Implication of gastric mucosal histamine in inhibition by isoprenaline of pentagastrin-induced gastric secretion

Effect of terbutaline, a beta 2-adrenoreceptor agonist, on gastric acid secretion and serum gastrin concentrations in humans

Because beta-adrenoreceptor agonists inhibit gastrin-stimulated gastric acid secretion in animals, we postulated that the beta 2-adrenoreceptor agonist, terbutaline, would inhibit pentagastrin-stimulated acid secretion in humans. Moreover, we hypothesized that terbutaline might inhibit food-stimulated acid secretion, as gastrin is a major mediator of food-stimulated acid secretion. Subcutaneous terbutaline (0.25 mg) reduced acid secretion during intravenous infusion of a submaximal dose of pentagastrin by 30%-40% (p less than 0.005), even though terbutaline increased serum gastrin levels (p less than 0.05). Furthermore, subcutaneous (0.25 mg) or oral (5 mg) terbutaline, given before a homogenized steak meal was infused into the stomach, lowered mean food-stimulated acid secretion rates, despite enhanced postprandial serum gastrin concentrations. Terbutaline also increased serum gastrin concentrations in patients with Zollinger-Ellison syndrome and in vagotomized individuals. Thus, beta 2-adrenoreceptor agonists enhance gastrin release while at the same time inhibiting gastrin-stimulated acid secretion in humans.

Inhibitory effect of isoprenaline on gastric acid secretion in the rat. The role of endogenous histamine

In dogs beta-adrenoreceptor agonists inhibit gastric acid secretion stimulated by exogenous gastrin to a much greater extent than acid secretion stimulated by exogenous histamine. One possible explanation for this observation is that endogenous histamine is important in gastrin-mediated acid secretion and that isoprenaline and related beta-adrenoreceptor agonists block gastric mucosal histamine release. This possibility was tested in the present study in gastric lumen-perfused anaesthetized rats. Intravenous infusion of isoprenaline (12 microgram kg-1 h-1) inhibited maximal, pentagastrin-stimulated acid output by 50-70% (P less than 0.01), but had no significant inhibitory effect on the maximal acid secretory response to histamine. In contrast to its inhibitory effect on gastrin-stimulated acid output, isoproterenol had no effect on gastric histamine output during pentagastrin infusion. We conclude that isoprenaline selectively inhibits gastrin-stimulated acid secretion in the rat, as in the dog, and by a mechanism other than inhibiting gastric histamine release.

Effect of omeprazole on gastric acid secretion by the rat isolated stomach

The inhibitory effect of omeprazole has been investigated on the isolated gastric fundus from immature rats. Omeprazole (10(-7)-10(-5) M) inhibited basal acid secretion, conversely from H2-receptor antagonists, antimuscarinic compounds and calcium antagonists; the effect was mimicked only by KSCN (3 X 10(-4)-3 X 10(-2) M). Omeprazole (10(-6)-10(-5) M) caused an insurmountable antagonism of the hypersecretion induced by histamine and bethanechol, whereas it competitively antagonized the secretory response to isoprenaline and dibutyryl cAMP. Experiments carried out in low calcium media showed that calcium ions did not significantly affect the inhibitory potency of omeprazole when tested on basal acid secretion, whereas low calcium solutions enhanced the action of omeprazole against histamine-induced hypersecretion. The above data confirmed the potent antisecretory activity of omeprazole in different experimental conditions in which the common antisecretagogues are without effect and pointed out the novel site of action in the control of gastric acid secretion at intracellular level.

Adrenergic stimulation of pepsinogen release from rabbit isolated gastric glands

Isolated gastric glands from the rabbit were used for studying the effect of catecholamines on the release of pepsinogen. Isoprenaline, adrenaline and noradrenaline stimulated pepsinogen release in a dose-dependent manner with similar maximal effects, but isoprenaline was significantly more potent than the other two agonists. The effect was mediated through beta-adrenoceptors, since the response was inhibited by the beta-adrenoceptor antagonist, propranolol, and since the alpha-adrenoceptor agonist, phenylephrine, was without effect in the concentration range 0.01 - 10 microM. Concentration-response curves for isoprenaline were shifted to the right in parallel by increasing doses of propranolol, and maximal response was not influenced by propranolol per se, which indicates a competitive type of antagonism. A Schild plot showed a pA2-value for propranolol of 7.70 and the slope of the regression line was 1.02. Studies with the beta 1-selective antagonist pafenolol and the beta 2-selective antagonist ICI 118.551 demonstrated that isoprenaline acted through beta 1-adrenoceptors. The results suggest an adrenergic component in the control of the peptic cells in rabbit gastric mucosa.

Potentiating action of hexoprenaline on 14C-aminopyrine uptake by isolated rat parietal cells

Hexoprenaline potentiated the 14C-aminopyrine uptake (a reliable index of H+ generation) of isolated rat gastric cells stimulated by 10(-6) -10(-4) mol/l carbachol, and inhibited that in response to 10(-4) mol/l histamine without and in the presence of propranolol. It is concluded that hexoprenaline acts as a partial agonist on parietal cell H2-receptors and that beta-adrenoceptor activation may functionally modulate gastric acid secretion.

Effects of beta-adrenoceptor drug stimulation on various models of gastric ulcer in rats

1. Experiments were designed to evaluate the effect of the pharmacological activation of beta-adrenoceptors on various models of gastric ulcer in the rat. 2. Pretreatment with the beta-adrenoceptor stimulant drugs, isoprenaline or salbutamol, significantly inhibited stress-induced gastric ulcers. This anti-ulcer effect was abolished by propranolol but not by atenolol, suggesting that beta 2-adrenoceptors mediate this response. 3. In the pylorus-ligation model, salbutamol inhibited lesion formation and reduced the intragastric content of hydrogen ions, histamine and pepsin although the latter was only affected with the higher dose of salbutamol. 4. Salbutamol also prevented the ulcerogenic action on the gastric mucosa of an exogenously perfused artificial gastric juice, showing that the anti-ulcer effect is not necessarily dependent on acid inhibition. 5. Salbutamol also reduced the formation of acute ulcers induced by various iatrogenic means (histamine, polymyxin B, reserpine and indomethacin). 6. Long-term treatment with salbutamol accelerated the healing of experimental chronic gastric ulcer. 7. In anaesthetized rats, salbutamol produced a dose-related increase in mucosal blood flow which may contribute to its mode of action. 8. It is concluded that beta-adrenoceptor agonists exert preventive and curative effects on gastric damage induced in the rat. This effect seems specific and mediated through beta-adrenoceptor activation.

Inhibitory alpha 2-adrenergic mechanism regulating gastric secretion in pylorus-ligated rats

Several alpha-adrenoceptor agonists given intracerebroventricularly or subcutaneously to rats were assessed for their effects on gastric secretion under condition of pylorus ligation. Intracerebroventricular injection of alpha-adrenoceptor agonists reduced gastric secretion, in the following order (relative potency, clonidine = 1): oxymetazoline (100) greater than clonidine (1) greater than methoxamine (0.024) greater than phenylephrine (0.003). The antisecretory effects of oxymetazoline and clonidine given intracerebroventricularly were antagonized with yohimbine administered by the same route. Subcutaneous injection of alpha-adrenoceptor agonists also reduced gastric secretion, in the following order (relative potency, clonidine = 1): clonidine (1) greater than oxymetazoline (0.3), much greater than phenylephrine (0.001) approximately methoxamine (0.0006). Oxymetazoline, when given intracerebroventricularly, was most effective in decreasing the volume and titratable acidity of gastric secretion. Pretreatment with 6-hydroxydopamine intracerebroventricularly (250 micrograms/rat x 2; 72 and 120 h before) reduced the antisecretory effect of clonidine given intracerebroventricularly. Thus, gastric secretion appears to be regulated in an inhibitory manner by alpha 2-, but not by alpha 1- adrenoceptors in the central and peripheral nervous systems, in pylorus-ligated rats.

A comparison of the effects of sympathomimetic agents on gastric acid secretion by the rat stomach in vivo and in vitro

1. The action of isoprenaline on gastric acid secretion in rats with Heidenhain pouches has been compared with its action in a rat isolated stomach preparation. 2. Isoprenaline (40 micrograms kg-1 h-1) inhibited the acid secretion in response to pentagastrin (20 micrograms kg-1 h-1) in conscious rate with Heidenhain pouches. 3. This inhibition could be abolished by propranolol (2 mg kg-1) and butoxamine (8 mg kg-1) and partially reversed by practolol (8 mg kg-1). 4. Propranolol (2 mg kg-1) significantly increased the response to pentagastrin (20 micrograms kt-1 h-1) but butoxamine and practolol (both at 8 mg kg-1) and the inactive isomer (+)-propranolol (2 mg kg-1) were without any effect on the pentagastrin response in the rats with pouches. 5. In the rat isolated stomach preparation isoprenaline stimulated acid secretion over the range 10(-7) M-10(-3) M whereas phenylephrine and methoxamine were without effect. 6. Propranolol (2 X 10(-5) M) inhibited this stimulatory effect of isoprenaline in vitro but (+)-propranolol (2 X 10(-5) M), practolol and butoxamine (both at 10(-4) M) had no effect on the response. 7. Propranolol (2 X 10(-5) M) did not have any effect on the response of the isolated stomach to pentagastrin (5 X 10(-7) M) or bethanechol (1.7 X 10(-5) M). 8. Phenylephrine (2 X 10(-5) M) did not affect the in vitro responses to pentagastrin (2.17 X 10(-7) M), bethanechol (1.7 X 10(-5) M) or histamine (5.4 X 10(-5) M). 9. It is concluded that isoprenaline has a direct stimulatory effect and an indirect inhibitory effect on gastric acid secretion in the rat. Both effects involve stimulation of beta-adrenoceptors. The relative predominance of one or other of these two opposing effects may help to explain the contradictory results in the literature regarding the actions of beta-adrenoceptor agonists on gastric acid secretion.

Effects of beta2-sympathomimetic on histamine-stimulated gastric acid secretion in dogs

The effect of a selective beta2-adrenoceptor agonist on histamine-stimulated gastric acid secretion was studied in conscious gastric-fistula dogs. A dose-dependent inhibition was found, most pronounced for the lowest doses of histamine. The inhibition was on the volume, whereas the acidity was unchanged. The pulse rate was increased by the beta2-agonist and by histamine. Propranolol prevented the inhibition of acid secretion as well as the increase in pulse rate. Practolol had no effect on the inhibition of acid secretion but reduced the increase in pulse rate. Dose-response experiments with six doses of histamine and two doses of the beta2-agonist showed unchanged calculated maximal response and an increase in D50. It is concluded that the beta2-agonist inhibits histamine-induced acid secretion in the dog, but to a lesser degree that pentagastrin-induced acid secretion, as found in earlier experiments. The inhibition follows competitive kinetics, and beta2-receptors are probably involved. The influence of beta2-sympathomimetics on the mechanism of gastric acid secretion is discussed.

The effect of (-)-isoprenaline and (+/-)-salbutamol on pepsinogen and acid secretion in the dog

1 The beta-adrenoceptor agonists, (-)-isoprenaline and (+/-)-salbutamol, reduced pepsinogen secretion induced by pentagastrin in conscious dogs with Heidenhaim pouches. 2 (-)-Isoprenaline and (+/-)-salbutamol also reduced gastric acid secretion while producing a moderate tachycardia.

Further studies on the mode of action of isoprenaline on gastric secretion in the conscious rat

1 The effect of isoprenaline on gastric secretion evoked by various means has been studied in conscious rats provided with Pavlov and Heidenhain pouches. 2 Interdigestive acid secretion in the Pavlov pouch was reduced by isoprenaline, whereas pepsin secretion was unaltered. 3 Central vagal stimulation effected by 2-deoxy-D-glucose injection evoked a gastric secretory response that was substantially reduced by isoprenaline. 4 2-Deoxy-D-glucose increased the mobilization of gastric mucosal histamine, an effect that was prevented by isoprenaline. 5 Isoprenaline infusion alone induced a slight increase in histamine mobilization and also a considerable elevation of immunoreactive serum gastrin concentration. 6 The secretory response to food in the Pavlov pouch was almost abolished by isoprenaline. 7 Although the acid response to histamine in the Heidenhain pouch was susceptible to isoprenaline inhibition, that to methacholine was not. 8 Pepsin secretion in the Heidenhain pouch preparation stimulated by histamine or methacholine seemed to be enhanced by isoprenaline.

Effects of isopropylnoradrenaline on tachyphylaxis of gastrin stimulated gastric acid secretion and mucosal blood flow in the anaesthetized cat

1. Gastric acid and mucosal blood flow responses to sustained gastrin stimulation were studied in anaesthetized cats. 2. Acid and mucosal blood flow showed identical rates of tachyphylaxis to both large (maximal) and small doses of gastrin. 3. Isopropylnoradrenaline infused either intravenously or close arterially to the stomach produced increases in blood flow unrelated to time following the peak response to large doses of gastrin, whereas increases in acid secretion appeared to be a part removal of the acid tachyphylaxis. 4. During stimulation by small doses of gastrin, isopropylnoradrenaline increased both blood flow and acid secretion above the peak responses to gastrin alone. 5. Increases in gastrin stimulated acid and mucosal blood flow were also produced by expansion of the blood volume by dextran-saline infusions. 6. The physiological significance of these findings is discussed. 7. It is concluded that tachyphylaxis of gastric acid secretion to gastrin may be a function of the primary tachyphylaxis of mucosal blood flow.

Alterations in secretory patterns following antrectomy in rats with Pavlov pouches

1. In conscious rats provided with Pavlov pouches, with the antrum retained or resected,the gastric secretory response to various stimuli has been studied. Each acid secretory response was related to that obtained with maximal doses of methacholine and histamine in combination, presumed to reflect the maximal secretory capacity of the mucosa. 2. Three weeks after the operation, the maximal acid secretory capacity was 60 percent lower in the antrectomized than in the intact Pavlov pouch rats; the difference was still larger at 6 weeks and 3-5 months, owing to a gradual increase in the rats with the antrum retained. 3. Antrectomy reduced interdigestive secretion of acid to the same degree as the concomitant reduction in maximal secretory capacity. 4. Acid secretion in response to a maximal infusion of pentagastrin was reduced by about 50 percent at 3 and about 65 percent at 6 weeks after antrectomy. No significant difference was, however, noted between the antrectomized and intact rats when the responses were related to the maximal secretory capacity. The dose response curve to pentagastrin revealed a redcued responsiveness to submaximal doses of this agent following antrectomy. 5. The maximal acid secretory response to histamine was reduced after antrectomy, although the sensitivity to submaximal infusions of histamine appeared to be increased. 6. The mean secretroy output to 2-deoxy-D-glucose was reduced by about 65 percent and that to food by about 85 percent following antrectomy. 7. After antrectomy a background infusion of pentagastrin enhanced the secretory responses to 2-deoxy-D-glucose and to food but did not restore the responses to the levels in the intact rats. The feeding responses as related to the maximal secretory capacity were, however, similar in the two groups on infusing pentagastrin in the antrectomized rats. 8. Interdigestive secretion of pepsin was reduced by about 60 percent after antrectomy, while the peak response to 2-deoxy-Dglucose was about twice the interdigestive level in both groups. Pepsin secretion in response to food showed an increased secretion above the interdigestive level of longer duration in the antrectomized than in the intact Pavlov pouch rats. 9. The irreversibily reduced responsiveness of the gastric mucosa after antrectomy is discussed in relation to known morphological and biochemical changes.