Energy metabolism of adult Haemonchus contortus in vitro: a comparison of benzimidazole-susceptible and -resistant strains.

In vitro biochemical characteristics of three strains of Haemonchus contortus, benzimidazole-susceptible, mebendazole-resistant and thiabendazole-resistant isolates, were investigated. Steady-state pool sizes of glucose and metabolic intermediates, including adenine nucleotides and end-products revealed no differences between adult worms resistant or susceptible to benzimidazoles in 30-60 min incubations. Possible regulatory steps in the glycolytic pathway are identified as those involving the enzymes hexokinase, phosphofructokinase and pyruvate kinase. The major component of carbohydrate reserves was trehalose, some glycogen was present and the glucose pool was small. On incubation for 18 h in vitro, carbohydrates were metabolised in all three strains. However, in the benzimidazole-susceptible worms there was a preferential use of the glycogen reserves to maintain energy metabolism. All three strains had similar levels of total lipid, total protein and free amino acid and these did not change on incubation. The major products found in the medium on incubation, in vitro, for 18 h were propionate, acetate and propanol, with smaller amounts of ethanol, lactate and malate. All three strains produced a similar sum total of end-products; however, in the mebendazole-resistant strain there appeared to be a diversion of carbon flow to the ethanol-producing pathway. Carbon dioxide production in 60 min incubations was measured using radioactively labelled glucose. A greater output of labelled CO2 was noted under aerobic than anaerobic conditions. This was particularly true of the mebendazole-resistant strain and, in this strain, was sensitive to cyanide. The extent to which metabolic differences noted in the three strains may be related to benzimidazole resistance is not readily apparent.

Observations on the fumarate reductase system in Haemonchus contortus and their relevance to anthelmintic resistance and to strain variations of energy metabolism.

Fumarate reductase activity in a thiabendazole-resistant strain of Haemonchus contortus was found to be significantly lower than that from a susceptible strain. However, the fumarate reductase activity in a mebendazole-resistant strain did not differ from that in the susceptible strain, even though it was cross-resistant to thiabendazole. Published reports of fumarate reductase activity in strains of H. contortus susceptible or resistant to benzimidazoles were reassessed. A second, unrelated Australian thiabendazole-resistant strain also proved to have significantly diminished fumarate reductase activity, whereas two American strains, one resistant to thiabendazole and one to cambendazole, possess fumarate reductase activities indistinguishable from corresponding susceptible strains. It therefore appears that the phenomenon of benzimidazole resistance cannot be generally correlated with diminished fumarate reductase activity, although in the specific case of the Australian thiabendazole-resistant strains it may be a contributory factor.

The effects of changes in the definitive host environment on the metabolism of Hymenolepis diminuta during growth and maturation.

Flexibility in the metabolism of Hymenolepis diminuta is associated with changing intrinsic requirements during maturation but is also influenced by extrinsic factors, that is, by the nature of the host environment. End-products of carbohydrate metabolism and enzyme activities in worm extracts were used as indicators of metabolic regulation in H. diminuta recovered at various times postinfection. The predominant end-product from 6-day-old worms is lactate, generated by cytosolic glycolysis. As the cestode matures in the host, lactate production by the whole worm decreases and greater amounts of the mitochondrial end-products, succinate and acetate, are detected. A stable, dichotomous carbon flow to lactate, succinate and acetate is observed from 12 days post-infection. A metabolic gradient along the length of individual strobila is also evident. It extends from glycolysis, in the anterior region, to mitochondrial dismutation in the posterior region. The transition from cytosolic to mitochondrial pathways during maturation and along the strobilus is delayed or suppressed in worms recovered from immunosensitized hosts. Four host environments were compared: unsensitized rats, rats immunosensitized with a primary infection of H. diminuta, rats immunosensitized with a primary infection of Nippostrongylus brasiliensis and mice concurrently infected with Heligmosomoides polygyrus. The specific activities of PK and PEPCK in whole worm extracts were similar in 10-, 21- and 35-day-old worms and did not differ in worms isolated from different host environments. However, the PEPCK/PK ratio is high in worms that utilize mitochondrial pathways and low in worms that produce predominantly lactate. LDH activity is high in lactate producers. It is concluded that the pattern of metabolism in H. diminuta is influenced by many effectors in the host environment.

The role of the host in the regulation of end-product formation in two strains of the rat tapeworm, Hymenolepis diminuta.

Individual worms from rats infected with different strains of Hymenolepis diminuta were incubated in vitro and the products lactate, succinate, acetate and ammonia assayed. Variability in excretion was not confined to differences between strains. Two metabolic types were identified. Where succinate was above 20 mumol g-1 h-1, lactate excretion was low. Where succinate was not detected, lactate excretion was high. Acetate excretion was variable. Lactate and ammonia excretion were positively correlated. All worms from one rat were of the same type but could be of either type from different rats. The host strain had no effect. A relationship was shown between lactate excretion and the number of worms from a standard inoculum present at 21 days of infection. The incidence of high lactate excretion was increased in worms from secondary infections. Components of the host immune response may thus exert effects on the metabolism of H. diminuta, manifest as shifts in emphasis on cytosolic and mitochondrial metabolism.