Inhibitory effects of tunicamycin on procollagen biosynthesis and secretion

Chick embryo cells were briefly exposed to the antibiotic, tunicamycin. Pre-exposed cells, compared to control cultures, showed a severe, progressive inhibition of the incorporation of glucosamine and mannose into total cellular macromolecules. Inhibition of the incorporation of glycine, leucine and proline was also progressive but not as marked as for the carbohydrates. Cellular secretion of all macromolecules was severely impaired. while comparison of the procollagens showed no difference in their subunit size or in their degree of glycosylation; the intracellular content of procollagen polypeptides was similar for both types of cells. In vitro studies showed that tunicamycin selectively inhibited glucosamine, but not mannose, incorporation into macromolecules. The composite results indicate that tunicamycin effectively inhibits protein synthesis, protein glycosylation and protein secretion in chick embryo cells.     

Inhibitory effects of tunacamycin on procollagen biosynthesis and secretion

Chick embryo cells were briefly exposed to the antibiotic, tunicamycin. Pre-exposed cells, compared to control cultures, showed a severe, progressive inhibition of the incorporation of glucosamine and mannose into total cellular macromolecules. Inhibition of the incorporation of glycine, leucine and proline was also progressive but not as marked as for the carbohydrates. Cellular secretion of all macromolecules was severely impaired, while comparison of the procollagens showed no difference in their subunit size or in their degree of glycosylation; the intracellular content of procollagen polypeptides was similar for both types of cells. In vitro studies showed that tunicamycin selectively inhibited glucosamine, but not mannose, incorporation into macromolecules. The composite results indicate that tunicamycin effectively inhibits protein synthesis, protein glycosylation and protein secretion in chick embryo cells.     

Bile secretion in the chicken: effects of secretin, cholecystokinin- pancreozymin and duodenal mucosal extract

The effects of i.v. administration of secretin, CCK-PZ, acid extracts from the duodenal mucosa and the duodenal acidification of the intestine on bile secretion were studied in anaesthetized chickens. Secretin and acid extracts from the duodenal mucosa, which increase bile flow, caused comparable modifications in bile composition; infusion of HCl to the duodenum only induced slight modifications. CCK-PZ caused a pronounced cholecystokinetic effect and, to a lesser degree, it also showed choleretic effects. The results suggest that in the hormonal regulation of bile secretion in the chicken CCK-PZ is more important than secretin and furthermore that the choleretic activity of the latter must be carried out by other secretin-like peptides.     

Catecholamine biosynthesis and vasopressin and oxytocin secretion in the spontaneously hypertensive rat: an in vitro study of localized brain regions

The in vitro synthesis of catecholamines and the secretion of vasopressin (AVP) and oxytocin (OT) was measured in localized regions of the hypothalamo-neurohypophyseal system in the spontaneously hypertensive rat (SHR). The posterior pituitary (PP), median eminence (ME) and supraoptic (SON) and paraventricular (PVN) nuclear regions were incubated in vitro in media containing 3H-tyrosine. Media and tissue levels of AVP and OT were measured as well as norepinephrine and dopamine content and biosynthesis. There were no differences in peptide release in either the PP, ME or SON. However, there was a marked increase in peptide release from the PVN of the SHR. Media AVP levels were 0.3 pg/ml/micrograms protein in the WKY as compared to 2.1 pg/ml/micrograms protein in the SHR. OT release was increased 2 fold, from 0.85 to 1.7 pg/ml/micrograms protein. PVN content of both AVP and OT was significantly lower in the SHR. ME and SON peptide levels were not changed, while neurohypophyseal AVP levels were increased in the SHR. With regard to the catecholamines appreciable norepinephrine synthesis was measured in the PVN and SON while there was little 3H-norepinephrine in the ME or PP. In the hypertensive rat, there was an increase in norepinephrine synthesis in the PVN with no change in the SON. These results provide further support for fundamental changes in the catecholaminergic and peptidergic systems of the hypothalamo-neurohypophyseal axis of the SHR.     

Minireview: physiological and pharmacological properties of vanadium

Vanadium is distributed extensively in nature. It is a trace element and is present in almost all living organisms including man. Even though vanadium was originally recognized for its ability to inhibit membrane Na+-K+-ATPase, various laboratory studies now document that this element has the capacity to affect the activity of various intracellular enzyme systems and may modify their physiological functions. Vanadium may be an essential element for normal development and may play an important role in various homeostatic mechanisms, and thus vanadium deficiency may prove to be an important concern. Abnormalities in biological disposition of vanadium may be involved in the pathogenesis of certain neurological disorders or cardiovascular diseases. While the essentiality of this element for living organisms is yet to be established with certainty, vanadium has become an increasingly important element and is used extensively in various heavy industries such as steel, oil, etc.; thus, the incidence of exposure to toxic levels of vanadium to industrial workers has been an increasing concern for toxicologists. To date, little information is available on the physiological or pharmacological actions of vanadium; hence, it is difficult to reach any definitive conclusion concerning its biological significance, essentiality and its role in pathological states. An attempt has been made in this review to broadly document what is known of various biological actions of vanadium.     

Comparative effects of vasoactive intestinal peptide and secretin on exocrine pancreatic secretion in the cat

The stimulatory effect of vasoactive intestinal peptide (VIP) and secretin has been compared on exocrine pancreatic secretion in anaesthetized cats. Both peptides were given by bolus intravenous injection and continuous intravenous infusion. After bolus injection, VIP stimulated pancreatic secretion only weakly. On the contrary, during intravenous infusion, the maximal effect of VIP did not differ significantly from that of secretin. Therefore, while the potency of VIP is always lower than that of secretin, its efficacy appears to be strictly dependent on the mode of administration.     

Heat and other physiological stress-induced analgesia: Catecholamine mediated and naloxone reversible response

Acute environmental heat (40±2°C) and other physiological stressful situations increased the pain threshold to radiant heat in rats and mice. Naloxone pretreatment or chronic exposure to stress antagonised this response. After pretreatment with catecholamine depleters, α-methyl-p-tyrosine, reserpine or with adrenoceptor blockers, haloperidol and chlorpromazine, the stress-induced analgesic effect was abolished. Cyproheptadine, a serotonin antagonist, also blocked this response. The results suggest the role of brain monoamines in stress-mediated analgesia.     

In vivo effects of five secretin analogs on fluid and potassium secretion from the rat pancreas

The importance of the N-terminal part of the secretin molecule for inducing fluid and potassium secretion from the pancreas was tested on anesthetized rats by comparing the biological capacity of bolus intravenous injections of secretin, secretin analogs, and the secretin (7–27) fragment. Except in one case, the relative potencies with which these peptides influenced fluid secretion correlated with the potencies on potassium secretion. [Glu³]secretin and [Asn³]secretin were 2–3 and 14 times less potent, respectively, than secretin. [Ala⁴]secretin, [D-Ala⁴]secretin and secretin were almost equipotent. [Val⁵]secretin was as potent as secretin on water secretion but 2-fold less potent on potassium secretion. Secretin (7–27) was at best a very weak agonist of secretin.     

Aluminium effects on thyroid gland function: iodide uptake, hormone biosynthesis and secretion

The effects of aluminium (Al) on thyroid function were evaluated in adult Wistar rats intraperitoneally (i.p) injected with 7 mg Al (as lactate)/kg body weight (b.w) per day during a six week period. The time-course kinetics of Na¹²⁵I (3 μCi per 100 g b.w, i.p) was analysed by measuring gamma-radioactivity of thyroid, serum, serum protein precipitate and bile, at times ranging from 2 to 96 h post-dosing. In Al-treated group the ¹²⁵I⁻ thyroid uptake at 24 h (15,840 ± 570 vs. 18,030 ± 630 dpm/mg, P < 0.05) as well as the rate of ¹²⁵I⁻ release from the gland, calculated as the slope of the plot between 24 and 96 h (84 ± 8 vs. 129 ± 11 dpm/mg/h, P < 0.05) were significantly reduced as compared to control. The biliary ¹²⁵I⁻ excretion was not modified at all studied times. The Al content and lipid peroxidation (69.1 ± 8.5 vs. 53.2 ± 7.0 nmol MDA/g wet weight, P < 0.05) of thyroid tissue were increased in Al-treated rats. The serum concentrations of total thyroxine (T4, 3.78 ± 0.14 vs. 4.68 ± 0.12 μg/dL, P < 0.05) and total triiodothyronine (T3, 47 ± 4 vs. 66 ± 5 ng/dL, P < 0.05) were decreased by effect of Al, but free-T4 (1.05 ± 0.05 vs. 1.04 ± 0.04 ng/dL, NS) and thyrotropin (TSH, 2.7 ± 0.4 vs. 2.6 ± 0.5 ng/ml, NS) remain unchanged. In spite of the Al could indirectly affect thyroid iodide uptake and hormones secretion by a mechanism involving the induction of an oxidative stress state, however, these changes could be managed by the hypothalamus-pituitary-thyroid endocrine axis. We can conclude that in adult rats the Al would not act as a thyroid disruptor.     

GABA-benzodiazepine interactions: physiological, pharmacological and developmental aspects

Many of the pharmacological actions of the benzodiazepines can be attributed to their actions on gamma-aminobutyric acid (GABA) systms in the brain. Electrophysiological studies on dorsal raphe neurons indicate that the benzodiazepines act postsynaptically to potentiate GABAergic inhibition in this midbrain nucleus. Direct binding studies have shown that both in vitro and in vivo binding of [3H]diazepam to a specific high affinity benzodiazepine binding site in cerebral cortical tissue are enhanced by the direct in vitro addition of GABA and GABA agonists or by pretreatment of animals with GABA analogs and agents that elevate GABA levels in brain. Ontogenic development of [3H]diazepam binding in brain parallels the development of the sodium-independent [3H]GABA binding. The ability of GABA to enhance benzodiazepine binding is present throughout development and inversely related to age. These data suggest that there is a functionally significant interaction between the benzodiazepines and GABA throughout development and at maturity. A model is proposed to relate these interactions to conformational changes in a benzodiazepine/GABA/Cl- ionophore complex.     

Cellular mechanism of sodium oleate-stimulated secretion of cholecystokinin and secretin

Long-chain fatty acids are potent stimulants of secretin and CCK release. The cellular mechanisms of fatty acid-stimulated secretion of these two hormones are not clear. We studied the stimulatory effect and mechanism of sodium oleate (SO) on secretin- and CCK-producing cells. SO stimulated the release of secretin or CCK from isolated rat mucosal cell preparations enriched in either secretin- or CCK-producing cells, respectively. SO also time- and dose-dependently stimulated secretin and CCK release from STC-1 cells. In STC-1 cells, SO-stimulated secretin and CCK release was potentiated by IBMX and inhibited by a protein kinase A-selective inhibitor and a cAMP-specific antagonist. SO-stimulated releases of the two hormones were also inhibited by downregulation or inhibitors of protein kinase C, a calmodulin antagonist and an inhibitor of calmodulin-dependent protein kinase II. Chelating of extracellular Ca(2+) or addition of an L-type calcium channel blocker diminished SO-stimulated hormone releases. SO caused an increase in intracellular Ca(2+) concentration that was partially reversed by diltiazem but had no effect on production of cAMP, cGMP, or inositol-1,4,5-triphosphate. These results indicate that SO acts on secretin- and CCK-producing cells. Its stimulatory effect is potentiated by endogenous protein kinase A and mediated by activation of Ca(2+) influx through the L-type channels and of protein kinase C and Ca(2+)/calmodulin-dependent protein kinase II.     

Effects of secretin on insulin secretion and glucose tolerance.

Effects of intravenous (IV) infusion of secretin during IV infusion of glucose were examined in normal men. Secretin was administered according to three schedules: with each schedule a comparable priming dose was delivered in the first minute, but this was followed by a maintained (120 min) infusion of secretin at a relatively high rate, or by maintained infusion at one-third that rate, or by brief (15 min) infusion at the lower rate. The lower infusion rate produced increments in secretin in the blood within the range attainable during endogenous secretion. By comparison with effects of glucose alone each secretin infusion enhanced the increments of immunoreactive insulin in the blood. Enhancement of the early release (0-5 min) of insulin was similar with each type of secretin infusion, but the integrated changes in insulin levels through the total infusion period were related to the total doses of secretin. With each dose of secretin glucose tolerance was improved but the three mean glucose curves observed during infusions of secretin were not distinguishable from one another in spite of widely different integrated insulin responses. Secretin did not modify suppression of immunoreactive glucagon or free fatty acids in the blood during hyperglycemia. The results suggest that the effect of continuous administration of secretin on glucose tolerance is not simply related to its integrated insulinotropic action. It is suggested that the effect may be highly dependent on enhancement of insulin secretion early in the response to glycemia, or that it may be due to effects of secretin on glucose production or disposal which are not mediated by insulin.     

Eicosanoid biosynthesis and action: novel opportunities for pharmacological intervention

Novel eicosanoid biosynthetic pathways and receptors are reviewed as potential targets for pharmacological intervention. In addition to the cyclooxygenase and lipoxygenase pathways of arachidonate metabolism, a cytochrome P450-dependent monooxygenase has been identified in corneal and renal epithelial cells. Elucidation of the enzymatic pathways of thromboxane (TX) disposition and development of analytical techniques for measuring urinary metabolites have allowed a reliable assessment of TXA2 biosynthesis in health and disease, and provide a rationale for the combined use of TX-synthase inhibitors and TXA2-receptor antagonists in the setting of platelet activation. Recent evidence for a transcellular metabolism of neutrophil-derived leukotriene (LT) A4 by other human blood cells might link platelet and neutrophil activation to the occurrence of vasospastic phenomena. Prostacyclin (PG1(2)), PGE2, PGD2, TXA2/PGH2, and sulfidopeptide-LT receptors are being characterized in terms of distribution, signal-transduction mechanisms, and agonist-mediated regulation. Development of relatively selective agonists and antagonists of these receptors is providing novel therapeutic strategies for several human diseases.     

Catecholamine secretion and adenylate cyclase activation in sea urchin eggs

The role of neurotransmitters in sea-urchin eggs was investigated by studying their effect on adenylate cyclase of the egg membrane. Maximal stimulation of enzyme activity occurs in the presence of dopamine and GTP. 5-hydroxytriptamine, 5-methoxytriptamine and acetylcholine have no effect on activity, despite a decrease in intracellular cAMP level in eggs treated with 5-hydroxytriptamine antagonists as previously reported (Renaud et al., 1983). High-performance liquid chromatography (HPLC) revealed that dopamine is released from the sea-urchin egg into the external medium following fertilization.     

MEK inhibits secretin release and pancreatic secretion: roles of secretin-releasing peptide and somatostatin

We investigated the mechanism of action of methionine enkephalin (MEK) on HCl-stimulated secretin release and pancreatic exocrine secretion. Anesthetized rats with pancreatobiliary cannulas and isolated upper small intestinal loops were perfused intraduodenally with 0.01 N HCl while bile and pancreatic juice were diverted. The effect of intravenous MEK on acid-stimulated secretin release and pancreatic exocrine secretion was then studied with or without coinfusion of naloxone, an anti-somatostatin (SS) serum, or normal rabbit serum. Duodenal acid perfusate, which contains secretin-releasing peptide (SRP) activity, was collected from donor rats with or without pretreatment with MEK, MEK + naloxone, or MEK + anti-SS serum, concentrated by ultrafiltration, and neutralized. The concentrated acid perfusate (CAP), which contains SRP bioactivity, was infused intraduodenally into recipient rats. MEK increased plasma SS concentration and inhibited secretin release and pancreatic fluid and bicarbonate secretion dose-dependently. The inhibition was partially reversed by naloxone and anti-SS serum but not by normal rabbit serum. In recipient rats, CAP increased plasma secretin level and pancreatic secretion. CAP SRP bioactivity decreased when it was collected from MEK-treated donor rats; this was partially reversed by coinfusion with naloxone or anti-SS serum. These results suggest that in the rat, MEK inhibition of acid-stimulated pancreatic secretion and secretin release involves suppression of SRP activity release. Thus the MEK inhibitory effect appears to be mediated in part by endogenous SS.     

Effect of acute hyperglycemia on basal, secretin and secretin + cholecystokinin stimulated exocrine pancreatic secretion in humans

Pancreatico-biliary secretion is reduced during acute hyperglycemia. We investigated whether alterations in pancreatico-biliary flow or volume output are responsible for the observed reduction in duodenal output of pancreatic enzymes and bilirubin during hyperglycemia. Eight healthy subjects were studied on two occasions during normoglycemia and hyperglycemia (15 mmol/l). Pancreatico-biliary output was measured by aspiration using a recovery marker under basal conditions (60 min), during secretin infusion (0.1 CU/kg.h) for 60 min and during secretin + CCK (0.5 IDU/kg.h) infusion for 60 min. Secretin was infused to stimulate pancreatico-biliary flow and volume output. Secretin significantly (P<0.005-P<0.05) increased volume and bicarbonate output and CCK significantly (P<0.01) increased the output of bilirubin, pancreatic enzymes, bicarbonate and volume, both during normoglycemia and hyperglycemia. During hyperglycemia basal, secretin stimulated and secretin + CCK stimulated total pancreatico-biliary output were significantly (P<0.005-P<0.05) reduced compared to normoglycemia. The incremental outputs, however, were not significantly different between hyper- and normoglycemia. Pancreatic volume output was significantly (P<0.05) reduced during hyperglycemia compared to normoglycemia under basal conditions (31+/-16 m/h versus 132+/-33 m/h) during secretin infusion (130+/-17 ml/h versus 200+/-34 m/h) and during secretin + CCK infusion (370+/-39 ml/h versus 573+/-82 ml/h). Plasma PP levels were significantly (P<0.05) reduced during hyperglycemia. It is concluded that 1) hyperglycemia significantly reduces basal pancreatico-biliary output 2) the incremental pancreaticobiliary output in response to secretin or secretin + CCK infusion is not significantly affected during hyperglycemia, 3) a reduction in volume output contributes to the inhibitory effect of hyperglycemia on pancreatico-biliary secretion, 4) hyperglycemia reduces PP secretion suggesting vagal-cholinergic inhibition of pancreatico-biliary secretion and volume during hyperglycemia.