Human ␣4␤2 nicotinic acetylcholine receptors (AChRs) expressed in Xenopus laevis oocytes or transfected cell lines are present as a mixture of two stoichiometries, (␣4) 2 (␤2) 3 and (␣4) 3 (␤2) 2 , which differ depending on whether a ␤2 or ␣4 subunit occupies the accessory subunit position corresponding to ␤1 subunits of muscle AChRs. Pure populations of each stoichiometry can be expressed in oocytes by combining a linked pair of ␣4 and ␤2 with free ␤2 to produce the (␣4) 2 (␤2) 3 stoichiometry or with free ␣4 to produce the (␣4) 3 (␤2) 2 stoichiometry. We show that the (␣4) 3 (␤2) 2 stoichiometry and the (␣4) 2 (␤2) 2 ␤3 and (␣4) 2 (␤2) 2 ␣5 subtypes in which ␤3 or ␣5 occupy the accessory positions have much higher permeability to Ca 2ϩ than does (␣4) 2 (␤2) 3 and suggest that this could be physiologically significant in triggering signaling cascades if this stoichiometry or these subtypes were found in vivo. We show that Ca 2ϩ permeability is determined by charged amino acids at the extracellular end of the M2 transmembrane domain, which could form a ring of amino acids at the outer end of the cation channel. ␣4, ␣5, and ␤3 subunits all have a homologous glutamate in M2 that contributes to high Ca 2ϩ permeability, whereas ␤2 has a lysine at this position. Subunit combinations or single amino acids changes at this ring that have all negative charges or a mixture of positive and negative charged amino acids are permeable to Ca 2ϩ . All positive charges in the ring prevent Ca 2ϩ permeability. Increasing the proportion of negative charges is associated with increasing permeability to Ca 2ϩ .

The serodiagnosis of Strongyloides stercoralis infection by enzyme-linked immunosorbent assays based on crude antigen (CrAg-ELISA), while useful, has been limited by the reliance on crude parasite extracts. Newer techniques such as the luciferase immunoprecipitation system assay (LIPS), based on a 31-kDa recombinant antigen (termed NIE) from S. stercoralis and/or the recombinant antigen S. stercoralis immunoreactive antigen (SsIR), or the NIE-ELISA have shown promise in controlled settings. We compared each of these serologic assays in individuals from both regions of the world in which S. stercoralis is endemic and those in which it is not. A comprehensive stool evaluation (sedimentation concentration, Baermann concentration with charcoal cultures, agar plate, and Harada-Mori) and four different serologic techniques using CrAg-ELISA or recombinant NIE-ELISA as well as LIPS using NIE alone or in combination with a second recombinant antigen (NIE/SsIR-LIPS) were compared among individuals with parasitologically proven infection (n ‫؍‬ 251) and healthy controls from regions of the world in which the infection is nonendemic (n ‫؍‬ 11). Accuracy was calculated for each assay. The prevalence of S. stercoralis infection was 29.4% among Argentinean stool samples (n ‫؍‬ 228). Sedimentation concentration and Baermann were the most sensitive stool-based methods. NIE-LIPS showed the highest sensitivity (97.8%) and specificity (100%) of the serologic assays. The calculated negative predictive value was highest for both the NIE-LIPS and CrAg-ELISA (>97%) irrespective of disease prevalence. No cross-reactivity with soil-transmitted helminths was noted. NIE-LIPS compares favorably against the current CrAg-ELISA and stool evaluation, providing additional accuracy and ease of performance in the serodiagnosis of S. stercoralis infections irrespective of disease prevalence.