Excretory/secretory products of Haemonchus contortus inhibit aminopyrine accumulation by rabbit gastric glands in vitro

Abstract

Abomasal parasites inhibit gastric acid secretion and reduce the number of acid secreting parietal cells either through physical contact with gastric tissue, the release of inhibitory excretory/secretory (ES) products or by initiating the host inflammatory response. To examine the role of parasite ES products, adult Haemonchus contortus were incubated in a medium based on Hank’s balanced salt solution and three incubates were tested for the presence of inhibitors of acid secretion by dispersed rabbit gastric glands in vitro, using the intracellular accumulation of 14 C -aminopyrine (AP) as an indirect measure of acid secretion. Acceptably sensitive rabbit preparations (80%) for testing ES products showed increased 14 C -AP accumulation with either 10 −5 or 10 −4 M histamine. Sheep glands proved unsuitable as a test system as only two of six preparations responded weakly to histamine. Inhibitory activity was demonstrated in all the three parasite incubates, although responses varied quantitatively between tests, even with the same batch of incubate. In single tests, 16% ES products significantly reduced the 14 C -AP ratio ( P<0.05) of 10 −4 M histamine-stimulated glands (15 of 19 tests with Batch 1 and two of six with Batch 3). Even at 1 and 5%, ES products (Batch 2) were inhibitory for 10 −5 M histamine-stimulated glands: at 1% the mean reduction was 26.0% (range 7.8–54.8%, n=10), four of which were statistically significant and at 5% was 17.6% (range −15.3–53.8%, n=17), four of which were statistically significant. ES products were overall inhibitory (−12%, P<0.05), when tested on glands exposed to increasing histamine concentrations from 10 −6 M to 10 –3 M (which increased the 14 C -AP ratio ( P<0.001)). Responses by individual gland preparations differed ( P<0.001). The active component(s) of the parasite ES products were less than 5000 molecular weight. Ammonium chloride, tested over the range of concentrations of ammonia present in worm incubates (0.2–1 mM, final concentrations in the gland incubations of 0.005–0.1 mM), had variable, but overall inhibitory effects on 10 −5 M histamine-stimulated glands ( n=3). When tested with a range of histamine concentrations, 0.01 mM NH 4Cl overall reduced the response by 8.6% (P<0.05) (n=4) . The similarity of responses of rabbit glands to parasite ES products and to NH 4Cl suggests that ammonia may be the small molecular weight ES product of adult H. contortus, which inhibited acid secretion in vitro.