Native Xenopus oocytes express two types of muscarinic receptors.

We have recently described two types of muscarinic responses in native Xenopus oocytes of different donors (common and variant) that display qualitative and quantitative differences (Lupu-Meiri et al., 1990). Here we characterized the muscarinic receptors mediating these two types. The anti-muscarinic toxins from Dentroaspis significantly inhibited responses in oocytes of common donors, but had little effect on responses in oocytes of variant donors, possibly indicating expression of different receptor subtypes. Using specific muscarinic antagonists, we found that oocytes of common donors exhibit a pattern compatible with the M3 subtype of muscarinic receptors, while oocytes of variant donors appear to possess receptors of the M1 subtype. To more directly determine the subtypes of muscarinic receptors in oocytes of both populations of donors, we have microinjected antisense oligonucleotides into native oocytes. Antisense oligonucleotides to unique sequences in the N-terminal and the third cytoplasmic loop of M3 muscarinic receptors caused a significant inhibition of the response of common oocytes, but had virtually no effect on responses in oocytes of variant donors. Conversely, oligonucleotides complementary to the unique sequences of the m1 muscarinic receptors inhibited the response in variant oocytes, but not in oocytes of common donors. We conclude that native Xenopus oocytes of different donors phenotypically express either M3-like (majority) or M1-like (minority) muscarinic receptor subtypes. The differences in receptor subtype expression may explain the different characteristics of responses in the two populations.