The hemispheric distribution of Torpedo nicotinic receptors expressed in Xenopus oocytes

The physical and the functional distribution of Torpedo nicotinic-cholinergic receptors expressed in Xenopus oocytes was assayed. Physical hemispheric receptor distribution was tested by binding of 125[I]-bungarotoxin. The density of the expressed nicotinic receptors was equal on both hemispheres (ratio animal/vegetal = 1.1 +/- 0.2). Functional distribution was tested either by whole hemispheric response assay or by monitoring responses from small areas on the two hemispheres. While the first method yielded results that suggested uniform receptor density distribution, the second method indicated two-fold higher responsiveness on the animal hemisphere, when compared with the vegetal hemisphere. Direct comparison on oocytes of the same donors did not reveal significant differences between the two assays. We did see, however, a high variability among the different donors (animal/vegetal activity ratio range 0.5-4.7). Overall, in 35 experiments in 18 donors, the animal/vegetal ratio of hemispheric responsiveness was 1.4. The possible source of this high variability may have been the large excess of bungarotoxin-binding sites over the number of active channels. We have also tested hemispheric responsiveness ratio with different concentrations of acetylcholine. When acetylcholine concentration was below 10 microM, the animal/ vegetal ratio was significantly lower than 1.0. Similar results were obtained with nicotinic receptors expressed after injection of RNA transcribed in vitro from cloned mouse nicotinic receptor subunits. These results imply that hemispheric membrane heterogeneity may affect receptor and/or channel activities to yield polarized channel activity despite nearly homogeneous receptor distribution.