L-glutamate binding sites of parasitic nematodes: an association with ivermectin resistance?
Nematode membrane preparations contain high amounts of low-affinity specific L-glutamate binding sites. The numbers of these sites were increased in 2 isolates, one field-derived and the other laboratory-derived, of ivermectin-resistant Haemonchus contortus and a field isolate of ivermectin-resistant Telodorsagia circumcincta, when compared to control, drug-sensitive isolates. Specific [3H]ivermectin binding to these membrane preparations showed no differences between ivermectin-sensitive and resistant isolates and the number of ivermectin binding sites was approximately 100-fold less than the number of L-glutamate binding sites. Kinetic analysis of L-glutamate binding suggested the presence of at least 2 classes of binding site. L-Glutamate binding was blocked by ibotenic acid, kynurenic acid and beta-hydroxyaspartate, but not by ivermectin, argiopine, kainate, quisqualate or NMDA. Competition assays with ibotenic acid suggested that there were 2 distinct populations of glutamate binding sites and that the site with the lower affinity for ibotenate was upregulated in the ivermectin-resistant nematodes. In the field isolate of resistant H. contortus we found no coding changes in the cDNAs encoding glutamate-gated chloride channel subunits HG2, HG3 and HG4, nor were any changes in channel expression detected using subunit-specific antibodies. The low-affinity binding site is unlikely to be associated with the ivermectin receptor in these nematodes.
Hejmadi
M V
MV
Department of Biology and Biochemistry, University of Bath, Claverton Down, UK.
Jagannathan
S
S
Delany
N S
NS
Coles
G C
GC
Wolstenholme
A J
AJ
D000818:Animals; D000969:Antinematodal Agents / Q000494:pharmacology*; D001665:Binding Sites; D018118:Chloride Channels; D004351:Drug Resistance; D005971:Glutamates / Q000378:metabolism*; D006189:Haemonchus / Q000187:drug effects / Q000254:growth & development / Q000378:metabolism; D015640:Ion Channel Gating; D007559:Ivermectin / Q000494:pharmacology*; D009348:Nematoda / Q000187:drug effects* / Q000254:growth & development / Q000378:metabolism*
D000969:Antinematodal Agents; D018118:Chloride Channels; D005971:Glutamates; D007559:Ivermectin