Basal and hormone-stimulated adenylate cyclase in liver plasma membranes: measurement by radioimmunoassay of cyclic AMP

Comparison of pharmacodynamics between carvedilol and metoprolol in rats with isoproterenol-induced cardiac hypertrophy: effects of carvedilol enantiomers

A recent clinical study has shown that carvedilol has a significantly more favorable effect than metoprolol on survival rate in patients with heart failure. This may be due to actions of carvedilol such as beta(2)-adrenoceptor blockade, alpha-adrenergic receptor blockade and other properties such as anti-oxidant effects that are not yet fully understood. We compared the effects of racemic carvedilol, metoprolol and carvedilol enantiomers on cardiac hypertrophy at similar heart rate in rats with isoproterenol-induced cardiac hypertrophy. Continuous administration of isoproterenol for 2 weeks produced heart failure, which is characterized by an increased heart rate, cardiac hypertrophy and downregulation of beta-adrenoceptors. The doses of racemic carvedilol and metoprolol were adjusted to obtain a similar heart rate in rats with isoproterenol-induced cardiac hypertrophy. The reduction of left ventricular weight and improvement of cAMP production induced by carvedilol were superior to those induced by metoprolol. Although heart rate, blood pressure and cAMP production were not affected by R-carvedilol, left ventricular weight was significantly reduced as a result of alpha-adrenoceptor blockade. The improvement of cAMP production by S-carvedilol was significantly higher than that induced by coadministration of R-carvedilol and metoprolol, suggesting that beta(2)-adrenoceptor blockade partly contributed to the improvement of signal transduction in rats with isoproterenol-induced cardiac hypertrophy. This study has demonstrated that the effects of carvedilol on cAMP production and cardiac hypertrophy in rats with isoproterenol-induced cardiac hypertrophy are superior to those induced by metoprolol at a similar heart rate.

Refractory hypothalamic adenylate cyclase in anorectic tumor-bearing rats: implications for NPY-induced feeding

Although isoproterenol stimulated adenylate cyclase activity in hypothalamic membranes taken from freely-feeding, food-restricted or nonanorectic tumor-bearing rats, the response was greatly reduced in anorectic tumor-bearing rats. The addition of NPY to the membrane preparation inhibited adenylate cyclase activity in hypothalamus taken from freely-feeding and food-restricted rats, but NPY-inhibitory activity was significantly reduced in both groups of tumor-bearing rats. These results suggest that cyclic AMP formation is refractory in anorectic tumor-bearing rats, and that NPY-induced inhibition of hypothalamic adenylate cyclase is reduced in tumor-bearing rats prior to the onset of significant anorexia. Therefore, NPY-induced feeding may be reduced in tumor-bearing organisms due to a dysfunction in the cyclic AMP second messenger system.

Mechanisms of insulin inhibition of ACTH-stimulated steroid secretion by cultured bovine adrenocortical cells

Results of previous studies indicated that insulin at levels comparable to those in humans during hyperinsulinemia decreased ACTH-stimulated cortisol and androstenedione secretion by bovine adrenal fasciculata-reticularis cells in primary culture. In the present studies this inhibitory action was examined further by comparing the effects of insulin on ACTH-stimulated corticosteroid secretion with its effects on 8-(4-chlorophenylthio)-cAMP (cpt-cAMP), forskolin- and [5val]angiotensin II (Ang II)-stimulated corticosteroid secretion. Effects on corticosteroid secretion were correlated with effects on cAMP accumulation and rates of cAMP production. Monolayers were incubated for 24 h in the absence or presence of each agonist alone or in combination with insulin. Insulin (1.7 x 10(-9) or 17.5 x 10(-9) M) caused about a 50% decrease in cortisol and androstenedione secretion in response to ACTH (10(-11) or 10(-8) M). Insulin also decreased ACTH-stimulated aldosterone secretion by cultured glomerulosa cells. Cpt-cAMP (10(-4) or 10(-3) M)-stimulated increases in cortisol and androstenedione secretion were inhibited by insulin, but to a lesser extent than those in response to ACTH. The inhibition of cpt-cAMP-stimulated steroid secretion was not related to increased degradation of the cyclic nucleotide. Increases in cortisol and androstenedione secretion caused by a submaximal concentration (10(-6) M) of forskolin were decreased 50-70% by insulin. In contrast, insulin failed to significantly affect cortisol or androstenedione secretion caused by a maximal concentration (10(-5) M) of forskolin. The secretory responses to Ang II (10(-8) M) were also unaffected by insulin. The effect of insulin to inhibit ACTH-stimulated steroid secretion was accompanied by a reduction in cAMP accumulation as well as an apparent inhibition of adenylate cyclase activation. These data indicate that the effect of insulin to attenuate ACTH-stimulated corticosteroid secretion results from both an inhibition of ACTH-stimulated adenylate cyclase activity and an antagonism of the intracellular actions of cAMP.

N-alpha-biotinylated-neuropeptide Y analogs: syntheses, cardiovascular properties, and application to cardiac NPY receptor visualization

Two monobiotinylated analogs of neuropeptide Y (NPY) were synthesized by coupling the N-hydroxysuccinimidyl esters of biotin and (6-biotinylamido)-hexanoic acid, respectively, to the free alpha-NH2 group of the side chain protected NPY peptide resin. Crude peptides obtained by HF cleavage were purified by RPLC and their integrities were confirmed by amino acid and mass spectral analysis. As with NPY, both biotinylated analogs inhibited 125I-NPY binding and adenylate cyclase activity of rat cardiac ventricular membranes in a dose-dependent manner. N-alpha-[(6-biotinylamido)-hexanoyl]-NPY exhibited potencies comparable to that of NPY whereas N-alpha-biotinyl-NPY was slightly less potent. In the in vivo experiments, however, both the biotinylated analogs exhibited responses comparable to NPY in increasing arterial blood pressure and decreasing heart rate in anesthetized rats. The responses of the biotinyl analogs were longer lasting than those of NPY. Histochemical studies revealed that N-alpha-[(6-biotinylamido)-hexanoyl]-NPY could label the NPY receptors in rat cardiac ventricular tissues. This labeling was specific since intact NPY inhibited the staining. These studies show that biotinyl-NPY analogs exhibit biological potencies comparable to intact NPY and can therefore be used to further probe the NPY-receptor interaction.

Pertussis toxin inhibits neuropeptide Y-induced feeding in rats

Neuropeptide Y (NPY) is the most powerful peptide drug stimulating feeding in rats. Rats with paraventricular hypothalamic (PVH) cannulae were used to investigate the mechanisms involved in NPY-induced feeding. Consistent with previous reports, injection of 2 micrograms of NPY into the PVH significantly increased the cumulative food intake over 1-, 2- and 4-hr periods. Ad lib feeding decreased significantly two days after pertussis toxin (PT) administration, but recovered to nearly normal levels on the fourth day. PT had no immediate effect on NPY-induced feeding; however, four days after PT was injected NPY (2 micrograms) did not increase the food intake compared to control. In vitro investigations showed that isoproterenol-stimulated adenylate cyclase activity in the hypothalamus of control rats was inhibited by NPY. In PT-treated rats, however, no inhibition of cAMP production was observed. These results suggest that cAMP may mediate NPY-induced feeding and that a PT-sensitive G protein may be involved in this signal transduction.

Pharmacological control of the human gastric histamine H2 receptor by famotidine: comparison with H1, H2 and H3 receptor agonists and antagonists

Histamine 0.1 microM-0.1 mM increased adenylate cyclase activity five- to ten-fold in human fundic membranes, with a potency Ka = 3 microM. The histamine dose-response curve was mimicked by the H3 receptor agonist (R) alpha-MeHA, but at 100 times lower potency, Ka = 0.3 mM. Histamine-induced adenylate cyclase activation was abolished by H2, H1 and H3 receptor antagonists, according to the following order of potency IC50: famotidine (0.3 microM) greater than triprolidine (0.1 mM) thioperamide (2 mM), respectively. Famotidine has no action on membrane components activating the adenylate cyclase system, including the Gs subunit of the enzyme stimulated by forskolin and cell surface receptors sensitive to isoproterenol (beta 2-type), PGE2 and VIP. The Schild plot was linear for famotidine (P less than 0.01) with a regression coefficient r = 0.678. The slope of the regression line was 0.64 and differs from unity. Accordingly, famotidine showed a slow onset of inhibition and dissociation from the H2 receptor in human cancerous HGT-1 cells. The results demonstrate that famotidine is a potent and selective H2 receptor antagonist with uncompetitive actions in human gastric mucosa. Consequently, famotidine might be a suitable drug with long-lasting actions in the treatment of Zollinger-Ellison syndrome. The results also confirm and extend the previous observations that (R) alpha-MeHA and thioperamide are two selective ligands at histamine H3 receptor sites. In the human gastric mucosa, these drugs are respectively 330 and 6700 times less potent than histamine and famotidine on the adenylate cyclase system. The possible involvement of histamine H3 receptors in the regulation of gastric secretion is proposed.

A binding site for [3H]glipizide in the rat cerebral cortex

[3H]glipizide, a 2nd generation hypoglycemic sulfonylurea, binds specifically to rat cerebral cortex membranes in a time- and temperature-dependent way. The binding is saturable and reversible. The maximal binding capacity is 110 fmol/mg protein and the dissociation constant 1.5 nM. The binding site was destroyed by proteolytic and lipolytic enzymes suggesting a lipoprotein nature. Active analogs of sulfonylureas are characterized by IC50 values in the cerebral cortex which parallel their insulinotropic activity. In the cerebral cortex, adenylate cyclase was not stimulated by glipizide but sulfonylureas could inhibit, at high doses, the cAMP-dependent phosphodiesterase. This central binding site for glipizide displays the characteristics of the recognition moiety of a biological receptor.

Epidermal growth factor mimics insulin effects in rat hepatocytes

Epidermal growth factor (EGF) mimicked the effect of insulin to activate glycogen synthase and stimulate glycogen synthesis in isolated rat hepatocytes. Both agents required glucose (greater than 5 mM) and had similar time courses of action. The maximum effect of EGF was approx. 70% of that of insulin, and the half-maximally effective concentrations were 9 nM and 4 nM respectively. Combinations of the two agents produced additive responses. EGF also resembled insulin in its ability to inhibit the effects of 0.1-1.0 nM-glucagon on cyclic AMP and glycogen phosphorylase in hepatocytes. The maximum effect of EGF was approx. 70% of that of insulin, and the half-maximally effective concentrations were approx. 5 nM and 0.5 nM respectively. EGF and insulin inhibited phosphorylase activation by exogenous cyclic AMP, and inhibited cyclic AMP accumulation induced by forskolin. They also inhibited phosphorylase activation provoked by phenylephrine, but not by vasopressin. EGF added alone rapidly activated phosphorylase and increased cytosolic [Ca2+], but the effects were no longer apparent at 5 min and were smaller than those of vasopressin. Insulin did not induce these changes. In hepatocytes previously incubated with myo-[3H]inositol, EGF did not significantly increase myo-inositol 1,4,5-trisphosphate. However, its ability to increase cytosolic [Ca2+] was blocked by neomycin, an inhibitor of phosphatidylinositol bisphosphate hydrolysis. It is concluded that some, but not all, of the effects of EGF in liver are strikingly similar to those exerted by insulin, suggesting that these agents may have some similar mechanisms of action in this tissue.

Functional receptors for VIP, GIP, glucagon-29 and -37 in the HGT-1 human gastric cancer cell line

Three separate sets of receptors sensitive to VIP, GIP and pancreatic/entero-glucagons, have been characterized in HGT-1 cells. The order of relative potencies of VIP receptor agonists was VIP greater than rh GRF-43, rh GRF-29 greater than PHI greater than hp GRF-40, secretin. G-37 was about 4 times less potent than G-29 in HGT-1 cells (G-29 greater than G-37), whereas it was about 20 times more potent than G-29 in rat fundic glands (G-37 greater than G-29). Adenylate cyclase in HGT-1 cells was stimulated by VIP, G-29, G-37 and GIP, over a concentration from 3.16 X 10(-9) to 3.16 X 10(-7) M GIP. The experimental data: (1) support the enterogastrone activity of GIP, via adenylate cyclase activation and somatostatin release by gastric D cells; (2) demonstrate that HGT-1 cells originating from a human fundic tumor are sensitive to the glucagon-like peptides G-29 and -37, as rat fundic glands; (3) indicate that the pharmacological properties of the VIP receptor in this human gastric cell line are similar to those characterized in normal human gastric glands.

Secretin receptor activity in rat gastric glands. Binding studies, cAMP generation and pharmacology

We measured 125I-secretin binding to membranes prepared from rat fundic glands and compared the abilities of natural and synthetic secretin (SN) analogs to inhibit 125I-secretin binding and to activate the cAMP generating system in glandular and subcellular preparations from the fundus and antrum. The natural peptides structurally related to porcine secretin (pSN) included: chicken secretin (cSN), vasoactive intestinal peptide (VIP), porcine peptide with N-terminal histidine and C-terminal isoleucine amide (PHI), helodermin, growth hormone releasing factors isolated from the rat hypothalamus (rhGRF-43, rhGRF-29) or from a human pancreatic tumour (hpGRF-40). These peptides inhibited the binding of 125I-secretin to rat fundic membranes: pSN greater than cSN greater than PHI, VIP and activated the cAMP generating system in fundic glands, according to the following order of potency; pSN greater than cSN greater than PHI, VIP greater than rhGRF-29 greater than rhGRF-43. Porcine peptide with N-terminal tyrosine and C-terminal tyrosine (PYY), GIP, SOM and hpGRF-40 were inactive. Structural requirements for secretin receptor activity were evaluated with four synthetic secretin analogs corresponding to porcine secretin substituted at the N-terminal end by sequence portion of VIP, GIP, GLU and SOM: Ala4-Val5-SN(VIP-SN); Tyr1-Ala2-Glu3-SN (GIP-SN); Gln3-SN (GLU-SN) and Phe1-Phe1-Trp3-Lys4-SN (SOM-SN). The relative potencies of the analogs in fundic and antral preparations were: pSN greater than VIP-SN greater than VIP, GIP-SN greater than GLU-SN greater than SOM-SN for 125I-secretin displacement and cAMP production (glandular cAMP generation and adenylate cyclase activation).(ABSTRACT TRUNCATED AT 250 WORDS)

Vasoactive intestinal peptide receptor activity in human fetal enterocytes

Functional and specific receptors for vasoactive intestinal peptide (VIP) (determined by their capacity to bind 125I-VIP and activate adenylate cyclase) and cyclic AMP-dependent phosphodiesterase activities were characterized in enterocytes of human fetal small intestine between 18 and 23 weeks of gestation. Half-maximal stimulation of the cyclase and inhibition of 125I-VIP binding in membrane preparations were respectively observed at 1.4 and 5 X 10(-10) M VIP. The peptides structurally related to VIP activated the cyclic AMP generating system at pharmacological doses (10(-7) M and above) in the following order of potency: VIP greater than PHI greater than GRF greater than secretin. Other peptides or test substances, including GIP, pancreatic glucagon, somatostatin-14, gastrin, CCK, neurotensin, pancreatic polypeptide, PYY, substance P, histamine and isoproterenol are inactive in this system, while the ubiquitous adenylate cyclase activators NaF, forskolin and prostaglandins were effective. These results, combined with the appearance of intestinal VIP in nerve fibers at 8 weeks and with the morphological and enzymatic maturation at 9-12 weeks of the intestinal mucosa, indicate that this neuropeptide may regulate either the differentiation or function of enterocytes during the early development of human intestinal mucosa.

Activation of the cAMP-generating system by vasoactive intestinal polypeptide (VIP) in the human laryngeal malignant cell line HEp-2

In the presence of 3-isobutyl-l-methylxanthine, VIP produced a dose-related (3 X 10(-9)-10(-7) M) increase (8-fold) in cAMP production in isolated HEp-2 cells incubated at 15 degrees C in KRP buffer. Among the peptides structurally related to VIP, including secretin (10(-7) M), pancreatic glucagon (10(-6) M), PHI, somatostatin-14 (10(-6) M), hpGRF (10(-8)-4 X 10(-6) M), GIP (2 X 10(-7) M), only PHI (3 X 10(-7) M and above) is able to activate the cAMP-generating system in HEp-2 cells, but at 10(2) times lower potency. Under the same conditions, histamine (10(-3) M) was also ineffective, while PGE2 (10(-7)-10(-4) M) increased (4-fold) basal cAMP levels in HEp-2 cells. The VIP effect is related to the interaction of the peptide on VIP recognition sites (125I-VIP-binding capacity), coupled to the membrane-bound adenylate cyclase. The results indicate that the transformed laryngeal cell line HEp-2 possesses a receptor-cAMP system preferentially activated by VIP (relative potencies: VIP greater than PHI much greater than other peptides of the secretin family), and suggest that this neuropeptide could modulate biological functions in normal laryngeal epithelia in man.

Desensitization by histamine of H2 receptor-mediated adenylate cyclase activation in the human gastric cancer cell line HGT-1

Short-term treatment of cultured HGT-1 cells with histamine produced a time-dependent (half-life: 20 min) and homologous desensitization of histamine H2 receptor activity mediating cAMP generation in HGT-1 cells and gastric acid secretion in normal gastric mucosa. Histamine treatment resulted in loss of response of the adenylate cyclase to histamine in purified plasma membranes, but had no effect on basal, vasoactive intestinal peptide (VIP)- or NaF-stimulated enzyme activities. We propose that the desensitization of gastric histamine H2 receptor by histamine evidenced in cellular or subcellular preparations from HGT-1 cells could be involved in the physiological regulation and pharmacological control of gastric cell function in man.

Forskolin stimulates adenylate cyclase in human colonic crypts: interaction with VIP

Forskolin in the 10(-8)-10(-4) M concentration range (ED50 2 microM) strongly stimulated the cyclic AMP production of epithelial crypts isolated from the human colon. At a maximal dose, production increased up to 500 and 700 times the basal cyclic AMP levels at 15 and 37 degrees C, respectively. Forskolin was thus much more efficient than VIP, which is the physiological regulator of this system. Forskolin (ED50 7 microM) also stimulated colonic membrane adenylate cyclase. The stimulation was immediate, did not require guanyl nucleotides and was inhibited by calcium (10(-5)-10(-3) M). In the concentration range between 10(-9) and 10(-5) M (ED50 0.04 microM), forskolin strongly potentiated the stimulation of adenylate cyclase by VIP. We conclude that: (1) forskolin exerts a double dose-dependent action on cyclic AMP production in human colonic crypts, i.e. direct activation of basal adenylate cyclase activity and potentiation of VIP effect; (2) forskolin may be a unique pharmacological tool to investigate the cyclic AMP-dependent processes in human intestine.

Modulation of hepatic protein kinase activity by indomethacin

We previously demonstrated that treatment with indomethacin in vivo significantly blunted the glucagon-induced glycemic response in the rat. This prostaglandin synthetase (cyclo-oxygenase) inhibitor also accentuated the evanescent effect of glucagon on hepatic glucose output in the intact, anesthetized rat. In this report, we present evidence that impairment of glucagon action in the rat liver by indomethacin is mediated through its inhibitory effect on both cAMP-dependent and cAMP-independent hepatic protein kinase. Indomethacin treatment did not have a measurable effect on any of the other components of the glucagon transducer system. Furthermore, infusion with glucagon for two hours that maintained plasma glucagon values at high physiological levels significantly reduced hepatic cAMP-dependent protein kinase activity without altering its Km. Glucagon infusion also down-regulated its own hepatic receptors and glucagon-stimulated cAMP production; prostaglandin E1-stimulated cAMP production was not affected. We concluded that prostaglandins may play a role in the regulation of hepatic protein kinases involved in the glucagon-stimulated glycogenolytic response and that glucagon-induced down-regulation extends at least to the hepatic protein kinases. However, a direct effect of indomethacin or protein kinase and the adenylate cyclase complex cannot be ruled out.

Glucagon-37 (oxyntomodulin) and glucagon-29 (pancreatic glucagon) in human bowel: analysis by HPLC and radioreceptorassay

A method for assaying specifically the biologically active peptides Glucagon-37 (G-37/Oxyntomodulin/bioactive Enteroglucagon) and Glucagon-29 (G-29/pancreatic Glucagon) has been developed by use of high performance liquid chromatography (HPLC) of crude tissue extracts followed by radioreceptorassay in liver membranes. The peaks observed with this method in samples from human bowel have also been analysed in two other assays: stimulation of cyclic AMP accumulation in gastric glands and radioimmunoassay. Owing to the different patterns of activity of porcine G-37 and G-29 in these assays, the comparison of the data obtained allows to discriminate between the two peptides. The same behaviour in both HPLC and the three assays of the human peaks on one hand and the porcine peptides on the other strongly suggests that human intestine contains a very similar or the same molecules as that isolated from the porcine tissues. Whatever the portion of small intestine, G-37 represented ca 90% of G-37 + G-29. A decreasing concentration gradient of both G-37 and G-29 was also observed from ileum to descending colon.

A specific gastrin receptor in human gastric mucosa

Biological active 125I-synthetic human gastrin (125I- SHG ) appeared to bind specifically on human gastric mucosal membrane preparations. Gastrin binding was reversible, saturable and of high affinity (Kd = 3.44 +/- 3.44 X 10(-10)M) with the binding site (3.66 +/- 0.34 fmol/mg protein) at 37 degrees C for 30 min (pH 7.4). Specific gastrin binding was present in the fundic mucosa and absent from the antral, duodenal, jejunal, and colonic mucosa. Gastrin analogues (tetragastrin, pentagastrin, caerulein, and synthetic human gastrin) inhibited 125I- SHG specific binding. However, the specific gastrin binding was not inhibited by glucagon, insulin, acetylcholine, atropine, histamine, or cimetidine. It was suggested that gastrin and histamine H2 receptor or gastrin and muscarinic cholinergic receptor did not share the same locus.

Isolation of basolateral membranes from columnar cells of the proximal colon of the guinea pig

A method for an analytical isolation of plasma membranes from columnar cells (colonocytes) of the proximal colon of the guinea pig is described. Isolation of the colonocytes was performed by a mild EDTA-chelation method. After homogenization, two subsequent sucrose gradient centrifugations (isokinetic and isopycnic) yielded a plasma-membrane fraction which was enriched 12-fold in (Na+ + K+)-ATPase activity and 8-fold in adenylate cyclase activity. It is suggested that the purified membrane fraction consists mainly of basolateral membranes of the colonocytes. Due to the lack of suitable marker enzymes, no evidence for enrichment of the brush-border membranes was obtained. Histochemical studies demonstrated that alkaline phosphatase is absent from the luminal membrane of the surface epithelial cells of the proximal colon of the guinea pig.

Decreased glucagon-stimulated cyclic AMP production by isolated liver cells of rats with type 2 diabetes

This study was undertaken to investigate the effect of experimental type 2 diabetes in the rat on the insulin and glucagon receptors and on the early steps of glucagon action. The binding of insulin and glucagon and the glucagon-stimulated cyclic AMP accumulation in the presence of a phosphodiesterase inhibitor (IBMX, 0.1 mmoles/l) were studied in liver cells isolated from 7-9-month-old rats with chronic type 2 diabetes and from control rats. No significant change was observed in [125I] insulin binding and [125I]glucagon binding of diabetic liver cells as compared to controls. Scatchard analysis of the competition experiments indicated that affinity and number of insulin and glucagon receptors were not significantly changed in the liver cells of diabetic rats. The basal cyclic AMP level was significantly lower in the diabetic hepatocytes (2.3 +/- 0.9 pmoles/10(6) cells) than in the controls (4.0 +/- 0.6 pmoles/10(6) cells). Cyclic AMP response to physiological concentrations of glucagon (0.1-1 nmoles/l) was about 2 times lower in the diabetic hepatocytes than in the controls. Furthermore, the basal liver membrane adenylate cyclase activity and the fluoride-activatable adenylate cyclase activity were about 2 times lower in the diabetics as compared to control rats, while the liver cyclic AMP and cyclic GMP phosphodiesterase activities were unchanged. The ability of glucagon to stimulate liver membrane adenylate cyclase over a 10(-12)-10(-6) M concentration range was decreased in diabetic rats. Taken together, these data are consistent with the thesis that the impairment of the liver cyclic AMP response to glucagon in rats with type 2 diabetes is caused by a decrease in the amount of adenylate cyclase in the liver plasma membranes.

Histamine and VIP interactions with receptor-cyclic AMP systems in the human gastric cancer cell line HGT-1

In HGT-1 cells incubated at 20 degrees C for 15 min with 1 mM 3-isobutyl-1-methylxanthine (IBMX), histamine (10(-4)M) increased basal cAMP levels from 2.12 +/- 0.14 to 22.9 +/- 2 pmol per 10(6) cells, with a potency of 6.4 X 10(-6)M. IBMX was added in order to inhibit cAMP degradation by low and high Km cAMP-phosphodiesterases (cAMP-PDE). The use of specific H1, H2 agonists or antagonists indicated that the histamine effect was due to an interaction with typical H2 -receptors that are involved in gastric acid secretion. Cyclic AMP levels were also increased (10-fold) by vasoactive intestinal peptide VIP (3 X 10(-11) - 10(-8)M). Porcine peptide having N-terminal histidine and C-terminal isoleucine amide (PHI) and secretin were respectively 80 and 3600 times less potent than VIP and did not produce additive effect when tested in combinations with VIP. This observation indicates that these two peptides, structurally related to VIP, are acting through the recognition sites for VIP. Combination of VIP and histamine results in additive stimulation on intact cells as well as on membrane-bound adenylate cyclase, suggesting the existence of two cell populations bearing respectively the two sets of receptors. Two other human cancer cell lines originating from nongastric tumors (HT-29 and HL-60) possess only VIP or histamine receptors, respectively, indicating the gastric cellular originality of the HGT-1 cells. It is concluded that HGT-1 cells possess both VIP and histamine H2 receptors with similar pharmacological properties to those characterized in normal human fundic glands (1,2). Therefore, this cell line can be a good model to study drugs used therapeutically during the treatment of patients for gastric ulcer or cancer.

Isolation of glucagon-37 (bioactive enteroglucagon/oxyntomodulin) from porcine jejuno-ileum. Isolation of the peptide

A 37 amino acid-peptide has been isolated from porcine jejuno-ileum on the basis of its glucagon-like activity in liver (interaction with glucagon-binding sites and activation of adenylate cyclase) using gel filtration, ion-exchange and high-performance liquid chromatography. Depending on the criteria chosen, this peptide is referred to as either 'bioactive enteroglucagon' (activity in liver), 'oxyntomodulin' (specific action in gastric oxyntic glands) or 'glucagon-37' (chemical structure).

A simple isolation method for basal-lateral plasma membranes from rat kidney cortex

Basal-lateral membranes were separated in a self-orienting Percoll (modified colloidal silica) gradient from a heavy microsomal membrane fraction by centrifugation at 48,000g for 0.5 h. The (Na+--K+)-ATPase activity as a marker enzyme for the basal-lateral plasma membrane was 20-fold enriched by this procedure. The adenylate-cyclase activity measured in the basal-lateral membrane fraction was stimulated 6-fold by parathyrin and only up to 1.5-fold by arginine-vasopressin, calcitonin, or isoproterenol. The yield of basal-lateral plasma membranes was 5 to 10 percent of the amount initially present in the homogenate. The method is also applicable to the pig kidney.

Secretin binding sites coupled with adenylate cyclase in rat fundic membranes

Specific binding sites for secretin have been identified in rat fundic membranes, using 125I-secretin. The binding was saturable, reversible, time and temperature dependent. Optimal pH for binding was around 7-7.5. Scatchard plots were compatible with the existence of 2 classes of receptors; the first class with a high affinity for secretin (apparent Kd of 4 x 10(-10) M) and a low binding capacity (150 fmol per mg membrane protein, i.e., 4,500 high affinity sites/cell) and a class of receptors with a lower affinity (Kd of 3 x 10(-9) M) and a higher binding capacity (580 fmol per mg membrane protein i.e., 17,400 sites/cell). Glucagon, gastric inhibitory polypeptide and somatostatin had no-effect on secretin binding. In contrast, VIP inhibited 125I-secretin binding and stimulated adenylate cyclase activity, in both cases at a 200-times lower potency than secretin (ID50 and Ka = 2 x 10(-7) M VIP). The properties of these secretin receptors strongly support the concept that secretin acts as a regulatory peptide on the rat gastric epithelium.

Hormonal regulation of amino acid transport and cAMP production in monolayer cultures of rat hepatocytes

The effects of insulin, glucagon of Dexamethasone (DEX) and of glucagon with insulin or DEX were examined on the uptake of 2-amino [1-14C]isobutyric acid (AIB) and N-Methyl-2-amino [1-14C]isobutyric acid (NMe AIB) in monolayer cultures of rat hepatocytes. Insulin and glucagon stimulated the uptake of both the amino acids and DEX inhibited it, showing that all three of these hormones regulate the A system (the sodium-dependent system that permits the transport of NMe AIB) for amino acid transport in these cultures. Experiments investigating the transport of aminocyclopentane-1-carboxylic acid, 1- [carboxyl-14C] in the presence of excess AIB or in the absence of sodium showed that insulin had no effect on the activity of the L system (the sodium-independent system that prefers leucine). Experiments on the uptake of AIB in the presence of excess NMe AIB showed insulin had no effect on the transport activity of the ASC system (the sodium-dependent system that does not transport NMe AIB). Insulin concentrations ranging from 0.1 nM to 100 nM did not antagonize the stimulatory effect of optimum or suboptimum concentrations of glucagon on the uptake of either AIB or NMe AIB. Similarly, glucagon did not antagonize the stimulatory effect of optimum or suboptimum concentrations of insulin on the uptake of both the amino acids. The combined effect of insulin and glucagon was additive on the rate as well as the cumulative uptake of both AIB and NMe AIB. DEX alone inhibited the transport of both AIB and NMe AIB by about 25%, while glucagon caused a 2--3-fold increase; however, the addition of glucagon to cultures containing DEX caused a 7--8-fold increase in the uptake of both AIB and NMe AIB when compared to cultures containing DEX alone. The effect of insulin on the levels of cAMP was also investigated. Insulin had no effect on the cAMP levels in cultures treated or untreated with optimum or suboptimum concentrations of glucagon.

Nicergoline, an anti-aggregating agent which inhibits release of arachidonic acid from human platelet phospholipids

Nicergoline is an inhibitor of human platelet aggregation induced by many agents. It inhibits platelet prostaglandin production induced by thrombin or collagen but not by arachidonic acid. It abolishes the decrease of platelet phospholipid induced by thrombin. Nicergoline appears as a drug with an antiphospholipase activity of human platelets.

Inversal relationship between basal adenylcyclase activity and maximal degree of stimulation in membranes of red blood cells from rats

The degree of stimulation of adenylcyclase activity, in membranes from immature red blood cells from rats, brought about by isoprenaline, guanylyl-imidodiphosphate and sodium fluoride is strongly dependent on the basal activity of the enzyme. The inversal relationship between basal activity and the maximal degree of stimulation by (--) isoprenaline, shows an apparent seasonal dependence.

Interaction of vasoactive intestinal peptide with isolated intestinal epithelial cells from rat. 2. Characterization and structural requirements of the stimulatory effect of vasoactive intestinal peptide on production of adenosine 3':5'-monophosphate

Porcine vasoactive intestinal peptide stimulated adenosine 3':5'-monophosphate (cyclic AMP) production in rat intestinal epithelial cells. The stimulation was dependent on time and temperature and was potentiated by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine. Under optimal conditions (at 15 degrees C, with 0.2 mM 3-isobutyl-1-methylaxanthine, at a cell concentration up to 18 microgram DNA/ml), the cyclic AMP production produced by vasoactive intestinal peptide was constant for 10 min and stopped after 15 min incubation, at either low (1 nM) or high (30 nM) concentration of the peptide. This plateau effect was demonstrated not to be due to an inactivation of vasoactive intestinal peptide in the medium nor to an alteration of receptors for the peptide. Cyclic AMP production was sensitive to a concentration as low as 0.1 nM vasoactive intestinal peptide. Maximal stimulation of cyclic AMP levels by vasoactive intestinal peptide was observed with 30 nM vasoactive intestinal peptide and represented an 11-fold increased above basal. The dorse-response curve was monophasic with a Km of 2.3 x 10(-9) M. No cooperative effects were detected by Hill analysis. The positive non-linear relationship observed between stimulation of cyclic AMP production and occupancy of binding site was not time-dependent as indicated by experiments performed after 15, 45 and 120 min incubation. Maximal and half-maximal responses were obtained at about 70% and 7% occupation of binding sites, respectively. Chicken vasoactive intestinal peptide and porcine secretin were agonists of porcine vasoactive intestinal peptide with a 6-times and a 120-times lower potency, respectively. Among secretin analogs that were found to have low affinity for vasoactive intestinal peptide binding sites, [4-alanine, 5-valine]secretin, that resembles vasoactive intestinal peptide at the first seven amino acids at the N-terminal end, was a partial agonist of vasoactive peptide at the first seven amino acids at the N-terminal end, was a partial agonist of vasoactive intestinal peptide and others failed to stimulate cyclic AMP production. Glucagon (10microM), gastric inhibitory peptide (0.1 microM), substance, P, neurotensin, octapeptide of cholecystokinin, bovine pancreatic polypeptide, human gastrin I with leucine at residue 15, Leu-enkephalinand somatostatin (1 microM) did not alter cyclicAMP levels. Non-peptide mediators such as dopamine, serotonin, acetylcholine and histamine, tested at 10 microM, were also ineffective. Prostaglandins E2, E1 and isoproterenol, tested at 10 microM, induced an increase of cyclic AMP levels above basal but were 9.5, 13.7 and 17.5 times less efficient than vasoactive intestinal peptide, respectively. Thus vasoactive intestinal peptide is a unique stimulus of cyclic AMP production in rat intestinal epithelial cells.

Involvement of calcium in the inhibition by insulin of the glucagon-stimulated adenylate-cyclase activity

1. The inhibitory effect of adenosine on the glucagon-stimulated adenylate cyclase activity of liver plasma membranes, prepared from PVG/c rats, was potentiated by insulin. In the presence of EGTA, such potentiating effect of insulin was lost. 2. Calcium (10 microM) potentiated the inhibitory effects of both adenosine and insulin on the glucagon-stimulated cyclase activity. The synergestic effect of calcium + insulin required the presence of adenosine as judged from the use of adenosine deaminase. 3. Insulin had no significant inhibitory effect on the glucagon-stimulated cyclase activity of liver plasma membranes, prepared from young Wistar rats, unless both adenosine (50 microM) and calcium (10 microM) were added externally. 4. Results demonstrate an interaction of calcium and insulin at membrane level that, in the presence of adenosine, results in the inhibition of the glucagon-stimulated adenylate cyclase activity.

Characterization of a vasoactive intestinal peptide-sensitive adenylate cyclase in rat intestinal epithelial cell membranes

A vasoactive intestinal peptide-sensitive adenylate cyclase in intestinal epithelial cell membranes was characterized. Stimulation of adenylate cyclase activity was a function of vasoactive intestinal peptide concentration over a range of 1 . 10(-10)-1 . 10(-7) M and was increased six-times by a maximally stimulating concentration of vasoactive intestinal peptide. Half-maximal stimulation was observed with 4.1 +/- 0.7 nM vasoactive intestinal peptide. Fluoride ion stimulated adenylate cyclase activity to a higher extent than did vasoactive intestinal peptide. Under standard assay conditions, basal, vasoactive intestinal peptide- and fluoride-stimulated adenylate cyclase activities were proportional to time of incubation up to 15 min and to membrane concentration up to 60 microgram protein per assay. The vasoactive intestinal peptide-sensitive enzyme required 5-10 mM Mg2+ and was inhibited by 1 . 10(-5) M Ca2+. At sufficiently high concentrations, both ATP (3 mM) and Mg2+ (40 mM) inhibited the enzyme. Secretin also stimulated the adenylate cyclase activity from intestinal epithelial cell membranes but its effectiveness was 1/1000 that of vasoactive intestinal peptide. Prostaglandins E1 and E2 at 1 . 10(-5) M induced a two-fold increase of cyclic AMP production. Vasoactive intestinal peptide was the most potent stimulator of adenylate cyclase activity, suggesting an important physiological role of this peptide in the cyclic AMP-dependent regulation of the intestinal epithelial cell function.

Impairment of platelet thromboxane A2 generation and of the platelet release reaction in two patients with congenital deficiency of platelet cyclo-oxygenase

Two cases of thrombocytopathia with congenital deficiency of platelet cyclo-oxygenase were investigated. The platelet release reaction was impaired. There was a marked decrease of aggregation with collagen and with adrenalin and a total absence of aggregation with sodium arachidonate. The platelet response to labile aggregation stimulating substance (LASS, mostly thromboxane A2) was normal. There was no biosynthesis of prostaglandin cyclic endoperoxides or of thromboxane A2 from arachidonic acid. Basal levels of platelet PGE1 were lowered although plasma levels were normal. Thrombin decreased the cyclic AMP content of patients' platelets and also that of control platelets pretreated with aspirin. The patients platelets showed no ultrastructural difference when compared with control platelets, except for a slight decrease of granule volume, but, in contrast to control platelets, thrombin (0.02 U/ml) did not provoke contraction of the patients' platelets.

A study of insulin binding sites in the chicken tissues

Specific binding of chicken and porcine insulin was demonstrated in isolated chicken hepatocytes, chicken liver plasma membranes and chicken erythrocytes. In the liver, the binding reaction was characterized by a sensitivity and an apparent affinity which were similar to those observed in rat liver and, in contrast, by a decreased number of binding sites. In chicken liver, there were about 5 times fewer binding sites per mg of membrane protein or per unit of cell surface area than in rat liver. In chicken erythrocytes, the number of insulin binding sites per cell was even lower than in chicken hepatocytes. This decreased insulin binding was not accounted for by a faster insulin degradation in chicken tissues. Glucagon binding sites also appeared to be less numerous in chicken than in rat liver, at least at low glucagon concentration; however, the decrease in maximal binding capacity in chicken liver involved insulin and not glucagon binding. That chicken cells are equipped with insulin receptors which are less numerous than in mammalian cells may explain, partly at least, the physiological state of insulin resistance observed in the chicken.

Cyclic nucleotide phosphodiesterases, insulin and thyroid hormones

The in vitro effects of insulin on different phosphodiesterase activities present in rat epididymal fat cells from normal and hypothyroid rats have been studied. Evidence is presented that insulin increases the maximum velocity of a particulate, low Km, cyclic adenosine-3', 5'-monophosphate (cyclic AMP) phosphodiesterase in both types of cells, this effect being more clearly evident with the fat cells from hypothyroid animals; combination of insulin and thyroidectomy resulted in a 400% stimulation with 10-10 - 10-9 M insulin. A clear and significant effect was apparent at 10-11 M insulin. However, the dose-response curve was biphasic, since stimulation by insulin was suppressed for doses of hormone higher 10-8 - 10-7 M. Moreover, insulin effects were very fast, since clear stimulation was observed after only 2 min of incubation; the maximal increase was obtained after 10 min. Insulin did not significantly affect the soluble cyclic AMP phosphodiesterase activity in normal cells, thus confirming results obtained by others. However, the soluble cyclic AMP phosphodiesterase activity was clearly stimulated by insulin when the fat cells were prepared from hypothyroid rats. Maximal stimulation was obtained with 10-9 M insulin; the response was again very fast. Soluble cyclic GMP phosphodiesterase activity was also increased additively by hypothyroidism and insulin, maximal stimulation being obtained with 10-9 M insulin. With this dose of insulin the additive effects of thyroidectomy and insulin produced a 5-fold stimulation. The effect of insulin on the soluble cyclic GMP phosphodiesterase was very fast (2-5 min). With both soluble cyclic nucleotide phosphodiesterase activities, insulin increased the maximal velocity but not apparent Km of the enzyme. Thus, hypothyroidism and insulin produced additive effects suggesting a different mechanism of action of these two hormonal situations on the degradation of the intracellular pools of cyclic AMP and cyclic GMP.