Gastrin secretion by ovine antral mucosa in vitro

The effect on gastrin and somatostatin release in sheep of stimulatory and inhibitory peptides and pharmacological agents was investigated using an in vitro preparation of ovine antral mucosa. Carbachol stimulated gastrin release in a dose-dependent manner but had no effect on somatostatin release. As atropine blocked the effect of carbachol, cholinergic agonists appear to stimulate gastrin secretion directly through muscarinic receptors on the G-cell and not by inhibition of somatostatin secretion. Both vasoactive-intestinal peptide (VIP) and gastric-inhibitory peptide (GIP) increased somatostatin release but did not inhibit basal gastrin secretion, although VIP was effective in reducing the gastrin response to Gastrin-releasing peptide (GRP). Porcine and human GRP were stimulatory to gastrin secretion in high doses but bombesin was without effect. The relative insensitivity to GRP (not of ovine origin) previously reported from intact sheep may be caused either by a high basal release of somatostatin or by the ovine GRP receptor or peptide differing from those of other mammalian species

Effect of Ostertagia circumcincta excretory/secretory products on gastrin release in vitro

It has been suggested that parasite excretory/secretory (ES) products may be capable of direct stimulation of gastrin secretion and of contributing to the hypergastrinaemia typical of abomasal parasitism. Ostertagia circumcincta ES products were tested on an ovine antral mucosal preparation which had been developed for a pharmacological study of gastrin secretion in the sheep. Its responsiveness to chemical stimulation was established by stimulation with amino acids and amines: tryptophan (0.1–5 mM) and phenylalanine (10–100 mM) stimulated gastrin release (151–160 and 117–129%, respectively), whereas glycine (0.1–100 mM) was without effect; ammonium sulphate, but not sodium sulphate, stimulated gastrin release in concentrations from 1 mM (122%) to 50 mM (148%). ES products were prepared by incubation of exsheathed third-stage larvae (L3) or parasites recovered on Day 8 p.i. (L4), Day 12 p.i. (10% L4, 90% immature adults), Day 21 p.i. (5% L4, 30% immature adults, 65% adults), Day 22 p.i. (20% immature adults, 80% adults), Day 30 p.i. (adults) and Day 35 p.i. (adults), or a mixed-age parasite population. Worms were recovered from agar and incubated in either distilled water or Hank's balanced salt solution (HBSS) adjusted to pH 2.5, 3.5, 4.5, 5.0 or 7.4. HBSS pH 7.4 was also prepared with antibiotics, without glucose, and with antibiotics but without glucose. Survival of Day 21 and 35 worms and exsheathed L3 in water or in a series of HBSS adjusted to pH 2.5, 3.5, 4.5, 5.0 or 7.4 was assessed from the percentage of motile parasites. L3 slowly became immotile over several days except in HBSS pH 2.5, in which survival was reduced, whereas adult worms did not tolerate incubation at 37 °C in water or HBSS at pH 2.5, retained motility for about 2 days at pH 3.5, but survived well at pH 4.5 and above. Incubates prepared from all stages of O. circumcincta, both in media favourable and unfavourable for parasite survival, failed to stimulate consistently the secretion of gastrin by tissue from both parasite-naive and previously exposed sheep, whereas a considerable number of incubates were significantly inhibitory. The inhibitor may not be produced by the nematodes, but by contaminating abomasal or environmental microflora, as inhibitory activity was predominantly generated by prolonged incubation, it was less potent when glucose was omitted and was not present in media containing antibiotics. This study did not find evidence for a gastrin stimulant in O. circumcincta ES products, but did demonstrate the acid intolerance of adult worms and suggests that abomasal microbes may be capable of modulating the secretory activity of the host digestive tract

Abomasal bacteria produce an inhibitor of gastrin secretion in vitro

Previously, proliferating microflora transferred with abomasal nematodes, were suspected to be the source of the gastrin inhibitor in some parasite excretory/secretory products. Aerobic cultures in HBSS of abomasal fluid from uninfected sheep became inhibitory during the static growth phase, unless antibiotics were present. Basal gastrin secretion was reduced by up to 90%. Rumen fluid and incubates and medium in which Streptococcus bovis and ovine rumen Actinomycete spp. had been grown also contained the inhibitor. Unlike abomasal cultures, rumen fluid and incubates also reduced the measurement of gastrin standards. Rumen incubates were less potent after exposure to pH 2–3, suggesting that inactivation normally occurs in the unparasitised abomasum. Contaminating bacteria which generate the gastrin inhibitor in parasite ES products are probably rumen organisms which survive in the abomasum and proliferate during subsequent incubation. Significantly, rumen bacteria have been shown to be capable of affecting the secretory activity of the gastric mucosa

Reduced Ostertagia circumcincta burdens in milk-fed lambs

Aims: To compare the susceptibility to parasitism by Ostertagia circumcincta of lambs fed entirely with bovine milk or weaned on to solid feed at 3 weeks of age. In addition, the effect of a single daily feed of milk on worm burdens was assessed.\ud \ud Methods: Eight lambs were assigned to each of the 3 diets: milk (M), milk plus solid feed (cereal-based pellets and lucerne chaff) (MS), or solid feed only (S). Those to be fed solid feed were converted from complete milk feeding to the designated diet during their third week of life. From 3 weeks of age, all lambs were infected with 1000 O. circumcincta larvae twice weekly for 6 weeks; 4 lambs from each diet group were given normal sheathed L3 and another 4 were infected with exsheathed larvae. Faecal egg counts (FEC) and serum gastrin and pepsinogen concentrations were monitored from Day 17 after first infection, and worm burdens and abomasal pH and morphology were determined at necropsy.\ud \ud Results: Total worm burdens and FEC were significantly lower in the M than MS and S groups, whereas there was no significant difference between those receiving sheathed and exsheathed larvae. The milk-fed lambs had a smaller reticulo-rumen and omasum and a more acidic abomasal pH. Serum gastrin and pepsinogen were increased in all groups, irrespective of diet or type of larvae used for infection.\ud \ud Conclusions: The cause of the lower worm establishment in lambs fed only milk was probably not failure to exsheath in the immature gastro-intestinal tract, as there were similar worm burdens in lambs whether sheathed or exsheathed larvae were administered. The lower pH of the abomasal contents of the preruminant lambs may have been a factor, as the parasites have previously been shown to die more rapidly in vitro at low pH. Alternatively, the milk itself had adverse effects on the parasites, but was ineffective when combined with solid feed. There was no benefit from feeding a milk plus solid diet over a solid diet

Effects of adult and larval Haemonchus contortus on abomasal secretion

Abomasal pH and serum pepsinogen and gastrin were increased in parasite-naive sheep by infection with either larval or adult H. contortus. Four sheep received 10 000 larvae intraruminally and 9000 adult worms were given directly via an abomasal cannula to another 4 sheep. The latter animals were dosed orally with 0.4 mg kg−1 ivermectin 4 days after worm transfer and their recovery was monitored for a further 8 days. Whereas the presence of adult worms rapidly induced changes in the secretory activity of the abomasum, the early larval stages had minimal effects up to the 4th day post-infection. After either larval or adult infection, the initial hypergastrinaemia began at the same time as the increase in abomasal pH, but serum gastrin continued to increase after abomasal pH had reached a maximum and returned more slowly to normal values after drenching. The increase in serum pepsinogen did not precede those in the other parameters, unlike the earlier hyperpepsinogenaemia which occurs with Ostertagia infection. Three of the 8 infected sheep had no detectable serum pepsinogen increases during the parasitism while showing typical effects on abomasal pH and serum gastrin. The rapidity of the responses to the transfer of adult worms and to their removal by treatment with anthelmintic supports a role for worm excretory/secretory (ES) products which possible are not produced by the early larval stages. The similarity of responses to H. contortus and O. circumcincta infection suggests the involvement of the same or very similar ES products