nucleotides in three population of human lymphocytes in culture

The level of cyclic nucleotides in three populations of cultured human lymphocytes were studied. An early conspicuous elevation of c-GMP level and a reciprocal relationship between c-AMP and c-GMP fluctuations were demonstrated in T cells from normal donors. Null cells from patients with ALL showed a constantly low level of c-AMP, while c-GMP fluctuated in apparent relationship with cell doubling time. Persistently low levels of c-AMP and persistently high level of c-GMP were found in B cells from patients with CLL. Possible significance of these findings is discussed.     

Role of adrenal steroids on the cardiac c-AMP response to isoprenaline in the rat

The effect of adrenalectomy on the basal cardiac c-AMP level and the elevated c-AMP level induced by intravenous administration of isoprenaline (10 mcg/kg) were determined by radio-protein assay. The basal cardiac c-AMP level was not altered by adrenalectomy. But the cardiac c-AMP responses to isoprenaline was significantly less in adrenalectomized rats, maintained with 1% sodium chloride solution for one week; this effect, however, disappeared when animals were maintained for 2 or 4 weeks. This could not be restored by acute administration of dexamethasone (100 mcg/rat). The possible reasons for these effects of adrenalectomy are discussed.