Glucagon-(19-29), a Ca2+ pump inhibitory peptide, is processed from glucagon in the rat liver plasma membrane by a thiol endopeptidase

Postabsorptive hyperglucagonemia in patients with type 2 diabetes mellitus analyzed with a novel enzyme-linked immunosorbent assay

Aims/introduction

The aims of the present study were to investigate the performance of a novel sandwich enzyme‐linked immunosorbent assay (ELISA) for measuring glucagon (1–29) with monoclonal antibodies against both the C‐ and N‐terminal regions of glucagon (1–29), and to analyze the differences in plasma levels and responses of glucagon (1–29) to oral glucose loading in normal glucose tolerance (NGT) subjects and patients with type 2 diabetes mellitus.

Materials and Methods

The cross‐reactivity against proglucagon fragments using the ELISA kit and two types of conventional radioimmunoassay (RIA) kits was evaluated. A 75‐g oral glucose tolerance test was carried out with NGT subjects and patients with type 2 diabetes mellitus, and the glucagon (1–29) concentration was measured using three types of kit.

Results

The ELISA kit clearly had the lowest cross‐reactivity against miniglucagon (19–29) and glicentin (1–61). The oral glucose tolerance test was carried out with 30 NGT and 17 patients with type 2 diabetes mellitus. The glucagon (1–29) levels measured by the ELISA kit after glucose loading were significantly higher at all time‐points in the type 2 diabetes mellitus group than in the NGT group. However, the glucagon (1–29) levels measured by one RIA kit were significantly higher in the NGT group, and those measured with the other RIA kit were approximately the same among the groups.

Conclusions

The novel sandwich ELISA accurately determines plasma glucagon (1–29) concentrations with much less cross‐reactivity against other proglucagon fragments than conventional RIA kits.

Method for characterization of PEGylated bioproducts in biological matrixes

PEGylation of peptides and proteins has been widely used to enhance stability and reduce immunogenicity of biotherapeutics. Characterizing the degradation of these PEGylated products in biological fluids can yield essential information to support pharmacokinetic evaluations and provide clues about their in vivo properties useful for further molecular optimization. In this paper, we describe a novel and uncomplicated approach to characterize PEGylated peptides or proteins and their related degradation products in biological matrixes. The method involves direct liquid chromatography/mass spectrometry (LC/MS) analysis of animal sera containing low nanograms to low micrograms per milliliter of PEGylated product with or without an acetonitrile precipitation sample treatment. Applying the methodology to analyze the model PEGylated peptides, 20K PEGylated-Pancreatic Polypeptide analogue (PPA) and 20K PEGylated-glucagon, we elucidated the decomposition pathways occurring in animal sera. The data provided direct evidence of cleavages within the peptide backbone. The identified degradation products were unambiguously confirmed by tandem mass spectrometry with high-energy C-trap dissociation (HCD) analysis, followed with in-source fragmentation. Additional spiking studies demonstrated nearly full recovery of PEGylated products, linear detection when the spiked concentration of PEGylated product was ≤1000 ng/mL, and a low ng/mL limit of quantitation (LOQ).

Is there a specific role for the plasma membrane Ca2+ -ATPase in the hepatocyte?

The plasma membrane Ca2+ -ATPase (PMCA) is responsible for the fine, long-term regulation of the cytoplasmic calcium concentration by extrusion of this cation from the cell. Although the general kinetic mechanisms for the action of both, well coordinated hydrolytic activity and calcium transport are reasonably understood in the majority of cell types, due to the complex physiologic and biochemical characteristics shown by the hepatocyte, the study of this enzyme in this cell type has become a real challenge. Here, we review the various molecular aspects known to date to be associated with liver PMCA activity, and outline the strategies to follow for establishing the role of this enzyme in the overall physiology of the hepatocyte. In this way, we first concentrate on the basic biochemical aspects of liver cell PMCA, and place an important emphasis on expression of its molecular forms to finally focus on the critical hormonal regulation of the enzyme. Although these complex aspects have been studied mainly under normal conditions, the significance of PMCA in the calcium homeostasis of an abnormal liver cell is also reviewed.

Differential regional metabolism of glucagon in anesthetized pigs

Glucagon metabolism under basal (endogenous) conditions and during intravenous glucagon infusion was studied in anesthetized pigs by use of midregion (M), COOH-terminal (C), and NH2-terminal (N)-RIAs. Arteriovenous concentration differences revealed a negative extraction of endogenous glucagon immunoreactivity across the portal bed (-35.4 +/- 11.0, -40.3 +/- 9.6, -35.6 +/- 16.9%, M-, C-, N-RIA, respectively), reflecting net secretion of pancreatic glucagon and intestinal glicentin and oxyntomodulin, but under exogenous conditions, a net extraction occurred (11.6 +/- 3.6 and 18.6 +/- 5.7%, C- and N-RIA, respectively). Hindlimb extraction of endogenous (17.4 +/- 3.7%, C-RIA) and exogenous (29.1 +/- 4.8 and 19.8 +/- 5.1%, C- and M-RIA) glucagon was detected, indicating M and C cleavage of the molecule. Renal extraction of glucagon was detected by all assays under endogenous (19.4 +/- 6.7, 33.9 +/- 7.1, 29.5 +/- 6.7%, M-, C-, N-RIA) and exogenous conditions (46.9 +/- 4.8, 46.4 +/- 6.0, 47.0 +/- 7.7%; M-, C-, N-RIA), indicating substantial elimination of the peptide. Hepatic glucagon extraction was undetectable under basal conditions and detected only by M-RIA (10.0 +/- 3.8%) during glucagon infusion, indicating limited midregional cleavage of the molecule. The plasma half-life determined by C- and N-RIAs (2.7 +/- 0.2 and 2.3 +/- 0.2 min) were similar, but both were shorter than when determined by M-RIA (3.2 +/- 0.2 min, P < 0.02). Metabolic clearance rates were similar regardless of assay (14.4 +/- 1.1, 13.6 +/- 1.7, 17.0 +/- 1.7 ml x kg-1 x min-1, M-, C-, N-RIA). Porcine plasma degraded glucagon, but this was not significantly affected by the dipeptidyl peptidase IV (DPP IV) inhibitor valine-pyrrolidide, and in anesthetized pigs, glucagon's metabolic stability was unchanged by DPP IV inhibition. We conclude that tissue-specific metabolism of glucagon occurs, with the kidney being the main site of removal and the liver playing little, if any, role. Furthermore, valine-pyrrolidide has no effect on glucagon stability, suggesting that DPP IV is unimportant in glucagon metabolism in vivo, in contrast to its significant role in the metabolism of the other proglucagon-derived peptides and glucose-dependent insulinotropic polypeptide.

Atrial natriuretic peptide attenuates Ca2+ oscillations and modulates plasma membrane Ca2+ fluxes in rat hepatocytes

BACKGROUND & AIMS

Oscillations in cytosolic free Ca2+ concentration are a fundamental mechanism of intracellular signaling in hepatocytes. The aim of this study was to examine the effects of atrial natriuretic peptide (ANP) on cytosolic Ca2+ oscillations in rat hepatocytes.

METHODS

Cyclic guanosine monophosphate (cGMP) was measured by enzyme immunoassay. Cytosolic Ca2+ oscillations were recorded from single aequorin-injected hepatocytes. Ca2+ efflux from hepatocyte populations was measured by using extracellular fura-2. Ca2+ influx was estimated by Mn2+ quench of fluorescence of fura-2 dextran injected into single hepatocytes.

RESULTS

ANP attenuated cytosolic Ca2+ oscillations through a decrease in their frequency. In addition, ANP dramatically stimulated plasma membrane Ca2+ efflux and modestly inhibited basal Ca2+ influx. All of the observed effects of ANP were mimicked by the cGMP analogue 8-bromo-cGMP (8-Br-cGMP), and were prevented by inhibition of protein kinase G. In contrast, activation of cytosolic guanylyl cyclase by sodium nitroprusside had no effect on Ca2+ efflux, Ca2+ influx, or Ca2+ oscillations.

CONCLUSIONS

ANP decreases the frequency of Ca2+ oscillations and modulates plasma membrane Ca2+ fluxes in rat hepatocytes. Attenuation of oscillatory Ca2+ signaling in hepatocytes may represent a key role for ANP in vivo.

Deficiency in plasma membrane calcium ATPase isoform 2 increases susceptibility to noise-induced hearing loss in mice

Susceptibility to noise-induced hearing loss (NIHL) is poorly understood at the genetic level. Mice homozygous for a null mutation in the plasma membrane Ca2+-ATPase isoform 2 (PMCA2) gene are deaf (Kozel et al., 1998). PMCA2 is expressed on outer hair cell stereocilia (Furuta et al., 1998). Fridberger et al. (1998) observed that the outer hair cell cytoplasmic Ca2+ concentration rises following acoustic overstimulation. We hypothesized that Pmca2+/- mice may be more susceptible to NIHL. Since the auditory brainstem response (ABR) thresholds of Pmca2+/- mice vary with the presence of a modifier locus (Noben-Trauth et al., 1997), Pmca2+/- mice were outcrossed to normal hearing CAST/Ei mice. The pre-exposure ABR thresholds of the resulting Pmca2+/+ and Pmca2+/- siblings were indistinguishable. Groups of these mice were exposed to varying intensities of broadband noise, and ABR threshold shifts were calculated. Fifteen days following an 8 h, 113 dB noise exposure, the Pmca2+/- mice displayed significant (P < or = 0.0007) permanent threshold shifts at 16 and 32 kHz that were 15 or 25 dB greater than those observed in Pmca2+/+ littermates. Pmca2 may be the first gene with a known mutated protein product that confers increased susceptibility to NIHL.

Miniglucagon (glucagon 19-29): a novel regulator of the pancreatic islet physiology

Miniglucagon, the COOH-terminal (19-29) fragment processed from glucagon, is a potent and efficient inhibitor of insulin secretion from the MIN 6 beta-cell line. Using the rat isolated-perfused pancreas, we investigated the inhibitory effect of miniglucagon on insulin secretion and evaluated the existence of an inhibitory tone exerted by this peptide inside the islet. Miniglucagon dose-dependently inhibited insulin secretion stimulated by 8.3 mol/l glucose, with no change in the perfusion flow rate. A concentration of 1 nmol/l miniglucagon had a significant inhibitory effect on a 1 nmol/l glucagon-like peptide 1 (7-36) amide-potentiated insulin secretion. A decrease in extracellular glucose concentration simultaneously stimulated glucagon and miniglucagon secretion from pancreatic alpha-cells. Using confocal and electron microscopy analysis, we observed that miniglucagon is colocalized with glucagon in mature secretory granules of alpha-cells. Perfusion of an anti-miniglucagon antiserum directed against the biologically active moiety of the peptide resulted in a more pronounced effect of a glucose challenge on insulin secretion, indicating that miniglucagon exerts a local inhibitory tone on beta-cells. We concluded that miniglucagon is a novel local regulator of the pancreatic islet physiology and that any abnormal inhibitory tone exerted by this peptide on the beta-cell would result in an impaired insulin secretion, as observed in type 2 diabetes.

Miniglucagon (glucagon 19-29), a potent and efficient inhibitor of secretagogue-induced insulin release through a Ca2+ pathway

Using the MIN6 B-cell line, we investigated the hypothesis that miniglucagon, the C-terminal () fragment processed from glucagon and present in pancreatic A cells, modulates insulin release, and we analyzed its cellular mode of action. We show that, at concentrations ranging from 0.01 to 1000 pM, miniglucagon dose-dependently (ID50 = 1 pM) inhibited by 80-100% the insulin release triggered by glucose, glucagon, glucagon-like peptide-1-(7-36) amide (tGLP-1), or glibenclamide, but not that induced by carbachol. Miniglucagon had no significant effects on cellular cAMP levels. The increase in 45Ca2+ uptake induced by depolarizing agents (glucose or extracellular K+), by glucagon, or by the Ca2+channel agonist Bay K-8644 was blocked by miniglucagon at the doses active on insulin release. Electrophysiological experiments indicated that miniglucagon induces membrane hyperpolarization, probably by opening potassium channels, which terminated glucose-induced electrical activity. Pretreatment with pertussis toxin abolished the effects of miniglucagon on insulin release. It is concluded that miniglucagon is a highly potent and efficient inhibitor of insulin release by closing, via hyperpolarization, voltage-dependent Ca2+ channels linked to a pathway involving a pertussis toxin-sensitive G protein.

Insulin, insulin-like growth factor-I (IGF-I) and glucagon: the evolution of their receptors

Insulin and glucagon, two of the most studied pancreatic hormones bind to specific membrane receptors to exert their biological actions. Insulin-like growth factors IGF-I and IGF-II are structurally related to insulin, although they are expressed ubiquitously. The biological functions of the IGFs are mediated by different transmembrane receptors, which includes the insulin, IGF-I and IGF-II receptors. The interaction of insulin, insulin related peptides and glucagon with the corresponding receptors has been studied extensively in mammals and continues to be so. At the same time, research on ectothermic animals has made enormous progress in the recent years. This paper summarizes current knowledge on insulin, IGF-I and glucagon receptors, from a comparative point of view with special attention to non-mammalian vertebrates. The review covers adult and mostly typical target tissues, and with very few exceptions, developmental aspects are not considered. Binding characteristics, tissue distribution and structure of insulin and IGF-I receptors will be considered first, because both ligands and receptors are structurally related and have overlapping functions. These sections will be followed by similar distribution of information on glucagon receptors. Readers interested in either structure or functions of insulin, IGFs and glucagon in nonmammalian vertebrates are referred to other reviews (Mommsen TP, Plisetskaya EM. Insulin in fishes and agnathans: history, structure and metabolic regulation. Rev Aquat Sci 1991;4:225-259; Mommsen TP, Plisetskaya EM. Metabolic and endocrine functions of glucagon-like peptides: evolutionary and biochemical perspectives. Fish Physiol Biochem 1993;11:429-438; Duguay SJ, Mommsen TP. Molecular aspects of pancreatic peptides. In: Sherwood NM, Hew CL, editors, Fish Physiology. vol 13. 1994:225-271; Plisetskaya EM, Mommsen TP. Glucagon and glucagon-like peptides in fishes. Int Rev Citol 1996;168:187-257.).

Miniglucagon: a local regulator of islet physiology

Miniglucagon, or glucagon-[19-29], is partially processed from glucagon in its target tissues where it modulates the glucagon action. In the islets of Langerhans, the glucagon-producing A cells contain miniglucagon at a significant level (2-5% of the glucagon content). We studied a possible control of insulin release by miniglucagon using as a model the MIN6 cell line. Miniglucagon, in the 10(-14) to 10(-9) M range, inhibited insulin release induced by glucose, glucagon, tGLP-1, or glibenclamide by 85-100% with an IC50 close to 1 pM. While no change in the cyclic AMP content was noted, Ca2+ influx was reduced in parallel with the inhibition of insulin release. Use of pharmacological modulators of L-type voltage-sensitive Ca2+ channels and bacterial toxins indicates that miniglucagon blocks insulin release by closing this type of channel via a pertussis toxin-sensitive G protein. Miniglucagon is a novel, possibly physiologically relevant, local regulator of islet function.

Effects on the hepatocyte [Ca2+]i oscillator of inhibition of the plasma membrane Ca2+ pump by carboxyeosin or glucagon-(19-29)

Single rat hepatocytes, microinjected with the Ca(2+)-sensitive photoprotein aequorin, respond to agonists acting through the phosphoinositide signalling pathway by the generation of oscillations in cytosolic free Ca2+ concentration ([Ca2+]i). The duration of [Ca2+]i transients generated is characteristic of the receptor species activated; the variability results in differences in the rate of fall of [Ca2+]i from its peak. It is conceivable that the plasma membrane Ca(2+)-ATPase (PM Ca2+ pump) may have an important role in the mechanism underlying agonist specificity. It has recently been shown that an esterified form of carboxyeosin, an inhibitor of the red cell PM Ca2+ pump, is suitable for use in whole cell studies. Glucagon-(19-29) (mini-glucagon) inhibits the Ca2+ pump in liver plasma membranes, mediated by Gs. We show here that carboxyeosin and mini-glucagon inhibit Ca2+ efflux from populations of intact rat hepatocytes. We show that carboxyeosin and mini-glucagon enhance the frequency of oscillations induced by Ca(2+)-mobilizing agonists in single hepatocytes, but do not affect the duration of individual transients. Furthermore, we demonstrate that inhibition of the hepatocyte PM Ca2+ pump enables the continued generation of [Ca2+]i oscillations for a prolonged period following the removal of extracellular Ca2+.

Arachidonic acid drives mini-glucagon action in cardiac cells

Recent studies have shown that glucagon is processed by cardiac cells into its COOH-terminal (19-29) fragment, mini-glucagon, and that this metabolite is an essential component of the contractile positive inotropic effect of glucagon (Sauvadet, A., Rohn, T., Pecker, F. and Pavoine, C. (1996) Circ. Res. 78, 102-109). We now show that mini-glucagon triggers arachidonic acid (AA) release from [3H]AA-loaded embryonic chick ventricular myocytes via the activation of a phospholipase A2 sensitive to submicromolar Ca2+ concentrations. The phospholipase A2 inhibitor, AACOCF3, prevented mini-glucagon-induced [45Ca2+] accumulation into the sarcoplasmic reticulum, but inhibitors of lipoxygenase, cyclooxygenase, or epoxygenase pathways were ineffective. AA applied exogenously, at 0. 3 microM, reproduced the effects of mini-glucagon on Ca2+ homeostasis and contraction. Thus AA: (i) caused [45Ca2+] accumulation into a sarcoplasmic reticulum compartment sensitive to caffeine; 2) potentiated caffeine-induced Ca2+ mobilization from cells loaded with Fura-2; 3) acted synergistically with glucagon or cAMP to increase both the amplitude of Ca2+ transients and contraction of electrically stimulated cells. AA action was dose-dependent and specific since it was mimicked by its non-hydrolyzable analog 5,8,11,14-eicosatetraynoic acid but not reproduced by other lipids such as, arachidic acid, linolenic acid, cis-5,8,11,14,17-eicosapentaenoic acid, cis-4,7,10,13,16, 19-docosahexaenoic acid, or arachidonyl-CoA, even in the micromolar range. We conclude that AA drives mini-glucagon action in the heart and that the positive inotropic effect of glucagon on heart contraction relies on both second messengers, cAMP and AA.

Synergistic actions of glucagon and miniglucagon on Ca2+ mobilization in cardiac cells

It has been recently shown that the physiological processing of glucagon into its C-terminal (19-29) fragment, miniglucagon, by cardiac cells was essential for the contractile positive inotropic effect of the hormone. However, the mechanisms underlying the effects of miniglucagon remained undetermined. In the present study, we assessed the effects of miniglucagon on Ca2+ homeostasis in embryonic chick ventricular myocytes. In quiescent cells, short-term applications of 0.1 nmol/L miniglucagon markedly increased the accumulation of 45Ca into intracellular compartments resistant to digitonin lysis and sensitive to caffeine. Ca2+ accumulation into the sarcoplasmic reticular (SR) store was further attested by fura 2 imaging studies on quiescent or prestimulated cells: miniglucagon potentiated Ca2+ release from the SR compartment triggered by caffeine and evoked a rise in cytosolic Ca2+ when applied on cells pretreated with 1 mumol/L thapsigargin, a specific inhibitor of the SR Ca2+ pump. Glucagon alone produced a small cytosolic Ca2+ signal that was considerably amplified by miniglucagon. The action of glucagon was mimicked by 8-bromo-cAMP and was blocked by isradipine, suggesting that it relied on the activation of L-type Ca2+ channels, via phosphorylation. We conclude that the combined actions of miniglucagon and glucagon on Ca2+ accumulation into SR stores and Ca2+ release from the same stores are likely to support the positive inotropic effect elicited in vivo by glucagon on heart contraction.

Proteolysis of glucagon within hepatic endosomes by membrane-associated cathepsins B and D

The acidic glucagon-degrading activity of hepatic endosomes has been attributed to membrane-bound forms of cathepsins B and D. Endosomal lysates processed full-length nonradiolabeled glucagon to 32 different peptides that were identified by amino acid analysis and full-length sequencing. These indicated C-terminal carboxypeptidase, endopeptidase as well as N-terminal tripeptidyl-aminopeptidase activities in endosomes. Glucagon proteolysis was inhibited 95% by E-64 and pepstatin A, inhibitors of cathepsins B and D, respectively. This was confirmed by the pH 6-dependent chemical cross-linking of [125I]iodoglucagon to a polypeptide of 30 kDa, which was immunodepleted by polyclonal anti-cathepsin B antibody, and the removal of greater than 80% of glucagon-degrading activity by polyclonal antibodies to cathepsins B and D. By similar criteria, insulin-degrading enzyme was ruled out as a candidate enzyme for endosomal proteolysis of glucagon. Lysosomal contamination was unlikely since all forms of cathepsin B in endosomes, i.e. the major 45-kDa inactive precursor as well as the lesser amounts of the 32- and 28-kDa active forms, were tightly bound to endosomal membranes. Furthermore the mature 29-kDa single-chain and 22-kDa heavy-chain forms of cathepsin L were undetectable in endosomes, although high levels of the 37-kDa proform were observed. Membrane association of the cathepsins B and D was not to the mannose 6-phosphate receptor since association was unaffected by mannose 6-phosphate and/or EDTA, thereby indicating a distinct endosomal receptor. Hence, a pool of active cathepsins B and D as well as a poorly defined tripeptidyl aminopeptidase is maintained in endosomes by selective membrane retention. These hydrolases degrade glucagon internalized into liver parenchyma early in endocytosis.

Role of amino acid sequences flanking dibasic cleavage sites in precursor proteolytic processing. The importance of the first residue C-terminal of the cleavage site

The amino acid sequences flanking 352 dibasic moieties contained in 83 prohormones and pro-proteins listed in a database were examined. Frequency calculations on the occurrence of given residues at positions P6 to P'4 allowed us to delineate a number of features which might be in part responsible for the in vivo discrimination between cleaved and uncleaved dibasic sites. These include the following: amino acids at these positions were characterized by a large variability in composition and properties; no major contribution of a given precursor subsite to endoprotease specificity was observed; some amino acid residues appeared to occupy preferentially certain precursor subsites (for instance, Met in P6 and P3, Asp and Ala in P'1, Pro in P6, Gly in P3 and P'2 etc.) whereas some others appeared to be excluded. Most amino acid residues occupying the P'1 position in these precursor cleavage sites were tolerated. But the beta-carbon branched side chain residues (Thr, Val, Leu, Ile) and Pro, Cys, Met and Trp were either totally excluded or poorly represented, suggesting that they might be unfavourable to cleavage. The biological relevance of these observations to the efficacy of dibasic cleavage by model propeptide convertases was in vitro tested using both pro-ocytocin convertase and Kex2 protease action on a series of pro-ocytocin related synthetic substrates reproducing the Pro7-->Leu15 sequence of the precursor in which the Ala13 residue (P'1 in the LysArg-Ala motif) was replaced by various amino acid residues. A good correlation was obtained on this model system indicating that P'1 residue of precursor dibasic processing sites is an important feature and may play the role of anchoring motif to S'1 convertase subsite. We tentatively propose that the present database, and the corresponding model, may be used for further investigation of dibasic endoproteolytic processing of propeptides and pro-proteins.

Miniglucagon production from glucagon: an extracellular processing of a hormone used as a prohormone

Glucagon is secondarily processed into its C-terminal (19-29) fragment, referred to as 'miniglucagon', which modulates the glucagon action. This extracellular processing, occurring at the level of of the glucagon target cells, is due to the presence at the cell surface of a new 100-kDa processing enzyme with characteristics of both thiol- and metalloprotease.

Interaction of substance P and its N- and C-terminal fragments with Ca2+: implications for hormone action

In an attempt to understand the role of Ca2+ on the bioactive conformation of peptide hormones, we have examined the interaction between Ca2+ and the neuropeptide substance P. Using CD spectroscopy to monitor conformational changes caused by Ca2+ binding, we found no significant binding of the cation by substance P in water. However, a substantial conformational change occurred in the hormone on Ca2+ addition in trifluoroethanol or an 80:20 (v/v) mixture of acetonitrile and trifluoroethanol. A biphasic binding of Ca2+ was observed in these solvents with saturation at 2 cations per hormone molecule. Mg2+ caused a relatively smaller conformational change in the hormone. A peptide corresponding to residues 1-7 at the N-terminal fragment of substance P showed a weak nonsaturating binding of Ca2+ in the nonpolar solvents whereas the 7-11 C-terminal fragment peptide displayed a binding indicative of an 1:1 Ca2+/peptide complex. Ca2+ binding by the hormone and the 7-11 fragment was also monitored by changes in fluorescence of the phenylalanyl residues. The results support the conclusion drawn from the CD data about a distinct Ca2+ binding site in the C-terminal part of substance P. The Kd values obtained from fluorescence data were 160 microM for Ca2+ and 1 mM for Mg2+ binding by substance P. The hormone and the two peptide fragments were also tested for their effect on the stability of dimyristoyl lecithin vesicles. Substance P and the N-terminal fragment caused no significant leakage of either fluorescent dyes or K+ trapped in the vesicles. Nor did they cause membrane fusion as monitored by the fluorescence quenching method.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxyntomodulin and related peptides control somatostatin secretion in RIN T3 cells

We studied the effects of oxyntomodulin (OXM), of its C-terminal (19-37) fragment (OXM (19-37)) and of glucagon (GLU) on somatostatin release, cyclic AMP accumulation and inositol phosphate turnover in somatostatin-secreting RIN T3 cells in culture. Rapid changes in cellular free Ca2+ were also measured using fura-2. Carbachol was used as a control test agent for the parameters involving the inositol phosphate/Ca2+ cascade. OXM, GLU and OXM (19-37) were all able to stimulate somatostatin release with relative ED50 of approx. 1, 22 and 45, respectively. OXM and GLU stimulated cyclic AMP levels with relative ED50 of approx. 1 and 30, respectively, whereas OXM (19-37) was totally ineffective on this parameter. In contrast to carbachol, none of the peptides significantly modified the inositol phosphate turnover or induced rapid changes in cellular free Ca2+. We conclude that the RIN T3 cells contain a receptor-cyclic AMP system similar to that found in gastric mucosa and that this system is linked to somatostatin release. Another receptor-second messenger mechanism linked to somatostatin release is triggered by the (19-37) fragment. This mechanism is not the inositol phosphate/Ca2+ cascade triggered in the same cells by cholinergic agents.

Oxyntomodulin and its (19-37) and (30-37) fragments inhibit histamine-stimulated gastric acid secretion in the conscious rat

Oxyntomodulin, a hormone released from jejuno-ileum and composed of the glucagon sequence extended by a C-terminal octapeptide displays original tissue specificity for the gastric mucosa. The aim of this study was to compare the effect of oxyntomodulin on histamine (0.4 mg/kg per h)-stimulated gastric acid secretion in the conscious rat to that of three of its fragments: oxyntomodulin-(19-37) produced from oxyntomodulin by enzymatic cleavage, oxyntomodulin-(30-37) corresponding to the molecular difference between oxyntomodulin and glucagon and oxyntomodulin-(32-37) produced during proglucagon processing to glucagon. The metabolic clearance rate (MCR) in the anesthetized rat was evaluated for each peptide. Oxyntomodulin, oxyntomodulin-(19-37) and oxyntomodulin-(30-37) inhibited the histamine-stimulated gastric acid secretion dose dependently. The dose-response curves for oxyntomodulin and oxyntomodulin-(19-37) were parallel. The potency of oxyntomodulin (19-37) (ED50 approximately 70 pmol/kg) was the same as that of oxyntomodulin (ED50 approximately 35 nmol/kg) after correction of the curve for the MCR (two-fold higher for the fragment). Although the dose-response curve for oxyntomodulin-(30-37) was not entirely parallel to that of the two other peptides, its maximal inhibition (at 15 nmol/kg) was the same as that of oxyntomodulin-(19-37). Thus, oxyntomodulin-(30-37) was an efficient as the longer molecules but approximately 150-fold less potent than oxyntomodulin. The MCR for oxyntomodulin-(30-37) was approximately 250 fold higher than that of oxyntomodulin, partly explaining the difference in potency. The (32-37) fragment, which is naturally released from the endocrine pancreas, was devoid of activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Miniglucagon [glucagon-(19-29)] is a component of the positive inotropic effect of glucagon

Glucagon is well known for its cardiotonic effect, but its mechanism of action remains undetermined. In the present study, we showed that glucagon, under minimal degradation conditions, had no effect on the amplitude of contractility of beating chick embryo ventricular cells. This raised the question of the contribution of the active metabolite of glucagon, glucagon-(19-29), referred to as miniglucagon, to the positive inotropic effect of glucagon. Incubation of glucagon with heart cells led to its rapid conversion into miniglucagon, as measured by radioimmunoassay. Accumulation of the metabolite was maximal after 8 min and remained stable until 15 min. reaching 6% of the initial glucagon concentration. Bacitracin inhibited this processing of glucagon into miniglucagon. Miniglucagon, from 0.1 pM to 1 nM, exerted a potent negative inotropic action. The most striking observation was a 45% increase in the amplitude of cell contractility elicited by the combination of 30 nM glucagon with 1 nM miniglucagon. A similar effect was obtained when glucagon was replaced by a low concentration (75 microM) of 8-bromoadenosine 3',5'-cyclic monophosphate. We conclude that glucagon processing into miniglucagon may be essential for the positive inotropic effect of glucagon on heart contraction.