Significance of acid amyloglucosidase in sulphonylurea-induced insulin release

Summary The effect of exogenous acid amyloglucosidase on sulphonylurea-induced insulin release was investigated in mice and rats. 1. Pretreatment of mice with acid amyloglucosidase enhanced insulin release induced by the different sulphonylurea derivatives, carbutamide, tolbutamide, glibenclamide, and glibornuride. 2. A dose-response relationship between glibenclamide-induced insulin response and amyloglucosidase dosage covering a 64-fold concentration range was established in mice. 3. Pretreatment of the animals with other macromolecules of similar physiological or chemical properties to acid amyloglucosidase such as-amylase,-glucuronidase and albumin did not influence glibenlamide-induced insulin release. 4. The effect of acid amyloglucosidase pretreatment on insulin release induced by different agents known to affect the islet-cell adenylate cyclase-cyclic AMP system such as secretin, L-isopropylnoradrenaline (L-IPNA), arginine, glibenclamide and 3-isobutyl-1-methylxanthine (IBMX) was tested. It was observed that in animals pretreated with acid amyloglucosidase, insulin release was enhanced when stimulated by glibenclamide, a phosphodiesterase inhibitor, but it was similarly enhanced by arginine, a phosphodiesterase activator. Insulin release induced by secretin, L-IPNA, and IBMX was unaffected. 5. Acid amyloglucosidase pretreatment in rats enhanced plasma immunoreactive insulin levels following glibenclamide injection not only in the peripheral veins but also in the portal vein. 6. Mice fasted for 24 hrs displayed a markedly depressed insulin response to tolbutamide injection. Pretreatment of the fasted animals with acid amyloglucosidase could restore the tolbutamide-induced insulin release to the same level as that recorded in a group of freely fed mice. It is suggested that acid amyloglucosidase plays an important role in insulin secretion induced by sulphonylureas. Most evidence suggests that this effect is exerted within the B-cell although an additional effect on the liver cannot be ruled out.     

Influence of isobutylmethylxanthine on the direct current electroretinogram of albino rabbit eyes

The direct current electroretinogram and the standing potential of the eye were recorded from both eyes of unilaterally vitrectomized albino rabbits. The effect of intraocular irrigation with the nonselective phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) was compared with that of a balanced salt control solution. IBMX (0.5mM) produced a reversible increase in the amplitudes of the a-wave (19%, p < 0.02), b-wave(21%, p < 0.001) and c-wave (12%, p < 0.01) of the electroretinogram. A small elevation of the standing potential was found (0.4 ± 0.2mV), but this increase was not statistically significant (p > 0.05). The results indicate a primary and principal influence of IBMX on the photoreceptors.     

Divergent effects of methylxanthines and adenylate cyclase agonists on monocyte cytotoxicity and cyclic AMP levels

The effects of theophylline, isobutylmethylxanthine (IBMX), prostaglandin E1 (PGE1), and isoproterenol on monocyte antibody-dependent cytotoxicity (ADCC) were compared with their effects on monocyte cyclic adenosine 3':5'-monophosphate (cAMP) levels. Theophylline (2 mmol/l) halved ADCC and gave a 2-fold increase in cAMP levels. At concentrations not elevating cAMP theophylline inhibited ADCC significantly. In comparison, incubation of monocytes with IBMX, PGE1 and isoproterenol ADCC was only modestly inhibited while these agents gave larger increments (3- to 8-fold) in cAMP levels than theophylline did. Low concentrations of IBMX (50 mumol/l) elevated cAMP without affecting monocyte ADCC whereas PGE1 and isoproterenol inhibited ADCC dose-dependently comparable to increases in cAMP. However, in doses giving similar inhibition of ADCC addition of PGE1 resulted in larger cAMP increments than isoproterenol. The effects of IBMX, PGE1 and isoproterenol was dependent on target cell to effector cell ratio and increased during preincubation with the agents. The inhibition of ADCC by the agents was accompanied by a depressed monocyte lysozyme release and depressed activation of hexose monophosphate shunt. However, only theophylline affected monocyte attachment to sensitized target cells. These results argue against the general inverse relationship between cAMP content and inhibition of monocyte ADCC and demonstrate that theophylline independent on increases in cAMP inhibits ADCC probably by abrogation of monocyte binding activity.     

The significance of the nongenomic pathway in mediating inflammatory signaling of the dioxin-activated Ah receptor to cause toxic effects

Evidence has been accumulating to indicate that the current classical model of dioxin's action based on the ligand-activated aryl hydrocarbon receptor (AHR) and AHR nuclear translocator (ARNT) dimer directly activating its target genes is not robust enough to explain many of the major toxic effects of this compound. In this review, efforts have been made to analyze the results of recent investigations in our laboratory in comparison to already existing evidence on the patterns of toxic actions of dioxin (=TCDD) from other laboratories from a specific viewpoint of elicitation of cellular inflammatory signaling by the ligand-activated AHR. The most salient features of the inflammatory action of TCDD are that its triggering events, such as the rapid increase in intracellular Ca²⁺ concentration, enzymatic activation of cytosolic phospholipase A2 (cPLA2) and that of Cox-2 are taking place through the nongenomic action of the ligand-activated AHR. This nongenomic pathway does not require ARNT. Therefore, this inflammation pathway is clearly discernable from the classical, genomic action pathway. The effect of such a nongenomic signaling persists for long time periods as shown by recent findings that artificial suppression of the early triggering events of this pathway, such as via suppression of cPLA2, Cox-2, or Src kinase indeed causes significant reduction of manifestations of hallmark toxicities of TCDD such as wasting syndrome and hydronephrosis. Together, the evidence strongly support the notion that the inflammatory action of the ligand-activated AHR that is mediated by the nongenomic pathway plays the major role in the inflammation inducing actions of dioxin-like chemicals.     

Histamine H2 receptor overexpression induces U937 cell differentiation despite triggered mechanisms to attenuate cAMP signalling

Knowing that cell-surface receptors that recognize and respond to extracellular stimuli are key components for the regular communication between individual cells required for the survival of any living organism, the aim of the present work was to investigate the effect of H2R overexpression on the U937 signal transduction pathway and its consequences on cell proliferation and differentiation. The overexpression of H2R led to an increase in cAMP basal levels, a leftward shift of agonist concentration-response curves, and similar maximal response to agonist treatment, suggesting that overexpressed H2Rs act as functional spare receptors. In this system cells triggered several mechanisms tending to restore cAMP basal levels to those of the na?ve cells. H2R overexpression induced PDE activity stimulation and GRK2 overexpression. In spite of the onset of these regulatory mechanisms, H2 agonist and rolipram treatments induced the terminal differentiation of the H2R overexpressed clone, conversely to the na?ve cells. Present findings show that stably H2R overexpression alters cAMP signalling as the result of not only the amounts of second messenger generated but also the activation or upregulation of various components of signalling cascade, leading to an adapted biologically unique system. This adaptation may represent an advantage or a disadvantage, depending on the biological system, but in any case, the existence of compensatory mechanisms should be considered when a clinical treatment is designed.     

Differential regulation of IL-4 expression and degranulation by anti-allergic olopatadine in rat basophilic leukemia (RBL-2H3) cells

Olopatadine hydrochloride (olopatadine) is an anti-allergic drug that functions as a histamine H(1) antagonist and inhibits both mast cell degranulation and the release of arachidonic acid metabolites in various types of cells. In this study, we examined the ability of olopatadine to inhibit the expression of cytokine genes in vitro via high-affinity receptors for immunoglobulin E in mast cells, using a rat basophilic leukemia (RBL-2H3) cell line and an in vivo mouse model. Levels of gene expression in RBL-2H3 cells were determined by semi-quantitative RT-PCR, and serum interleukin-4 (IL-4) level in mice was quantified by ELISA. Olopatadine inhibited significantly the induction of IL-4 expression by mast cells both in vivo and in vitro. Olopatadine inhibited Ca(2+) influx through receptor-operated channels (ROC) without affecting Ca(2+) release from intracellular stores. Comparative analysis of olopatadine with other anti-allergic drugs and the ROC blocker SKF-96365 demonstrated that the potency of inhibition of Ca(2+) influx correlated with the degree of suppression of degranulation and arachidonic acid release. Inhibition of Ca(2+) influx decreased phosphorylation of p38 mitogen-activated protein kinase and c-Jun NH(2)-terminal kinase, which participate in regulation of cytokine (e.g. IL-4) gene expression. However, the rank order of inhibition of Ca(2+) influx did not correspond to reduction of IL-4 expression, suggesting that an unknown mechanism(s) of action, in addition to inhibition of Ca(2+) influx, is involved in the expression of cytokines in mast cells.     

D-1 and D-2 dopaminergic receptors regulate protein phosphorylation in the rat neurohypophysis

Dopamine stimulates the phosphorylation of the neuron-specific synaptic vesicle proteins Synapsin I, Protein IIIa and Protein IIIb in the posterior pituitary gland of the rat [Tsou and Greengard (1982) Proc. natn. Acad. Sci. U.S.A. 79, 6075-6079]. This effect has been characterized in the present investigation. The stimulatory effect of dopamine was mimicked by the selective D-1 receptor agonist SKF 38393 and was competitively and potently inhibited by the selective D-1 receptor antagonist SKF 83509 as well as by the mixed D-1/D-2 antagonist fluphenazine. Conversely, the effect of dopamine was attenuated by a D-2 receptor agonist (LY 141865) and potentiated by a D-2 receptor antagonist (sulpiride). Norepinephrine also stimulated phosphorylation of the synaptic vesicle proteins, apparently through activation of the D-1 receptor. D-1 and D-2 dopaminergic receptors may play a role in the regulation of hormone secretion from the neurohypophysis. Evidence exists that in the isolated neurophypophysis activation of D-1 receptors facilitates, while activation of D-2 receptors inhibits, release of vasopressin. Further work will be required to determine whether the regulation by D-1 and D-2 receptors of the protein phosphorylation in the neurohypophysial peptidergic terminals is related to the regulation by those receptors of the neurohypophysial hormone secretion.