Cholecystokinin-B/Gastrin Receptor Blockade Suppresses the Activity of Rat Stomach ECL Cells

Abstract: Gastrin controls the histamine- and chromogranin A- producing enterochromaffinn-like (ECL) cells, the predominant endocrine cell population in the acid-producing part of the rat stomach. They are responsible for most of the circulating pancreastatin, a chromogranin A-derived peptide. The present study examines the ability of two potent and highly selective cholecystokinin-B/gastrin receptor antagonists. RP73870 and YM022, to incapacitate the ECL cells. The two antagonists were given by continuous subcutaneous infusion to otherwise untreated rats and to hypergastrinaemic rats treated with gastrin-17 (continuous subcutaneous infusion) or omeprazole (orally) for 7 days. Several parameters reflecting ECL cell activity were measured: The oxyntic mucosal histidine decarboxylase activity, the histamine concentration, the histidine decarboxylase mRNA and chromogranin A mRNA concentrations, and the serum pancreastatin concentration. In addition, the serum gastrin concentration was measured. RP73870 and YM022 greatly lowered the oxyntic mucosal histidine decarboxylase activity and the histidine decarboxylase mRNA and chromogranin A mRNA concentrations, and also reduced the oxyntic mucosal histamine concentration and the serum pancreastatin concentration. Moreover, they raised the serum gastrin concentration. With respect to blockade of histidine decarboxylase activity, 1.0 μmol· g^?1 · hr^?1 was an almost maximally effective dose for both RP73870 and YM022. The corresponding ID₅₀ values were 0.04 and 0.05 μmol- kg^?1 · hr^?1. RP73870 and YM022 inhibited the hypergastrinaemia-evoked rise in all ECL-cell parameters. The results suggest that sustained cholecystokinin-B/gastrin receptor blockade causes lasting deactivation of the ECL cells.     

Sustained Cholecystokinin–B/Gastrin Receptor Blockade Does Not Impair Basal or Histamine–Stimulated Acid Secretion in Chronic Gastric Fistula Rats

Abstract: Gastrin is a physiologically important secrelagogue. It is thought to stimulate parietal cells indirectly by mobilizing histamine from enterochromaffin–like (ECL) cells in the oxyntic mucosa. Gastrin stimulates the secretory activity and growth of the ECL cells via an action on cholecystokinin–B/gastrin receptors. Acute cholecystokinin–B/gastrin receptor blockade is known to inhibit gastrin–stimulated acid secretion but whether sustained cholecystokinin–B/gastrin receptor blockade will impair basal, gastrin– and histamine–stimulated acid secretion remains uncertain. The present study was designed to study the effect of long–term (4 weeks) cholecystokinin–B/gastrin receptor blockade on basal and stimulated acid secretion in conscious rats. The selective cholecystokinin–B/gastrin receptor antagonist YM022 (3 μmol kg^–1 hr^–l) was given to gastric fistula rats by continuous subcutaneous infusion via osmotic minipumps for various times from 2 hr to 4 weeks. Basal, gastrin– and histamine– stimulated acid secretion were examined during and after cessation of treatment. Basal and histamine–stimulated acid secretion was not affected by YM022 during the 4 week period of administration, whereas gastrin–induced acid secretion was inhibited. YM022 induced hypergastrinaemia in freely fed rats but did not affect the serum gastrin level in fasted rats. The serum gastrin concentration and gastrin–induced acid secretion returned to control levels 3–7 days after termination of YM022 administration.     

Evaluation of the Specificity and Potency of a Series of Cholecystokinin-B/Gastrin Receptor Antagonists in vivo

Abstract: The potency and specificity of five proposed cholecystokinin-B receptor antagonists, YM022, RP73870, L-740,093, L-365,260 and LY288513, were studied in rats and mice. Gastrin activates rat stomach histidine decarboxylase via cholecystokinin-B/gastrin receptors. To examine cholecystokinin-B receptor-mediated effects of the five drugs, they were infused intravenously to fasted rats and the histidine decarboxylase activity in the oxyntic mucosa was determined. While YM022, RP73870, L-740,093 and L-365,260 failed to activate histidine decarboxylase, they dose-dependently antagonized the gastrin-induced histidine decarboxylase activation. LY288513 had no effect in the doses tested. The maximal inhibitory effect of L-365,260, L-740,093, RP73870 and YM022 on histidine decarboxylase, activated by the intravenous infusion of an ED₅₀ dose of gastrin (0.4 nmoles/kg/hr), was seen at doses of 3, 0.3, 0.1 and 0.1 μmoles/kg/hr, respectively; the corresponding ID₅₀ values were 0.4, 0.02, 0.007 and 0.004 μmoles/kg/hr. In a follow-up study, YM022 and RP73870 were found to produce a rightward shift of the gastrin dose-response curve, which is consistent with competitive inhibition. The effect of the five drugs on a cholecystokinin-A receptor-mediated response was examined by studying gastric emptying in mice. Cholecystokinin-8s, given by a subcutaneous bolus injection, dose-dependently inhibits gastric emptying. The specific cholecystokinin-A receptor antagonist devazepide (given intravenously as a bolus injection) antagonized the effect of cholecystokinin-8s in a dose-dependent manner, with an ID₅₀ value of 28 nmoles/kg. None of the drugs inhibited the gastric emptying or prevented the cholecystokinin-8s-induced effect at the doses tested. The results indicate that YM022, RP73870, L-740,093 and L-365,260 act as cholecystokinin-B receptor antagonists in vivo, being without measurable agonistic activity. Furthermore, they do not interact with cholecystokinin-A receptors at the doses tested. Among the cholecystokinin-B receptor antagonists studied YM022 and RP73870 are superior, the rank order of potency being YM022±RP73870>L-740,093>L-365,260.     

Pharmacological analysis of CCK2 receptor antagonists using isolated rat stomach ECL cells.

1. Gastrin stimulates rat stomach ECL cells to secrete histamine and pacreastatin, a chromogranin A (CGA)-derived peptide. The present report describes the effect of nine cholecystokinin2 (CCK2) receptor antagonists and one CCK1 receptor antagonist on the gastrin-evoked secretion of pancreastatin from isolated ECL cells. 2. The CCK2 receptor antagonists comprised three benzodiazepine derivatives L-740,093, YM022 and YF476, one ureidoacetamide compound RP73870, one benzimidazole compound JB 93182, one ureidoindoline compound AG041R and three tryptophan dipeptoids PD 134308 (CI988), PD135158 and PD 136450. The CCK1 receptor antagonist was devazepide. 3. A preparation of well-functioning ECL cells (approximately 80% purity) was prepared from rat oxyntic mucosa using counter-flow elutriation. The cells were cultured for 48 h in the presence of 0.1 nM gastrin; they were then washed and incubated with antagonist alone or with various concentrations of antagonist plus 10 nM gastrin (a maximally effective concentration) for 30 min. Gastrin dose-response curves were constructed in the absence or presence of increasing concentrations of antagonist. The amount of pancreastatin secreted was determined by radioimmunoassay. 4. The gastrin-evoked secretion of pancreastatin was inhibited in a dose-dependent manner. YM022, AG041R and YF476 had IC50 values of 0.5, 2.2 and 2.7 nM respectively. L-740,093, JB93182 and RP73870 had IC50 values of 7.8, 9.3 and 9.8 nM, while PD135158, PD136450 and PD134308 had IC50 values of 76, 135 and 145 nM. The CCK1 receptor antagonist devazepide was a poor CCK2 receptor antagonist with an IC50 of about 800 nM. 5. YM022, YF476 and AG041R were chosen for further analysis. YM022 and YF476 shifted the gastrin dose-response curve to the right in a manner suggesting competitive antagonism, while the effects of AG041R could not be explained by simple competitive antagonism. pK(B) values were 11.3 for YM022, 10.8 for YF476 and the apparent pK(B) for AG041R was 10.4.     

Evaluation of Three Novel Cholecystokinin-B/Gastrin Receptor Antagonists: A Study of their Effects on Rat Stomach Enterochromaffin-Like Cell Activity

Abstract: Gastrin stimulates rat stomach enterochromaffin-like (ECL) cells via activation of cholecystokinin-B/gastrin receptors. The stimulation is manifested in the activation of the histamine-forming enzyme histidine decarboxylase and in the secretion of histamine and pancreastatin, a chromogranin A-derived peptide. We have examined the short-term effects of three novel cholecystokinin-B/gastrin receptor antagonists (YF476, JB93182 and AG041R) on the ECL cells in intact fasted rats. The drugs and/or gastrin were infused intravenously for 3 hr and the oxyntic mucosal histidine decarboxylase activity and the serum pancreastatin concentration were measured. We also studied the effects of the three drugs on gastric emptying in mice, a cholecystokinin-A receptor-mediated response. YF476, JB93182 and AG041R antagonized the gastrin-evoked histidine decarboxylase activation in a dose-dependent manner. YF476, JB93182 and AG041R induced maximal inhibition at 0.03, 0.1 and 0.1 μmol kg^?1 hr^?1, respectively; the corresponding ID₅₀ values were 0.002, 0.008, and 0.01 μmol kg^?1 hr^?1. YF476 was selected for further analysis. It produced a rightward shift of the gastrin dose-response curve, consistent with competitive inhibition. Moreover, it antagonized the omeprazole-evoked histidine decarboxylase activation and the gastrin- and omeprazole-induced rise in the circulating pancreastatin concentration. None of the three drugs tested inhibited gastric emptying or prevented the cholecystokinin-8s-induced inhibition of gastric emptying at the doses tested. The results show that YF476, JB93182 and AG041R are potent and selective cholecystokinin-B/ gastrin receptor antagonists, and that YF476 is 4-5 times more potent than JB93182 and AG041R.     

Responses to arachidonic acid and other dilator agonists and their modification by inhibition of prostaglandin synthesis in the canine hindlimb

The influence of indomethacin, in doses that completely inhibit the response to arachidonic acid, has been examined on canine hindlimb vascular responses to intra-arterial administration of bradykinin, histamine, nitroglycerin, isoprenaline and papaverine. The hindlimb was perfused at constant flow. Dose-response curves to intra-arterial administration of arachidonic acid (12 to 200 μg kg^?1), bradykinin (0·4 to 100 ng kg^?1), histamine (1·8 to 120 ng kg^?1), nitroglycerin (15 to 100 ng kg^?1), isoprenaline (12 to 100 ng kg^?1) and papaverine (1·2 to 160 μg kg^?1) (all n = 4) were compared before and 30 min after indomethacin (5 mg kg^?1 i.v.). All the drugs produced dose-related decreases in hindlimb perfusion pressure. After indomethacin, responses for all dilator agonists except arachidonic acid, were significantly greater than control (P < 0·05), both in terms of absolute (mmHg) or percent change. Doseresponse curves after indomethacin had a left upward shift compared with control. Arachidonic acid responses were completely blocked by indomethacin. These findings suggest that indomethacin produces a non-specific increase in responsiveness of the hindlimb vascular bed to dilator substances, except arachidonic acid. The data presented do not support the hypothesis that the peripheral vasodilatation produced by bradykinin, nitroglycerin and histamine could be mediated by endogenous prostaglandin release.     

Rolipram inhibits airway microvascular leakage induced by platelet-activating factor,histamine and bradykinin in guinea-pigs

Abstract— Rolipram (0·1–1000 μg kg^?1, i.v.) reduced the increase in microvascular permeability induced by platelet-activating factor (PAF; 50 ng kg^?1, i.v.) at different sites of the guinea-pig airways. Rolipram (1–100μg kg^?1, i.v.) inhibited histamine (30μg kg^?1, i.v.)-and bradykinin (0·3 μg kg, i.v.)-induced airway microvascular leakage. These effects of rolipram were obtained at doses which inhibit histamine (7–20 μg kg^?1 min^?1)-induced bronchoconstriction (IC50 = 3 ± 1 μg kg, i.v.) without depressing arterial blood pressure in the guinea-pig. Aminophylline (50 mg kg^?1) did not change the effect of PAF. The anti-exudative effect of rolipram is of potential therapeutic value in asthma.     

Caffeine metabolism in liver slices during postnatal development in the rat

The metabolism of caffeine was investigated in liver slices of young and adult rats. Liver slices from adult male rats metabolized caffeine at an initial rate of 48.31 ± 3.71 nmoles · (g liver)^?1 · hr^?1 to four main metabolite fractions. By a combination of thin-layer radiochromatography and high performance liquid chromatography, theophylline, paraxanthine and 1, 3, 7-trimethyldihydrouric acid were identified as caffeine metabolites. Apparent V_max of the overall reaction was 83.30 nmoles caffeine metabolites formed · (g liver)^?1 · hr^?1. Theophylline competitively inhibited caffeine metabolism [the apparent K_m was 19.20, μM in the absence of theophylline, the apparent k_i was 36.50 μM in the presence of theophylline (100 μM)]. SKF 525-A inhibited caffeine metabolism; the formation of all of the metabolite fractions was inhibited to a similar extent. Allopurinol (100 μM) had no effect. The specific activity of the enzyme system was extremely low when liver slices of 2-day-old-rats were used [1.46 ± 0.08 nmoles caffeine metabolites formed · (g liver)^?1 · hr^?1]; the reaction velocity increased gradually with increasing age and reached a peak [52.26 ± 1.41 nmoles caffeine metabolites formed · (g liver)^?1 · hr^?1]at 30 days of age. Changes in the formation of the four metabolite fractions with age followed the pattern of the overall caffeine metabolism. These results demonstrate that the liver of the newborn rat has an extremely limited capacity to metabolize caffeine in vitro and are consistent with the proposed involvement of the liver microsomal cytochromes P-450 monooxygenase system in the metabolism of caffeine. N-Demethylation is the main pathway of in vitro caffeine metabolism in the rat liver at all ages.     

The Adenosine Agonist NECA Inhibits Intestinal Secretion and Peristalsis

Abstract— This study aimed to determine whether the antidiarrhoeal effect of the mixed A₁/A₂ adenosine agonist NECA (5′-N-ethylcarboxamido adenosine) is due to inhibition of intestinal fluid transport or to contractility. Intestinal secretion was stimulated in anaesthetized rats by intra-arterial infusions of PGE₂ (4 μg min^?1) or vasoactive intestinal peptide (0·8 μg min^?1). NECA reversed PGE₂-induced secretion in the jejunum (ED50 16 μg kg^?1) and ileum (ED50 21 μg kg^?1, i.v.) and inhibited VIP-induced secretion in the jejunum (ED50 21·5 μg kg^?1). NECA inhibited twitch responses (0·1 Hz, 1 ms, IC50 11·2Nm ) but not tetanic contractions at 10 Hz of the transmurally stimulated guinea-pig ileum. Likewise, NECA (10 μm ) did not inhibit frequency-related contractions over the range of 2·5 to 40 Hz of rat jejunum or ileum. However, NECA was shown to be a potent inhibitor (30 Nm ) of the peristaltic reflex in the rat ileum. The results indicate that adenosine receptors are involved in modulating peristalsis as well as the secretory activity of the mucosa in the rat small intestine.     

The actions of cimetidine hydrochloride and mepyramine maleate in rat adjuvant arthritis

Adjuvant arthritis in rats was induced by a single subcutaneous injection of Freund adjuvant into a hind paw. The injected paws' mean volume increased continuously by 80±26% after 6 h (acute local response). The injected paw displayed a biphasic response peaking at day 1–2 decreasing to day 5, then increased to day 14 by 123% while the contralateral non-injected paw showed little increase until day 10 and increased by 42% at day 14. The lateral diameter of both knee-joints also increased biphasically, peaking at day 4, decreasing to day 6, then increasing to day 14 (early and late systemic phase). Mepyramine maleate, 1 mg·kg^?1 (i.p.) daily, suppressed only the early systemic phase by 50%. Daily cimetidine hydrochloride, 1 mg·kg^?1·day^?1, almost completely abolished the inflammatory response in the knee joints up to day 12, while at day 14 the supression was 71%. The increase in mean paw volume was also suppressed by orally administered cimetidine. Intraperitoneal doses of cimetidine (3.5 μmol·kg^?1) totally suppressed paw oedema produced by subplantar injection of histamine (1.8 mM) but had no action on equiactive dose of the specific H₁-agonist, 2-pyridylethylamine. Mepyramine at 3.5 μmol·kg^?1 or greater produced no more than 50% suppression of the histamine response. A component of both adjuvant and histamine-induced responses thus appears to be histamine H₂-receptor mediated.     

Genetic dissection of the signaling pathways that control gastric acid secretion

Gastric acid secretion is regulated by endocrine, paracrine and neurocrine signals via at least three pathways, the gastrin-histamine pathway, the CCK-somatostatin pathway and the neural pathway. Genetically-engineered mice, subjected to targeted gene disruption (i.e., knockout mice), have been used to dissect the signaling pathways that are responsible for the complexity of the regulation of acid secretion in vivo. Both gastrin knockout and gastrin/CCK₂ receptor knockout mice displayed greatly impaired acid secretion, presumably because of the loss of the gastrin-histamine pathway. Gastrin/CCK double-knockout mice had a relatively high percentage of active parietal cells with a maintained ability to respond with copious acid secretion to pylorus ligation-evoked vagal stimulation and to a histamine challenge. The low acid secretion in gastrin knockout mice and gastrin/CCK₂ receptor knockout mice and the restoration of acid secretion in gastrin/CCK double-knockout mice suggest that CCK plays an important role as inhibitor of the parietal cells via the CCK-somatostatin pathway by stimulating the CCK₁ receptor of the D cell. In the absence of both the gastrin-histamine and the CCK-somatostatin pathway (as in gastrin/CCK₂ receptor double-knockout mice), the control of acid secretion is probably taken over by neural pathways, explaining the high acid output. The observations illustrate the complexity and plasticity of the acid regulatory mechanisms. It seems that one pathway may be suppressed or allowed to dominate over the others depending on the circumstances.     

The effect of submaximal doses of pentagastrin on gastric acid secretion, histamine and histidine decarboxylase in rats

In Lai-rats gastric mucosal histamine and histidine decarboxylase were estimated after stimulation of gastric acid secretion by intravenous infusions of submaximal doses of pentagastrin for 1 or 2 h. Pentagastrin produced a dose-dependent acid response with a maximum of 26 μ equiv H⁺ per 10 min at a dose of 2.56 μg kg^?1 min^?1. There was a linear relation between the log dose of pentagastrin and the activation of gastric histidine decarboxylase. The highest dose of pentagastrin yielded a histidine decarboxylase activity of 200% of the unstimulated level when infused for 1 h and of 290% when infused for 2 h. No reduction of gastric mucosal histamine could be detected whatever the dose of pentagastrin or the duration of infusion. It was concluded (1) that stimulation of gastric histidine decarboxylase is a physiological function of gastrin-like peptides, (2) that a reduction of gastric mucosal histamine by gastrin or pentagastrin is a pharmacological rather than a physiological effect, and (3) that no negative feedback relation exists beween gastric mucosal histamine and the activation of histidine decarboxylase.     

Reversibility of cholecystokinin-B/gastrin receptor blockade: a study of the gastrin-ECL cell axis in the rat

Gastrin acts via cholecystokinin-B/gastrin receptors to control histamine- and chromogranin A-producing ECL cells, which constitute the quantitatively predominant endocrine cell population in the acid-producing part of the rat stomach. Cholecystokinin-B receptor blockade is known to suppress the activity of ECL cells and to prevent their ability to respond to gastrin stimulation. The present study examines the reversibility of long-standing cholecystokinin-B receptor blockade of ECL cells. YM022, a potent and selective cholecystokinin-B receptor antagonist, was administered in a maximally effective dose by continuous subcutaneous infusion for 4 weeks (via osmotic minipumps). The resulting receptor blockade was manifested in elevated serum gastrin concentration (due to the ensuing acid inhibition), while the serum pancreastatin concentration, oxyntic mucosal histidine decarboxylase activity, histidine decarboxylase- and chromogranin A- mRNA levels and histamine and pancreastatin concentrations were lowered. After withdrawal of YM022, all these parameters returned to normal after varying lengths of time. The serum gastrin concentration and the oxyntic mucosal histidine decarboxylase activity returned to normal within a week after termination of treatment. The serum pancreastatin concentration and the mucosal histidine decarboxylase- and chromogranin A-mRNA levels returned to normal within 2 weeks of drug withdrawal. The mucosal pancreastatin and histamine concentrations remained unchanged for about a week before gradually returning to control levels within the next two weeks. Hence, the various effects of cholecystokinin-B receptor blockade of the ECL cells are fully reversible within 1-3 weeks of drug withdrawal.     

Anti-ulcer Activity of Cromakalim (BRL 34915), a Potassium-channel Opener,Against Experimentally Induced Gastric and Duodenal Ulcers in Rats and Guinea-pigs

The effect of cromakalim, a potassium-channel opener, was studied on pylorus ligation-induced, aspirin-induced and water-immersion plus restraint stress-induced gastric ulcers in rats and on histamine-induced duodenal ulcer in guinea-pigs. Pretreatment with cromakalim (50–500 μg kg^?1, p.o.) resulted in a significant reduction in the incidence of gastric and duodenal ulceration in each model. The anti-ulcer activity of cromakalim was comparable with that of cimetidine. Cromakalim at 100, 250 and 500 μg kg^?1 caused a reduction in the volume of the gastric content in pylorus-ligated rats, and a dose of 250 μg kg^?1 resulted in a significant reduction in total acidity (28.81 ± 11.73 mEq L^?1, P < 0.02) in the pylorus ligation model. A significant reduction in total acid output was observed at doses of 250 μg kg^?1 (84.27 ± 22.33 mEqH+, P < 0.02) and 500 μg kg^?1 (120.17 ± 24.49 mEq H+, P < 001) in pylorus-ligated rats. A significant reduction in the ulcer index in pylorus-ligated rats was observed at all cromakalim doses: 50 μg kg^?1 (0.23 ± 009, P < 0.05), 100 μg kg^?1 (0.15 ± 0009, P < 0.02), 250 μg kg^?1 (0.12 ± 0.05, P < 0.01) and 500 μg kg^?1 (0.14 ± 0.03, P < 0.02). A significant reduction in the ulcer index of aspirin-treated rats was also observed at all cromakalim dose levels: 50 μg kg^?1 (0.39 ± 0.03. P < 0.01), 100 μg kg^?1 (0.28 ± 0.06, P < 0.01), 250 μg kg^?1 (0.22 ± 0.04, P < 0.001) and 500 μg kg^?1 (0.28 ± 0.03, P < 0.01). In the water-immersion plus restraint stress-induced gastric ulcer model, cromakalim significantly reduced gastric ulceration at all the dose levels: 50 μg kg^?1 (28.2 ± 2.12, P < 0.001), 100 μg kg^?1 (20.24 ± 1.71, P < 0.01), 250 μg kg^?1 (19.95 ± 1.46, P < 0.001) and 500 μg kg^?1 (21.61 ± 3.00, P < 0.001) but there was no consistent reduction of gastric bleeding. In addition to gastric ulcers, duodenal lesions were also reduced by pretreatment with cromakalim at all dose levels: 50 μg kg^?1 (97.87 ± 20.03 mm², P < 0.02). 100 μg kg^?1 (70.72 ± 12.82 mm², P < 0.02), 250 μg kg^?1 (48.32 ± 8.42 mm², P < 0.01) and 500 μg kg^?1 (55.50 ± 12.50 mm², P < 0.01). Cromakalim at a dose of 100 μg kg^?1 also reduced total acidity (99.36 ± 9.12 mEq L^?1, P < 0.02) and total acid output (172.22 ± 45.33 mEq of H+, P < 0.05) in this model. These findings demonstrate the anti-ulcer activity of cromakalim in different experimental models and suggest its potential use in ulcer therapy.     

The action of morphine and naloxone on acid secretion by the rat isolated stomach

The action of morphine and naloxone on acid secretion by the rat isolated stomach has been studied. Morphine (10^?7 to 10^?4 M) had no effect upon spontaneous acid secretion. Morphine (10^?6 M) did not modify the acid output in response to sub-maximal stimulation by pentagastrin, histamine, bethanechol or isoprenaline. Naloxone (10^?6 M) was without effect on the response to pentagastrin or histamine. Our results suggest that opiate receptors do not modify acid secretion in this preparation.     

The effect of SC-15396, atropine and mepyramine on gastrin-, bethanechol- and histamine-stimulated gastric acid secretion in rats and guinea-pigs

The effect of SC-15396 (“antigastrin”; 2-phenyl-2-(2-pyridyl)thioacetamide), atropine and mepyramine on gastrin-, bethanechol- and histamine-stimulated gastric acid secretion was studied in rats and guinea-pigs. For all three stimulants parallel dose response curves were obtained except in guinea-pigs where bethanechol even in very high doses displays a poor activity in stimulating gastric acid secretion. The maximal secretory response was found to be 12·7 ± 5·0 μ-equiv HCl/10 min in rats and 53·2 ± 27£9 μ-equiv HCl/10 min in guinea-pigs. All stimulating effects on gastric acid secretion were reduced by SC-15396; atropine abolished the secretory responses to bethanechol. Mepyramine was ineffective. In accordance with these findings the mechanism of action of gastrin and a receptor model on the oxyntic cell are discussed.     

The Disposition of Thioperamide,a Histamine H3-Receptor Antagonist,in Rats

Abstract— An HPLC method using an ovomucoid-conjugated column has been developed for measurement of thioperamide, a histamine H₃ antagonist, with a minimum quantitation limit of 0·05 μg mL^?1 The assay was used to study the disposition of thioperamide in rats. After bolus intravenous administration of thioperamide (10 mg kg^?1), the plasma concentration decreased monoexponentially with a half-life of 26·9 min. The apparent total body clearance of thioperamide from rat plasma was 74·6 mL min^?1 kg^?1. Although thioperamide was quickly transferred to various tissues, its concentrations in peripheral tissues were higher than that in the brain. However, the brain regional tissue/plasma ratios of thioperamide increased continuously after its injection.     

Pharmacokinetics and Tissue Distribution of (±)-3′-Azido-2′,3′-dideoxy-5′-O-(2-bromomyristoyl)thymidine,a Prodrug of 3′-Azido-2′,3′-dideoxythymidine (AZT) in Mice

The in-vivo biodistribution and pharmacokinetics in mice of 3′-azido-2′,3′-dideoxythymidine ( 1 , AZT), 2-bromomyristic acid ( 2 ) and their common prodrug, (±)-3′-azido-2′,3′-dideoxy-5′-O-(2-bromomyristoyl)thymidine ( 3 ) are reported. The objectives of the work were to enhance the anti-human immunodeficiency virus and anti-fungal effects of 1 and 2 by improving their delivery to the brain and liver. The pharmacokinetics of AZT (βt1/2 (elimination, or beta-phase, half-life) = 112.5 min; AUC (area under the plot of concentration against time) = 29.1 ± 2.9 μmol g^?1 min; CL (blood clearance) = 10.5 ± 1.1 mL min^?1 kg^?1) and its ester prodrug ( 3 , βt1/2 = 428.5 min; AUC = 17.3 ± 4.7 μmol g^?1 min; CL = 17.6 ± 4.8 mL min^?1 kg^?1) were compared after intravenous injection of equimolar doses (0.3 mmol kg^?1) via the tail vein of Balb/c mice (25.30 g). The prodrug was rapidly converted to AZT in-vivo, but plasma levels of AZT (peak concentration 0.17 μmol g^?1) and AUC (12.3 μmol min g^?1) were lower than observed after AZT administration (peak concentration 0.36 μmol g^?1; AUC 29.1 μmol min g^?1). The prodrug also accumulated rapidly in the liver immediately after injection, resulting in higher concentrations of AZT than observed after administration of AZT itself (respective peak concentrations 1.11 and 0.81 μmol g^?1; respective AUCs 42.5 and 12.7 μmol min g^?1). Compared with doses of AZT itself, 3 also led to significantly higher brain concentration of AZT (25.7 compared with 9.8 nmol g^?1) and AUCs (2.8 compared with 1.4 μmol min g^?1). At the doses used in this study the antifungal agent 2-bromomyristic acid was measurable in plasma and brain within only 2 min of injection. Hepatic concentrations of 2-bromomyristic acid were higher for at least 2 h after dosing with 3 than after dosing with the acid itself. In summary, comparative biodistribution studies of AZT and its prodrug showed that the prodrug led to higher concentrations of AZT in the brain and liver. Although the prodrug did not result in measurably different concentrations of 2-bromomyristic acid in the blood and brain, it did lead to levels in the liver which were higher than those achieved by dosing with the acid itself.     

Catecholamine uptake blockade in anaesthetized dogs: influence on cardiovascular responses

The effects of differential and combined catecholamine uptake antagonism on cardiovascular responses of anaesthetized dogs to isoprenaline, noradrenaline, and electrical stimulation of the left ansa subclavia nerve have been studied. Uptake 1 inhibition by cocaine HCl (5 mg kg^?1 and 1 mg kg^?1 every 45 min) enhanced responses to noradrenaline (0·1 to 2·0 μg kg^?1 i.v.) and sympathetic nerve stimulation (1 to 20 Hz), but did not affect those to isoprenaline. Uptake 2 inhibition by metanephrine (40 μg kg^?1 min^?1) enhanced cardiac responses to isoprenaline (0·05 to 1·0 μg kg^?1 i.v.), but did not significantly alter those to noradrenaline or nerve stimulation. Responses to all agonist interventions were increased by the combined administration of cocaine and metanephrine. Cocaine preferentially enhanced the positive chronotropic cardiac response to noradrenaline, but metanephrine did not differentiate between heart rate and contractility. These results have been discussed in the light of the mechanism of drug action involved.     

Stimulation of spinal dopaminergic receptors: Differential effects on tail reflexes in rats

The tail-flick reflex to heat and tail-withdrawal reflex to touch were measured in spinal and intact rats given either apomorphine (0.02–1.75) μmol · kg^?1) or dopamine (0.02–1.76 μmol · kg^?1) in the lumbar subarachnoid space. In spinal rats both apomorphine and dopamine suppressed thermal tailflick reflex and enhanced tactile tail-withdrawal response in a dose-dependent way. The effect of apomorphine in spinal rats was counteracted by dopaminergic receptor antagonists (cis-flupenthixol and (+)-butaclamol), but not by their stereoisomers. Phenoxybenzamine, propranolol, methysergide and naloxone failed to counteract the effects of apomorphine on tail-reflex responses in spinal rats. (+)-Butaclamol also counteracted effects of dopamine in spinal rats. Neither apomorphine nor dopamine influenced tail reflexes in intact rats, which suggests that effects of spinal dopaminergic mechanisms on these reflexes are influenced by descending supraspinal pathways.