Fig. 1

Heterotaxia induced by H⁺-V-ATPase inhibitors. Xenopus embryos were soaked in inhibitors of various ion transporters. The percent of embryos considered heterotaxic (defined as a reversal of at least one from the heart, gut or gall bladder) is calculated relative to the total number of embryos, most of which showed normal laterality. Inhibitors of H⁺-V-ATPase (A) caused significant levels of heterotaxia, while inhibitors of other proton pumps (B,C) or of other transporters (D-J) had no effect on laterality. Inhibitor names and sample sizes are listed above the bars; targets and doses of drugs in B-J are listed in Table 1. Complete randomization of three organs would lead to a maximum heterotaxia rate of 87.5%, as, by chance, organ situs will appear to be wild type in 12.5% of embryos. (K) A wild-type embryo, ventral view, showing the normal arrangement of the gut (yellow arrowhead), heart apex (pink arrowhead) and gall bladder (green arrowhead). (K') Higher magnification of normal heart. (L) A heterotaxic embryo (ventral view) showing reversal of all three organs, i.e. situs inversus. (L') Close-up of reversed heart. Image contrast has been enhanced for clarity, and the loop of the heart has been outlined with black dots in K and L. Drugs used for this screen were titered to determine a dose that will cause heterotaxia without causing other morphological defects; (M) an example titration curve for concanamycin. The asterisk in M corresponds to the datum used in A (115 nM). There is a degree of variability among sensitivity of embryos obtained from different females; toxicity (defined as the percent of embryos dying post-gastrulation and/or developing with significant morphological defects) increases at larger concentrations, and there is only a narrow range of useful doses. The dose that is toxic to 50% of embryos (TD₅₀; corrected for control background lethality of 9%) was 234 nM.