Two types of intrinsic muscarinic responses in Xenopus oocytes. II. Hemispheric asymmetry of responses and receptor distribution.

Fully grown Xenopus laevis oocytes display marked morphological asymmetry. The giant cell is divided into animal (pigmented) and vegetal hemispheres. We have developed methodology aimed at easy determination of hemispheric responses to the application of acetylcholine (ACh) and determination of the distribution of muscarinic receptors. Oocytes of common donors exhibit muscarinic responses that are similar when either the animal or the vegetal hemisphere of the cell is exposed to ACh. Oocytes of variant donors, however, exhibit markedly larger muscarinic responses when the animal hemisphere is exposed to ACh (ratio animal/vegetal, 5.8). The differences in hemispheric responsiveness correlate well with the hemispheric distribution of muscarinic receptors. While oocytes of common donors exhibit a modest excess of receptor number at the animal hemisphere (ratio animal/vegetal hemispheres, 1.4), oocytes of variant donors exhibit a large excess of receptors on the animal hemisphere (ratio animal/vegetal, 5.6). Upon further examination, we have found that the distribution of muscarinic receptors is non-homogeneous in either hemisphere in oocytes of both common and variant donors. The asymmetric distribution of receptors may be related to increased efficiency of signal transduction coupling in oocytes of variant donors.