Stimulation by cholera toxin of ADP-ribosylation of membrane proteins, adenylate cyclase and insulin release in pancreatic islets

In rat pancreatic islet membranes exposed to [alpha-32P]NAD, cholera toxin stimulated the labelling of three peptides with Mr close to 22 000, 42 000 and 48 000, respectively. In the islets, the toxin-stimulated ADP-ribosylation of the heavy form of the Ns alpha-subunit predominated over that of the light form, in mirror image of the situation found in the exocrine pancreas. When intact islets were preincubated with cholera toxin, the adenylate cyclase activity of a subcellular particulate fraction was increased. The responsiveness of adenylate cyclase to GTP was also augmented, but that to NaF was decreased. In intact islets, the production of cyclic AMP and the glucose-stimulated release of insulin were also enhanced after pretreatment with cholera toxin. These findings reveal the presence in pancreatic islets of the guanyl nucleotide regulatory protein of adenylate cyclase, with an unusual predominance of the heavy form of the Ns alpha-subunit.

Binding in vitro of vasoactive intestinal peptide on isolated acini of rat parotid glands

Binding of 125I-labelled vasoactive intestinal peptide (VIP) to rat parotid acini was saturable, temperature-dependent and reversible, and reflected interaction with a single class of binding sites. Parotid glands possessed approx. 400 fmol binding sites per mg protein and binding of the tracer to these sites could be inhibited by VIP [concentration for half-maximal effect (KD), 24 nM], by the peptide histidine isoleucine (KD, 140 nM), by secretin (KD, 470 nM) and by the human pancreatic growth hormone-releasing factor (hpGRF; KD, 3200 nM). In the same acini preparation, 10 microM VIP also stimulated amylase release 4-fold and increased cyclic AMP 11-fold. Thus, VIP might be a neurotransmitter in the rat parotid gland.

Primary structure of helodermin, a VIP-secretin-like peptide isolated from Gila monster venom

The complete amino acid sequence of helodermin isolated from the venom of Gila monster was elucidated. The peptide was shown to be a basic pentatriacontapeptide amide: His-Ser-Asp-Ala-Ile-Phe-Thr-Gln-Gln-Tyr-Ser-Lys-Leu-Leu-Ala-Lys-Leu-Ala- Leu-Gln-Lys- Tyr-Leu-Ala-Ser-Ile-Leu-Gly-Ser-Arg-Thr-Ser-Pro-Pro-Pro-NH2. A high degree of sequence similarities to secretin/VIP/PHI/(PHM)/GRF from mammal and bird was observed over the entire N-terminal 1-27 sequence. In particular, the amino acid residues in positions 3, 6 and 7 were found to be common to 9 peptides of the family. Another interesting feature of the structure of helodermin was its C-terminal -Pro-Pro-Pro-NH2 sequence. Isolation of helodermin was the first demonstration of the existence of a secretin/VIP-related peptide in an animal that is neither mammal nor bird.

Cleaving of disulfide bridges and apparent molecular weight of human prolactin variants as revealed by immunoperoxidase electrophoresis

Five highly purified preparations of human pituitary prolactin, widely used in radioimmunoassays, were analyzed by immunoperoxidase electrophoresis with rabbit antisera raised against ovine and rat prolactin. When unreduced, the prolactin content of these preparations was separated into one major prolactin-like immunoreactive band of Mr 23,000, and four fainter immunoreactive bands of Mr's 16,000, 22,000, 25,000, and 45,000. Increasing the 2-mercaptoethanol concentration modified the relative proportions and the Mr of immunoreactive bands so that, after extensive reduction, only two immunoreactive bands were separated, one major band of Mr 25,000 and one fainter band of Mr 17,000. A direct relationship existed between the load of prolactin submitted to electrophoresis in the 0.25 to 75 ng range and the optical density of the separated bands.

Effect of Bordetella pertussis toxin on ADP-ribosylation of membrane proteins, adenylate cyclase activity and insulin release in rat pancreatic islets

Exposure of rat pancreatic islet membranes to [alpha-32P]-NAD+ in the presence of Bordetella Pertussis toxin (islet-activating protein) reveals the ADP-ribosylation of a peptide with a Mr close to 41 kDa, which corresponds to the alpha-subunit of the guanine nucleotide regulatory protein Ni. Islets removed from rats pretreated with the Bordetella Pertussis toxin display a specific increase in adenylate cyclase responsiveness to GTP and are characterized by a resistance to the inhibitory action of alpha2-adrenergic agonists upon either adenylate cyclase activity or glucose-induced insulin release.

Inhibitory effects of quinidine on rat heart muscarinic receptors

Quinidine inhibited binding of the labelled agonist [3H]oxotremorine M [( 3H]Oxo-M) and the labelled antagonist [3H]N-methylscopolamine [( 3H]NMS) to rat heart muscarinic receptors. Kinetic studies demonstrated that quinidine decreased the association rates (I50: 4 and 7.5 microM) and dissociation rates (I50: 100 and 68 microM) of [3H]Oxo-M and [3H]NMS, with different potencies. These cooperative effects explained the low Hill coefficients and apparent selectivity of quinidine competition curves.

Effects of HIS1 modifications on the ability of vasoactive intestinal peptide to stimulate adenylate cyclase from rat and human tissues

The importance of the N-terminal His residue of VIP for stimulating adenylate cyclase was appreciated by estimating the intrinsic activity and EC50 of four VIP analogues on membranes from rat lung, liver, brain, anterior pituitary, and pancreas, and on human heart membranes. In all tissue preparations tested except one, the order of efficacy (and often potency) was: VIP greater than (Ac-His1)VIP greater than (Phe1)VIP = (3-Me-His1)VIP greater than (D-His1)VIP. In rat heart membranes, the order of efficacy was somewhat different: VIP greater than (Ac-His1)VIP = (Phe1)VIP greater than (D-His1)VIP greater than (3-Me-His1)VIP. These data demonstrated the key role of His1 in VIP in activating adenylate cyclase. They suggest that a given VIP analogue might act as full agonist in tightly coupled adenylate cyclase systems (such as those of rat lung and liver membranes) whereas the same analogue could not promote full activity in poorly coupled systems (such as that present in rat brain synaptic membranes).

One-step isocratic high-performance liquid chromatographic purification of radioiodinated and radioiodinated-photoactivable derivatives of cholecystokinin

N-Hydroxysuccinimidyl-3-(4-hydroxy-3-[125I]iodophenyl)propionate (the Bolton-Hunter reagent) was conjugated with (Thr 34, NLeu 37) cholecystokinin (CCK) 31-39 in anhydrous dimethylformamide-pyridine in 20% yield. The radiolabelled peptide was purified from the reaction mixture in one step, by isocratic elution from a C18 high-performance liquid chromatographic (HPLC) column with 35% aqueous acetonitrile-0.13% heptafluorobutyric acid as eluent. The concentration of the radiolabelled peptide was estimated by UV monitoring. The acylating photoactivable radioiodinated reagent N-hydroxysuccinimidyl-N-(4-azido-2-nitrophenyl)-3-[125I]iodotyrosi ne was synthesized, purified on a C18 HPLC column by isocratic elution with 65% acetonitrile-1 mM hydrochloric acid, then conjugated with (Thr 34, NLeu 37) CCK31-39. The resulting photoactivable radioiodinated CCK analogue was purified by isocratic elution on a C18 HPLC column with 39% aqueous acetonitrile-0.1 M triethylamine phosphate (pH 3.5). The binding ability of both tracers and their non-radioactive analogues to CCK receptors was tested on rat pancreatic plasma membranes. As compared to a KD of 4.5 nM for unmodified (Thr 34, NLeu 37) CCK31-39, the KD of the radioiodinated Bolton-Hunter derivative was 3 nM, and that of the photoactivable radioiodinated derivative was 19 nM.

Phospholipase A2 activity of pancreatic secretory factor, a new secretagogue isolated from the venom of Heloderma suspectum

Pancreatic secretory factor (PSF), an efficient pancreatic secretagogue recently isolated from the venom of Heloderma suspectum, is shown to exert phospholipase A2 activity towards phosphatidylcholine. This activity is strictly dependent on calcium (apparent Ka 40 nM) and has an optimum pH around 9. At pH 7.4 and in the presence of calcium, PSF retains 40% of its phospholipase A2 activity. These results are compared to the calcium dependency of the secretory effect of PSF on rat pancreatic acini. Taken collectively, the present data on PSF suggest that a similar endogenous phospholipase A2 activity might be involved in the late steps of stimulus-secretion coupling in the exocrine pancreas.

Importance of disulfide bonds in receptors for vasoactive intestinal peptide and secretin in rat pancreatic plasma membranes

Vasoactive intestinal peptide (VIP), secretin, and C-terminal octapeptide of cholecystokinin (CCK-8) receptors were identified in rat pancreatic plasma membranes by the ability of these peptides to stimulate adenylate cyclase activity. The membrane preparation procedure was conducted through a series of steps including discontinuous sucrose density gradient fractionation. 5 mM beta-mercaptoethanol was added stepwise. Membrane preparations obtained stepwise were preincubated for 10 min at 25 degrees C in the presence of various concentrations of beta-mercaptoethanol or dithiothreitol before assaying adenylate cyclase. The use of the reducing agents exerted no effect on p[NH]ppG-, NaF-, and CCK-8- stimulated activities. By contrast, stimulation of adenylate cyclase by low VIP concentrations was specifically altered when beta-mercaptoethanol was used during tissue homogeneization at 5 degrees C. In addition, both VIP and secretin responses were highly sensitive towards a preincubation of 10 min at 25 degrees C in the presence of dithiothreitol. These results were likely to reflect alterations at the receptor level. 125I-VIP binding was, indeed, reduced after dithiothreitol preincubation, low concentrations of the thiol reagent decreasing the apparent number of high-affinity VIP receptors and higher dithiothreitol concentrations reducing the affinity of VIP receptors.

Specific labelling by [125I]helodermin of high-affinity VIP receptors in rat liver membranes

Helodermin, a newly isolated peptide from Gila Monster venom, is structurally related to VIP and secretin. When used as radioligand, [125I]helodermin bound rapidly and reversibly to crude rat liver membranes, the dissociation being accelerated by GTP. Competition binding curves of [125I]helodermin and [125I]VIP with unlabelled peptides showed the following order of decreasing affinity: VIP greater than helodermin greater than secretin greater than hpGRF(1-29)-NH2. The shape of binding curves and of concurrent adenylate cyclase activation is compatible with the specific labelling, by [125I]helodermin, of a class of high-affinity VIP receptors that is capable to stimulate adenylate cyclase.

Effects of verapamil on the binding properties of rat heart muscarinic receptors: evidence for an allosteric site

The calcium channel antagonist verapamil is known to inhibit competitively antagonist binding to rat heart muscarinic receptors. The present data suggest that this drug recognized two binding sites on the muscarinic receptors: 1) an allosteric site modulating the tracer dissociation rates and 2) the muscarinic drug binding site. The affinity of verapamil for the allosteric site and the efficacy of its effect on muscarinic ligand dissociation rates depended on the ligand studied.

Secretin-induced changes in rate, contractility and adenylate cyclase activity in rat heart atria

Secretin stimulated adenylate cyclase activity in crude membrane preparations from right and left rat atria, when tested in the presence of the potentiating activator forskolin. Its maximal effect on adenylate cyclase activity was greater in the right atrium than in the left atrium but the peptide efficiency was lower, in both atria, than that of D,L-isoproterenol. Secretin stimulated the rate of contraction of the spontaneously beating right atrium, but less efficiently than D,L-isoproterenol. Although this positive chronotropic action of secretin was inhibited by D,L-propranolol, it was probably not mediated by the release of endogenous catecholamines as: a) the inhibitory effects of D- and L-propranolol were similar and b) the efficiency of secretin on the in vitro beating of right atrium was the same in control and reserpinized rats. Secretin stimulated the force of contraction of spontaneously beating right atrium, even in the presence of propranolol, i.e. when the chronotropic effect of secretin was abolished. The hormone exerted also a positive inotropic effect on the electrically stimulated rat atrium. This effect was blocked neither by tetrodotoxin nor by propranolol. When comparing the dose-effect curves of secretin and D,L-isoproterenol on adenylate cyclase activation on the one hand, and on the stimulation of rate and contractility on the other hand, it is tempting to suggest that cyclic AMP might be involved in the modulation by secretin of the mechanical properties of rat atria.

Multiple effects of short-chain alcohols on binding to rat heart muscarinic receptors

Short-chain alcohols inhibited the equilibrium binding of agonists and antagonists to rat heart muscarinic receptors. Methanol, ethanol, propan-2-ol and propan-1-ol, when used at low concentrations, behaved as pseudo-competitive antagonists. Their rank order of potency paralleled their relative partition coefficients, suggesting that this inhibition was simply due to the interaction of the alcohols with a hydrophobic part of the receptor or with membrane lipids. The four alcohols increased the dissociation rate constant of [3H]oxotremorine M from the high-affinity agonist receptors and decreased the stability of this receptor state. These effects might reflect increased membrane fluidity and/or decreased hydrophobic interactions (see below). By contrast, the effects of alcohols on the association and dissociation rates of N-[3H]- methscopolamine (an antagonist) were not correlated to their relative octanol/water partition coefficient (a measure of their affinity for biophases ). Alcohols, at the relatively high concentrations necessary for increased membrane 'fluidity', are known to affect the relative stability of various protein conformations. We believe that the effects of alcohols on antagonist binding to rat heart muscarinic receptors reflected changes in the activation energy of association and dissociation reactions, the inhibition of equilibrium binding being mainly due to decreased 'hydrophobic interactions'.

The in vitro chronotropic and inotropic effects of vasoactive intestinal peptide (VIP) on the atria and ventricular papillary muscle from Cynomolgus monkey heart

The in vitro chronotropic and inotropic effects of vasoactive intestinal peptide (VIP) and of isoproterenol, two agents known to stimulate cardiac adenylate cyclase were compared on the heart from Cynomolgus monkey using the spontaneously beating right atrium, the electrically stimulated left atrium, and the electrically-stimulated ventricular papillary muscle. VIP increased concentration-dependently the rate of beating of the right atrium as well as the contractility of both atria but its efficiency was lower than that of D,L-isoproterenol. VIP also stimulated concentration-dependently, and this time as efficiently as D,L-isoproterenol, the contractility of papillary muscle. These VIP effects were unaltered by the neuronal blocker tetrodotoxin. In addition, the moderate inhibition exerted by the beta-adrenergic blocker D,L-propranolol on VIP effects argued against the implication of beta-adrenergic receptors in VIP effects. These results indicate that VIP exerts a direct stimulatory influence on the rate and contractility of Cynomolgus monkey heart.

Heart receptors for VIP, PHI and secretin are able to activate adenylate cyclase and to mediate inotropic and chronotropic effects. Species variations and physiopathology

We have assessed the presence of VIP/PHI/secretin receptors in heart by: (1) testing the ability of the corresponding peptides to activate adenylate cyclase in cardiac membranes from rat, dog, Cynomolgus monkey and man, and (2) examining the ability of the same peptides to exert inotropic and chronotropic effects on heart preparations from rat and Cynomolgus monkey in vitro. Based on their affinity for natural peptides and synthetic analogs, two types of VIP/PHI/secretin receptors were characterized: the relatively nonspecific "secretin/VIP receptor" of rat heart (that is "secretin-preferring" only in that secretin was more efficient than VIP in stimulating adenylate cyclase), and the "VIP/PHI-preferring" receptor of man, monkey and dog heart. Four physiopathological situations affecting secretin/VIP receptors in rat heart were explored: In male rats from the Okamoto strain and the Lyon strain, two strains presenting spontaneous hypertension, heart membranes exhibited a markedly decreased response of adenylate cyclase to secretin/VIP, with lesser alterations in the responses to isoproterenol and glucagon. This impairment developed in parallel with the occurrence of hypertension and was reproduced in normotensive rats submitted to chronic isoproterenol treatment (but not in Goldblatt hypertensive rats). These findings are consistent with a hyperactivity of norepinephrine pathways in spontaneously hypertensive rats, leading to a reduced number of cardiac post-junctional secretin/VIP receptors bound to adenylate cyclase. Heart membranes from genetically obese (fa/fa) Zucker rats also exhibited severely decreased responses to secretin/VIP with lesser alterations in the responses to glucagon and isoproterenol. These anomalies were specific for the heart, and developed in concomitance with obesity. The first anomaly could not be corrected by severe food restriction. Secretin stimulation of heart adenylate cyclase was also selectively altered in streptozotocin-diabetic rats. Thus, two types of diabetic cardiomyopathy were characterized by a severe local alteration of secretin/VIP receptors coupled to adenylate cyclase. Hypothyroidism, provoked in rat by thyroidectomy or propylthiouracil treatment, again induced a marked decrease in secretin-stimulated cardiac adenylate cyclase activity. In rat papillary muscle electrically stimulated in vitro, secretin exerted a positive inotropic effect. This effect was reduced in obese (fa/fa) Zucker rats. In rat right atrium, secretin also exerted a positive chronotropic effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Specific decrease of secretin/VIP-stimulated adenylate cyclase in the heart from the Lyon strain of hypertensive rats

The cardiac adenylate cyclase activity of genetically hypertensive rats from the Lyon strain (LH) was compared to that of Lyon normotensive rats (LN) and that of low blood pressure Lyon rats (LL). The major finding was a 30-35% decrease of secretin- and VIP-stimulated adenylate cyclase activity in cardiac membranes of LH rats that was already obvious in 5 week-old prehypertensive animals: this alteration was apparently specific for the cardiac secretin/VIP-stimulated adenylate cyclase activity, the same activity in membranes from brain, anterior pituitary, and liver being similar in LH, LN and LL rats. It is tempting to conclude that a selective alteration of functional cardiac secretin/VIP receptors in LH rats reflects a local hyperactivity of the sympathetic adrenergic system.

Stimulatory effects of Gila monster venom on rat pancreatic acini

The effects of Gila monster venom on dispersed rat pancreatic acini were compared with those of secretin and VIP. The efficacy of the venom in terms of amylase release was much higher (a 24-fold increase over basal secretion) than that of secretin (a 4-fold increase) and VIP (+ 40% only). On the other hand, cyclic AMP levels increased 12-fold with the venom, as compared to 18-fold with secretin and 16-fold with VIP. The venom, VIP and secretin all displaced 125I-VIP and the competition curve with the venom was steeper, suggesting that all VIP-recognizing receptors bound the venom with the same affinity. VIP receptors were, however, not responsible for the release of amylase provoked by the venom since VIP (and secretin) did not inhibit the secretory action of the venom. The venom exerted no effect on 45Ca efflux and its secretory effect did not depend on the presence of external calcium. Besides, the effect of CCK-8 on amylase release was additive with the effect of the venom. A first exposure to the venom induced a refractoriness to itself with respect to amylase release but not in terms of cyclic AMP increase. In conclusion, Gila monster venom may contain one component binding to VIP/secretin receptors with resulting cyclic AMP elevation. A second venom component may be responsible for the high secretory efficacy, without involving cyclic AMP or calcium efflux.

Phosphorylation of 3 particulate proteins in rat pancreatic acini in response to vasoactive intestinal peptide (VIP), secretin and cholecystokinin (CCK-8)

Rat pancreatic acini were preincubated with 0.4 mM 32Pi for 45 min at 37 degrees C, then exposed for 15 min to VIP, secretin or CCK-8. The incubation was terminated with a stop solution and a fraction rich in mitochondria and zymogen granules was separated from a microsome-rich fraction by differential centrifugation. After heating in the presence of SDS, beta-mercaptoethanol was added and the pattern of equivalent amounts of 32P-labelled proteins was examined by autoradiography of SDS-PAGE gels. VIP, secretin, and CCK-8 stimulated the phosphorylation of a Mr=33 K microsomal protein and that of two proteins of Mr=21 K and Mr=25 K mostly present in a fraction rich in mitochondria and zymogen granules. Stimulations were dose-dependent, the highest stimulant concentrations tested allowing 2- to 3-fold increases of phosphorylation over basal. When 1 nM CCK-8 was used simultaneously with 1 microM VIP, the cyclic AMP levels attained and the pattern of protein phosphorylation were similar to those obtained with VIP alone, and there was a potentiation of amylase secretion; when a supra-maximal 0.1 microM CCK-8 concentration was added, the VIP-induced elevation in cyclic AMP levels and the phosphorylation of the Mr=21 K and Mr=25 K proteins were partially antagonized, and no potentiation any more of secretion occurred. To conclude the in vitro phosphorylation of three particulate proteins (Mr=33 K, 25 K, and 21 K) was similarly increased in rat pancreatic acini in response to secretin and VIP (acting through cyclic AMP) and to CCK-8 (acting mostly through Ca2+).

Interaction of Gila monster venom with secretin receptors in rat pancreatic membranes

The stimulatory effect of Gila monster venom on adenylate cyclase activity in rat pancreatic membranes was compared to that of porcine secretin and porcine VIP. The maximal effect exerted by the venom was identical to that of VIP but significantly lower than that of secretin. The effect of Gila monster venom could, however, be attributed to its interaction with secretin receptors rather than with VIP receptors, at variance with its previously described action on guinea pig pancreatic acini. Adenylate cyclase activation by both Gila monster venom and secretin in rat pancreatic membranes was, indeed: (1) dose-dependently inhibited by two secretin fragments secretin-(4-27) and secretin-(7-27), and (2) more severely depressed than VIP stimulation, after pretreating pancreatic membranes with dithiothreitol (DTT).

A comparison between muscarinic receptor occupancy, adenylate cyclase inhibition, and inotropic response in human heart

Binding to muscarinic receptors was compared with adenylate cyclase inhibition in membranes derived from human heart auricles, and with inhibition of the contraction of auricular muscle fibers. In the absence of GTP, agonists recognized two classes of receptors both of which bound antagonists with the same affinity. In the presence of GTP, both classes of receptors for agonists were converted into a single low affinity state. Carbachol and oxotremorine inhibited adenylate cyclase activity by 43%, pilocarpine being less efficient (-28%). The 3 agonists exerted similar inhibitory effects on the inotropic response, in 7 out of 9 preparations of electrically- and norepinephrine-stimulated fibers. Dose-effect curves suggested that spareness (or an amplification mechanism) was implicated in the occupancy of low affinity binding sites by carbachol and oxotremorine (but not by the partial agonist pilocarpine) and the resulting inhibition of both adenylate cyclase activity and contractile force.

Evidence that helodermin, a newly extracted peptide from Gila monster venom, is a member of the secretin/VIP/PHI family of peptides with an original pattern of biological properties

Helodermin, a newly isolated peptide from the venom of Gila monster (Heloderma suspectum) was shown to stimulate the adenylate cyclase activity of rat pancreatic membranes as efficiently as secretin and VIP. It also increased cyclic AMP levels and inhibited [125I]VIP binding in rat pancreatic acini. Finally, helodermin activated adenylate cyclase in membranes from rat heart, rat brain, and human heart, showing properties analogous yet distinct from those of secretin, VIP and PHI.

Pancreatic secretory factor (PSF), a protein from Gila monster venom stimulating enzyme secretion from rat pancreatic acini

Pancreatic secretory factor (PSF), a 17.5-kDa protein purified from the venom of Gila monster (Heloderma suspectum), stimulated amylase secretion from dispersed rat pancreatic acini more efficiently than CCK-8, bombesin, carbachol and secretin, and without increasing 45Ca2+ efflux and cyclic AMP levels. The secretory action was dependent on the presence of extracellular calcium and was additive to the secretion induced by agents acting via cyclic AMP or via Ca2+ efflux.

Purification of a novel pancreatic secretory factor (PSF) and a novel peptide with VIP- and secretin-like properties (helodermin) from Gila monster venom

A combination of three HPLC procedures applied to the venom of Gila monster (Heloderma suspectum) has led to the purification to homogeneity of two bioactive components: (i) a 17.5 kDa protein, isolated on the basis of its potent secretory effect on dispersed rat pancreatic acini, was accordingly designated PSF (pancreatic secretory factor); (ii) a 5.9-kDa peptide, designated helodermin, was purified on the basis of its ability to stimulate adenylate cyclase in rat pancreatic membranes. PSF was unable to activate adenylate cyclase and, conversely, helodermin was devoid of secretory action.