Gastric secretion during pregnancy and lactation in the rat

Redistribution of tissue zinc pools during lactation and dyshomeostasis during marginal zinc deficiency in mice

Zinc (Zn) requirements are increased during lactation. Increased demand is partially met through increased Zn absorption from the diet. It is estimated that 60-80% of women of reproductive age are at risk for Zn deficiency due to low intake of bioavailable Zn and increased demands during pregnancy and lactation. How Zn is redistributed within the body to meet the demands of lactation, and how Zn deficiency affects this process, is not understood. Female C57bl/6J mice were fed a control (ZA; 30mg Zn/kg) or a marginally Zn deficient (ZD; 15mg Zn/kg) diet for 30 days prior to mating through mid-lactation and compared with nulliparous mice fed the same diets. While stomach and plasma Zn concentration increased during lactation in mice fed ZA, mice fed ZD had lower stomach Zn concentration and abrogated plasma Zn levels during lactation. Additionally, femur Zn decreased during lactation in mice fed ZA, while mice fed ZD did not experience this decrease. Furthermore, red blood cell, pancreas, muscle and mammary gland Zn concentration increased, and liver and adrenal gland Zn decreased during lactation, independent of diet, while kidney Zn concentration increased only in mice fed ZD. Finally, maternal Zn deficiency significantly increased the liver Zn concentration in offspring but decreased weight gain and survival. This study provides novel insight into how Zn is redistributed to meet the increased metabolic demands of lactation and how marginal Zn deficiency interferes with these homeostatic adjustments.

Gastroprotective effect of histamine and acid secretion on ammonia-induced gastric lesions in rats

BACKGROUND

Previous studies have shown that ammonia produced by Helicobacter pylori urease or administrated intragastrically exhibits a toxic effect on the gastric mucosa. In the present study we investigated the influence of histamine and gastric acid secretion on ammonia (NH4OH)-induced gastric lesions.

METHODS

The gastric mucosa in rats was exposed to NH4OH (1.5 ml of 250 mM solution) under basal conditions, after administration of histamine (1 mg/kg), urea with urease, and ranitidine (40 mg/kg subcutaneously) given alone or in combination. We measured the area of gastric lesions, gastric blood flow (GBF), plasma gastrin concentration, DNA synthesis, gastric acid secretion and gastric luminal concentration of PGE2.

RESULTS

Application of NH4OH resulted in the formation of acute gastric lesions. This effect was accompanied by a fall in GBF, a rise in gastric pH, and a reduction in mucosal DNA synthesis. Administration of histamine 30 min prior to NH4OH reduced the area of gastric lesions. This was accompanied by an increase in GBF, DNA synthesis, and prostaglandin E2 (PGE2) production. Ranitidine given prior to NH4OH enhanced gastric mucosa damage, and reduced GBF and DNA synthesis. This effect was accompanied by a reduction in gastric acid secretion. Ranitidine given prior to histamine abolished gastric acid secretion and the protective effect of histamine against NH4OH-induced damage; these effects were accompanied by a decrease in GBF, DNA synthesis, and concentration of PGE2. Pretreatment with 2% urea with urease given prior to NH4OH reduced NH4OH lesions. This effect was associated with an increase in gastric acid secretion, gastric generation of PGE2, GBF, and DNA synthesis. Ranitidine given prior to urea with urease inhibited gastric acid secretion and the gastroprotective effect of urea-urease gastroprotection.

CONCLUSIONS

Histamine and gastric secretion exhibit a protective effect against ammonia-induced gastric lesions. This effect appears to depend upon the stimulation of gastric acid secretion and PGE2 production, and the improvement of gastric microcirculation.

Pathogenesis of digitoxin-induced duodenal ulcers in pregnant rats. Roles of gastric acid and duodenal alkaline secretion

Late-stage pregnant rats (day 17) had higher rates of gastric acid secretion (45-55 mu eq/15 min) as compared to nonpregnant and middle-stage pregnant (day 10) rats (20-25 mu eq/15 min). In contrast, basal rates of duodenal alkaline secretion were significantly lower (2-3 mu eq/15 min) in pregnant rats (day 10 and 17) than those in nonpregnant rats (approximately 5 mu eq/15 min), although the duodenal mucosa responded to acid with a significant rise in HCO3- output in these three groups of rats. In pregnant rats (day 17), a single injection of digitoxin, a Na+ K+-ATPase inhibitor (10 mg/kg, subcutaneously), had no effect on basal acid and alkaline secretions, but significantly blocked the acid-induced HCO3- secretion for more than 18 hr from 6 hr after administration. This drug, when given once daily for four days (10 mg/kg, subcutaneously), produced well-defined ulcers in the proximal duodenum with few lesions in the stomach of female rats, and the severity and incidence were significantly higher in late-stage pregnant rats than in the other two groups of rats. Following repeated administration of digitoxin (10 mg/kg) to late-stage pregnant rats (days 17-20), acid secretion significantly declined after two days of treatment, while the acid-induced HCO3- secretion was significantly attenuated after one day of treatment and remained inhibited during the whole period. These results suggest that an impairment of the mechanisms related to acid-induced HCO3- secretion may be associated with the induction of duodenal ulcers caused by digitoxin in female rats, and the high incidence of these ulcers in late-stage pregnant rats may be due to acid hypersecretion.

Plasma levels of gastrin, somatostatin, and cholecystokinin immunoreactivity during pregnancy and lactation in dogs

Plasma levels of gastrinlike, somatostatinlike, and cholecystokininlike immunoreactivity were determined by radioimmunoassay in peripheral blood from 11 beagles during pregnancy and in response to suckling during lactation. Cholecystokinin was determined in pooled and in individual plasma samples after separation of cholecystokinin and gastrin by high-performance liquid chromatography. Gastrin and somatostatin levels were significantly increased during the 8-wk-long dog pregnancy; peak values were recorded in week 7 and week 4, respectively. The cholecystokinin level rose and peaked at week 3, and remained elevated. In response to suckling, maternal gastrin levels rose significantly when tested in week 1 and 3 of lactation. Somatostatin levels did not respond significantly to suckling, but basal levels were elevated in week 1 compared to week 3 of lactation. Cholecystokinin rose in response to suckling both in week 1 and 3 of lactation. We suggest that the increased levels of gastrointestinal hormones during pregnancy and lactation may serve to increase digestive and anabolic capacity.

Sex related differences in serum gastrin concentrations and G- and D-cell populations of the gastric mucosa in guinea-pigs (experimental RIA and immunocytochemical study)

In the present study antral G-cells which secrete gastrin and antral and fundic D-cells which secret somatostatin, an inhibitor of gastric acid secretion were revealed immunocytochemically and the population size estimated along with serum gastrin levels in ten male and eight female guinea pigs. Serum gastrin, antral G-cells, antral D-cells and fundic D-cells were 41.90 +/- 6.10 SD pg/ml, 210 +/- 18.03 SD cells/cm, 128.20 +/- 17.64 SD cells/cm and 121 +/- 17.91 SD cells/cm respectively in males and 35 +/- 4.62 SD pg/ml, 176 +/- 13.80 SD cells/cm, 108.40 +/- 6.90 SD cells/cm and 106.8 +/- 6.50 SD cells/cm respectively in females. The differences in serum gastrin levels, antral G-cell population and antral D-cell population between the two sexes were statistically significant (P less than 0.05, P less than 0.001, P less than 0.01). It is possible that endogenous androgens induce a relative hyperplasia and endogenous oestrogens a relative hypoplasia of G-cells. Antral D-cell differences may reflect an adaptive hormonal mechanism to the possible different states of gastric secretory functions.

Influence of suckling and feeding on insulin, gastrin, somatostatin and VIP levels in peripheral venous blood of lactating sows

Blood samples were collected in peripheral venous blood of seven lactating sows, when their piglets were suckling. In four of the experiments samples were also taken when the sows were fed a meal. Gastrin, insulin, somatostatin and VIP levels were measured by radioimmunoassay. Insulin levels increased by approximately 100% for about 10 min in response to suckling, in some experiments even before the suckling occurred, i.e. when the sows saw, heard and smelled their piglets. In four of the sows suckling caused a biphasic twofold increase in gastrin levels - one immediate peak which lasted for a few min and a second peak of longer duration (about 30-60 min), whereas gastrin levels remained unchanged in three animals. Somatostatin levels usually reflected gastrin levels in a reciprocal way. Thus, a biphasic decrease of somatostatin levels occurred in the high gastrin responders. In contrast, somatostatin levels increased in the experiments, in which gastrin levels did not change. Immediate and short-lasting (a few minutes long) increases of VIP levels were also induced by suckling. Large litters and long suckling periods appeared to be related to greater changes of the levels of all the peptides measured. Feeding influenced insulin, gastrin and somatostatin levels in the same way as did suckling from both a qualitative and a quantitative point of view. In contrast, VIP levels were not increased by feeding. The possible functional effects of the suckling-induced release of gastrointestinal hormones and possible mechanisms of their release are discussed.

Factors related to gastric hypersecretion during pregnancy and lactation in rats

We attempted to elucidate the factors involved in gastric hypersecretion of rats during pregnancy and lactation. Acid secretion in pylorus-ligated and vagally denervated fistula rats stimulated with histamine, tetragastrin, and methacholine increased from midterm pregnancy and persisted during lactation. Pepsin secretion remained unaltered during pregnancy but increased during lactation. Vagal denervation itself abolished this hypersecretion. In late pregnancy, a delayed appearance of maximal acid response to histamine was apparent, as compared to nonpregnant rats, and was abolished by aminoguanidine treatment. There was a delay in the maximal response to tetragastrin but not to methacholine. Serum histamine concentrations were 3-4 times higher in late pregnancy, as compared to nonpregnant, lactating and nonlactating rats. Gastric DNA and protein concentrations were significantly increased in lactating rats with concomitant elevation of food intake and serum gastrin levels. Those changes disappeared in nonlactating rats, and gastric secretion was much the same in the nonpregnant rats. These results indicate that acid hypersecretion during pregnancy was exclusively associated with vagal innervation plus high serum histamine levels, while acid and pepsin hypersecretion in lactating rats were associated with vagal innervation plus hyperplastic gastric mucosa and high serum gastrin levels.

Release of gastrointestinal peptides in response to vagal activation induced by electrical stimulation, feeding and suckling

This paper contains a discussion and presentation of evidence concerning vagal control of the release of some gastrointestinal peptides, the secretion of gastrin and somatostatin in particular. Some aspects of the regulation of vasoactive intestinal peptide (VIP), substance P, bombesin and insulin release by the vagus nerve are also reviewed. Data were obtained not only by electrical stimulation of the vagi but also by monitoring effects of feeding and suckling. The involvement of vagal afferents, transmitters, neurohormones in release of other hormones and humorally carried activators and inhibitors is analyzed. Particular attention is paid to the chains of reactions induced by physiological stimuli: feeding and suckling.

Release of gastrin and insulin in response to suckling in lactating dogs

The concentration of gastrin and insulin was determined in peripheral blood of 4 lactating dogs during suckling. Suckling induced an immediate rise of gastrin and insulin levels, twofold and threefold, respectively. A peak was reached at approximately 5 min after suckling was started, and basal levels were reached again within 15-20 min. The increase in gastrin and insulin levels coincided with the let-down reflex, i.e. it occurred when the puppies started to obtain milk from their mothers. Sham feeding and feeding induce a vagally mediated increase of gastrin and insulin release in dogs. We suggest that the changes in the gastrin and insulin levels observed by us in the lactating dogs, reflect a similar vagal activation induced by suckling. Possible effects of the suckling induced release of gastrin and insulin on milk ejection and secretion are discussed.

Changes in gastrin levels, food intake, and duodenal mucosal growth during lactation

Gastrointestinal epithelial cell proliferation is stimulated during lactation in the rat. Serum gastrin levels and food intake are also increased during lactation. We investigated the role of gastrin and food intake as possible mediators of duodenal mucosal growth during the first 15 days of lactation. As the lactation period progressed, significant increases in the following crypt properties were noted: 1) crypt length; 2) cells/crypt; 3) labeling index; and 4) dimensions of the proliferative zone. Maternal serum gastrin levels rose abruptly by the first day of birth and remained elevated throughout lactation. The increases in crypt cell proliferation significantly correlated with food intake but not with serum gastrin levels during lactation. Mucosal mass and villus-crypt dimensions were also significantly increased above virgin levels in lactating antrectomized rats. These results suggest that the increase in duodenal growth during lactation most probably is not mediated by postpartum hypergastrinemia and that the increase in cell proliferation may be a direct response to an enhancement in food intake.

Influence of pregnancy on the development of various gastric lesions in rats

The influence of pregnancy, and to some extent lactation, on various gastric lesions in rats were studied. Shay ulceration and gastric lesions induced by cold-restraint stress and ulcerogenic agents, such as acetylsalicylic acid (ASA), reserpine, or epinephrine, in rats were significantly aggravated by pregnancy (day 20 of pregnancy). Gastric hypersecretion, which was observed during pregnancy in pylorus ligation preparation, appears to contribute to the mechanism of aggravation of gastric lesions. ASA induced a marked back-diffusion of acid in pregnant rats which might resulted in the aggravation of ASA-induced gastric lesions. In contrast, histamine-induced gastric lesions were markedly inhibited according according to the progress of pregnancy but after 10 days' lactation returned to the level seen in nonpregnant rats. The histaminase inhibitor aminoguanidine strongly aggravated the histamine-induced gastric lesions in pregnant rats as compared with the non-pregnant ones.

Effects of brocresine (NSD-1055) and cycloheximide on amino acid decarboxylase activities in gastric mucosa of normal and vagally denervated rats

1. Histidine decarboxylase activity of rat stomach fluctuates depending upon the functional state of the stomach. This varying enzyme activity poses special problems in assessing the effectiveness of enzyme inhibitors. After vagal denervation gastric histidine decarboxylase is markedly activated and remains at a high, stable level, which is unaffected by the functional state of the stomach. Thus it appears that vagally denervated rats are well suited for studies on histidine decarboxylase inhibitors.2. In vivo, brocresine (NSD-1055) was found to be a more effective inhibitor of gastric DOPA decarboxylase than of gastric histidine decarboxylase. With the fairly high dose given (200 mg/kg) the inhibition of histidine decarboxylase was at most 75-85% and quite short-lasting. The DOPA decarboxylase activity, which was not affected by vagal denervation, was inhibited more than 95% by brocresine; this inhibition was longer-lasting.3. Cycloheximide, which probably lowers gastric histidine decarboxylase activity by inhibiting enzyme synthesis, was maximally effective at a dose level as low as 1 mg/kg. Gastric DOPA decarboxylase was not inhibited by cycloheximide. Vagotomized rats and control rats responded similarly.4. Combined treatment of vagally denervated rats with brocresine and cycloheximide resulted in a rapid and persistent reduction of the histidine decarboxylase activity. It is concluded that the failure of brocresine alone to induce a lasting inhibition of histidine decarboxylase is due to continuous, rapid synthesis of new enzyme.5. The calculated half-life of gastric histidine decarboxylase was 75 min in vagally denervated rats and 45 min in normal fasted rats. The results suggest that the increased enzyme activity after vagal denervation is caused by an increased rate of enzyme synthesis.

Hyperplasia of the gastric mucosa during pregnancy and lactation in the rat

1. The growth of the gastric mucosa during pregnancy and afterwards has been studied by comparing the stomachs of rats, killed at weekly intervals after impregnation, with the stomachs of virgin rats of the same age and starting weight. The rate of growth of the gastric mucosa in both groups was determined from the changes observed in the weight of the whole stomach, the weight and surface area of the fundus, the volume (mass) of the fundic mucosa and the total parietal and total peptic cell populations.2. Hyperplasia of the gastric mucosa occurred during pregnancy and lactation, the effect being characterized by increases in the surface area and volume (mass) of the gastric mucosa, and in the total parietal and total peptic cell populations.3. The data suggest that the effect developed shortly after conception. Hyperplasia of the gastric mucosa continued throughout pregnancy and reached maximal values after the second week of lactation and waned thereafter; the maximal changes obtained for individual observations such as surface area and the total parietal and total peptic cell populations represented increases of the order of 40% above corresponding control values.4. From the observations that were made on food intake it seemed unlikely that the hyperplasia of the gastric mucosa was due to hyperphagia. It also seemed unlikely that the effect could be accounted for by the increase that occurred in body weight during pregnancy.

The role of the antrum and the vagus nerve in the formation of gastric mucosal histamine

1. The effects of the vagus nerve and of antral gastrin on the rate of histamine formation (histamine forming capacity, HFC, i.e. histidine decarboxylase activity) in the parietal cell region of the gastric mucosa has been investigated in the following stomach preparations: gastric fistula, denervated Heidenhain pouch, antral resection with gastrojejunostomy, gastrojejunostomy with exclusion of the duodenum and in the intact stomach. The determinations of mucosal HFC were made on fasting rats and on re-fed animals when the effect of feeding was studied.2. In fasting rats with a gastrojejunostomy and the antrum intact the mucosal HFC of the innervated stomach was about 4 times higher than in the corresponding preparation with the antrum resected. In the innervated main stomach the mucosal HFC was about twice as high as in the denervated pouch, indicating that the vagus and endogenous antral gastrin each contribute to maintaining mucosal HFC in the fasting state.3. Acidifying the stomach caused a substantial lowering of the mucosal HFC presumably by inhibiting antral gastrin release, whereas acid in the stomach did not interfere with the elevation of mucosal HFC evoked by injection of gastrin.4. Injection of gastrin elevated mucosal HFC in the innervated main stomach and in the denervated pouch to approximately equal levels. With the dose of gastrin employed there was about a fourfold increase in the HFC of the pouch mucosa.5. In antrectomized rats enhanced vagal influence, evoked by insulin injection or by feeding, raised the mucosal HFC. In rats with the antrum intact and the stomach acidified, insulin injection produced an increased HFC. Thus, a vagal effect on mucosal HFC exists independent of participation of antral gastrin.6. The stable choline esters carbachol and methacholine act directly on the parietal cell without involving mucosal HFC. The vagus nerve and gastrin, however, are assumed to provide secretory stimulation by means of accelerated histamine formation.7. The interrelation between increased histamine formation and hydrochloric acid secretion is discussed.

The response of isolated cat muscle spindles to passive stretch

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